Crop Scouting Basics for Corn and Soybean
CPN 4007. Published January 5, 2021. DOI: doi.org/10.31274/cpn-20201214-0
Adam J. Sisson, Iowa State University; Daren S. Mueller, Iowa State University; Shawn P. Conley, University of Wisconsin-Madison; Corey K. Gerber, Purdue University; Scott H. Graham, Auburn University; Erin W. Hodgson, Iowa State University; Travis R. Legleiter, University of Kentucky; Paul P. Price, Louisiana State University; Kristine J. Schaefer, Iowa State University; Ed J. Sikora, Auburn University; Tessie H. Wilkerson, Mississippi State University; and Kenneth L. Wise, Cornell University.
Integrated Pest Management (IPM) can help corn and soybean farmers obtain higher yields and increase profits. Assembling information from a variety of crop-related disciplines and emphasizing recordkeeping and field scouting, IPM begins with gaining knowledge. Thus, the focus of this text is helping a field scout to understand what is occurring in a corn or soybean field. Knowing what a healthy crop looks like and how to assess it, knowing what insects, diseases, or weeds are present, and knowing the risk associated with these issues are integral parts of crop production. We hope that through this text, your knowledge about crop scouting and pest management will increase, and that this will lead to stronger corn and soybean production systems through increased yields, profitability, and resource stewardship.
Check out the Crop Scout School from CPN, which consists of 22 webinars from crop specialists across the Midwest on a range of crop scouting topics.
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© 2021 Crop Protection Network unless otherwise noted. All rights reserved.
This educational resource was made possible by contributions from the National Corn Growers Association, Iowa State University Extension and Outreach; Iowa State University Integrated Pest Management; and the United States Department of Agriculture - National Institute of Food and Agriculture (USDA-NIFA).
The use of Integrated Pest Management (IPM) can help farmers obtain higher yields and increase profits in corn and soybean. IPM is a set of multidisciplinary tools; it assembles information from agronomy, entomology, plant pathology, economics, agricultural engineering, and climatology. IPM uses preventive and curative tactics to manage disease, insects, and weeds (collectively referred to as "pests" for this publication).
There are two sides to IPM. The first is collecting information about what you are up against in the field. Gathering information is accomplished by activities such as field scouting, looking at previous year's records, observing pest development, and paying attention to regional and national crop reports.
The second part of IPM is taking action in response to a pest to prevent crop damage. This action is informed by scouting, recordkeeping, observed thresholds, market price, and other pertinent information. Any management tactic put into action in the field should be based on information. Afterwards, observation and information gathering begins again to determine if the management tactic worked. It pays to keep track of what you find out.
Observing pest activity is important to make informed management decisions. This device tracks spores of soybean rust.
Crop scouts can use many different tools and techniques when scouting a field. Knowledge and experience are the best tools, however.
One kind of action taken to manage pests is the application of chemicals.
Daren Mueller
As you scout a field, look at the field level and the plant level, and collect and record information so it can be used in management planning both now and in subsequent growing seasons. Remember you are looking for patterns that can help you diagnose problems. Scout different cultivars and fields with different tillage, rotation, or fertilization histories separately.
Before scouting a field, you should have an idea of where you will scout and what to expect in a particular field. Some companies provide aerial images of fields, or online mapping software can be used to view aerial images. If you do not have access to aerial maps, you can drive around the field as best as possible to get a field level assessment. Look for parts of the field that may be problematic or areas with differences in height, color, form, or other unevenness in the crop from place to place. If you see variations, then walk into the odd areas and examine them more closely to determine the cause. Certain areas of fields can be more prone to infection or infestation than others; it is important to pay special attention to these parts of the field.
Good questions to ask while looking at the field level:
Is the problem scattered randomly through the field or occurring in a pattern?
Is the problem more prevalent along a fence or field edge, the entrance of a field, or along a waterway?
Is the problem in the affected area more severe in certain soil types, low areas, or on exposed slopes?
Does the pattern correspond to tillage, planting, spraying, harvesting, or other field activities?
Big picture view of soybean field - notice the yellowish streak at the right hand side.
Early season cornfield showing poor emergence.
Aerial image from drone of lodged corn.
Nitrogen skips are apparent as light green strips in this field view.
Aerial image from drone of lodged corn.
Big picture view of crop fields, helping a scout to determine places of interest to make sure to scout.
Field level scouting
Certain areas of fields can be more prone to infection or infestation than others; it is important to pay special attention to these parts of the field. These are a few examples of areas to pay attention to.
Iowa State University Integrated Pest Management
It is helpful to follow a general pattern when scouting to gain a systematic understanding of the condition of the field. Several different patterns are useful for this purpose. Once you start walking through the field, remember the scouting pattern you are following, but also be sure to deviate from this pattern to check out any potential problem areas. The pattern is there to make sure you are looking at a good representation of the field and not just the field edges or only one side of the field. At multiple points in the field, either predetermined or triggered by an visible problem, you should stop to make closer, systematic examinations of individual plants. Regardless of your sampling plan––part of what is called a protocol––you are trying to answer two questions:
How often does the problem occur?
How much damage is the problem causing––both now and in the future?
The simplest scouting pattern is a transect, which is just walking a somewhat straight line from one point to another and looking at the plants along the way. Another pattern is the "Z" or "W" pattern, called a zig-zag. A diamond pattern takes you to different quadrants of the field. The diamond pattern allows you to enter and leave the field at the same place, which is sometimes useful for access to vehicles, etc. Deviate from the pattern when neccesary to check out potential problem areas.
Iowa State University Integrated Pest Management
Assess a minimum of 50 to 100 plants per field in a systematic way. For example, walk a transect through the field, and stop every "x"; number of steps and look at 5 to 15 plants at each stop, then compile the information. Scouting precision increases as you look at more plants. However, the time you have available to scout should be considered when examining plants.
Iowa State University Integrated Pest Management
When you’re in the field, check for basic agronomic information such as the growth stage and plant population of the crop. It is important to know what a healthy plant looks like to help identify diseased or damaged plants. Healthy corn and soybean plants will be discussed in depth in Chapter 2. Be sure to check the entire plant and environment around it including leaves, stems, roots, internal tissues, soil, pests not directly on the plant, competition, and other aspects that occur to you.
A sick plant will give indicators of the problem called symptoms. Symptoms may include stunting, leaves and other plant parts changing color, crinkling of leaves, or loss of tissue. If you know what a healthy plant looks like, you can identify the symptoms better and begin to know what might be wrong as a foundation to correct the issue.
Proper identification of the problem is an obvious, yet very important step. When determining what could be problematic, consider the time of the year as different pests often occur at different, yet predictable times during the season. Once you narrow your options, the problems in the field usually are plant diseases, insect pests, or weeds. Knowing what diseases and insects may be in your field, along with the type and location of the injury on the plants, will help better identify the plant disease and insect pest in question.
Direct injury to plant tissue is a symptom. This shows soybean plants after a hail storm.
Grain can also show symptoms. This is purple seed stain resulting from fungal infection of the soybean plant.
Lesions can develop on leaves and other plant tissue. This is frogeye leaf spot of soybean.
Plant structure and function can be diminished as a result of pest issues. This corn is goosenecked.
Plant tissue deformity can indicate disease or disorder. This is an infection by soybean mosaic virus.
Yellowing of plant tissue is a symptom that can be caused by many different diseases or disorders. This is nitrogen deficiency on corn.
Plant symptoms
Some problems may not be biotic (i.e., caused by diseases, insects, or weeds). Problems that are abiotic (i.e., a living organism does not cause them) are referred to as disorders. Some common ways to determine if problems are disorders are to: 1) look for patterns – disorders often follow a pattern while biotic problems will follow topography or be randomly distributed across the field; 2) look at the timing of problems showing up. Abiotic problems often show up all at the same time while biotic problems develop over time; and 3) look at symptoms across different plant species. Abiotic disorders may cause similar symptoms on several different plant species while biotic problems are usually specific to one plant species.
Something else to consider before entering a field is recent or planned pesticide applications. Look at spray records if possible, or ask the farmer. Be aware of what was sprayed in the field, and don’t enter if it is not safe. Pesticides have a Re-Entry Interval (REI) telling when it is safe to go back into a field. If you need to go into a recently sprayed field before the REI has passed, special protective equipment is required. Also, you do not want to be in a field during a pesticide application, so communicate with the farmer or local cooperative responsible for applying pesticides.
Look for patterns to help determine if symptoms are caused by biotic or abiotic factors. Lightning kills plants in a defined patch in the field, whereas a spot with diseased plants may be ringed with different stages of diseased plants.
Adam Sisson
Understanding the importance of different types of pests is important when scouting a field. Pests can be classified by their status or severity or by the injury they cause. Examples of pests classified by their status or severity include key pests, major pests, minor or occasional pests, and secondary pests.
A key pest is one that dictates or influences the overall management program. An example of a key pest is an herbicide resistant weed that cause a grower to choose a crop variety based on the herbicide trait offered.
A major pest is one that routinely occurs at damaging levels. In many southern areas, stink bugs are a routine insect pest of soybean that must be dealt with. White mold of soybean can be a major issue when conditions favor disease development.
Minor, or occasional pests, are those that are rarely found at population or severity levels that justify pesticide applications. Grasshoppers and eyespot are examples of minor pests found in many cornfields, however rarely at treatable levels.
Secondary pests typically reach damaging levels due to the influence of control measures employed for other pests. In some cases, the use of broad-spectrum insecticides may cause a rapid increase in soybean loopers, leading to population levels that require treatment.
Pests may also be characterized as direct or indirect pests, depending on where they cause injury. Direct pests are those that directly affect the marketable commodity. These include insects such as corn earworms, which feed on developing corn kernels. On the other hand, indirect pests feed on non-marketable portions of the commodity. These include foliage feeders or vectors of diseases.
A scout can save a considerable amount of time by knowing the key and major pests of the crop and geographic location and focusing scouting efforts with this information. Another point is that not all pests will be major pests the entire season. For example, corn earworms are highly attracted to corn during the silking stage, so intense scouting for corn earworm larvae would not be necessary at the V3 stage. Instead, scouts should look for seedling pests of corn, such as cutworms.
As previously mentioned, there are many physical tools available to help with scouting in the field. Later, in Chapter 2: Know Your Field, we will cover the knowledge that you can equip yourself with before entering the field. Most of the tools are small enough to be carried into the field. Tools useful to the scout include:
A hand lens or mobile digital microscope will allow you to see small insects or disease lesions normally not as visible to the naked eye.
Sample collection bags, vials, or a cooler may be useful for collecting diseased plants or insect specimens to have professionally identified. A small bucket can also be used to carry samples and a magic marker can be used to label samples.
A way to mark field locations such as flags or flagging tape.
A pocket knife can be used to split or cut pods, stalks, stems, etc. while searching for pests.
A trowel or shovel is used to sift through soil surrounding plants to look for insects or to dig up roots.
A sweep net can be used to collect insect samples.
A soil probe is used to collect soil samples for nematode or nutrient tests.
Pen and paper are used to record information, as are electronic devices such as cameras, smartphones, or other mobile devices.
A tape measure
Personal protective gear is sometimes required and always useful. For example, special protective gear may have to be used when scouting a field shortly after a pesticide application, depending on the re-entry interval. Safety glasses, sunglasses, a hat, a long sleeve shirt, long pants, and boots help protect eyes, arms, and legs from abrasive plant surfaces such as the leaf edges or dried pods.
Iowa State University Integrated Pest Management
Brandon Kleinke
Printed publications such as field guides are available to help identify pests by outlining the best times to scout for a particular problem or by defining visual or behavioral characteristics of an organism. Some publications are pocket-sized and produced on weather-resistant pages which makes it easy to bring them into the field. These resources are available from your local Extension service and may be provided by agribusiness as well. Many printed publications are also available as PDFs, making them accessible from an electronic device. Good resources for identifying diseases of corn and soybean include the Farmer's Guide to Soybean Diseases and the Farmer's Guide to Corn Diseases.
Recently, apps have been created for tablets and smartphones that can make scouting and recordkeeping easier. These go beyond electronic publications or PDFs by increasing interactivity. Field recordkeeping information can be easily and automatically kept on file for future reference.
The Internet is another great source of information. Several universities provide up-to-date information on crop management issues throughout the growing season on their Extension websites. Other websites track the occurrence of disease or insects as they move across the country or at what time they appear. An example of this is the Corn ipmPIPE, which reports on multiple pests in corn. Finding out what is in a neighboring state may be helpful to identify that issue when walking through a field. There are also apps that help crop scouts keep track of what is going on in the field. An example includes a white mold forecasting tool known as Sporecaster.
What do you do if you find a pest or disorder and can’t figure it out? If you are unsure of the cause of the problem, plants with disease, weeds, or insects can be sent to a diagnostic laboratory. When submitting samples, there are some guidelines to follow to make sure they get to the diagnosticians undamaged.
Submitting Plants
Provide plenty of fresh material. When possible, send the entire plant, including roots and top growth. Stalks can be folded to fit the entire plant into a box. Include enough plant material to show a range of symptoms.
Provide appropriate background information for the field.
Include photos when possible.
Wrap specimens in dry paper towels or clean newspaper (do not add moisture).
Do not send dead tissue.
Include as much background as possible, rather than the example shown here.
Include as much background as possible, rather than the example shown here.
When sending plants or samples to a diagnostic center, there are helpful things to remember.
Plant-feeding stink bugs can be confused with the beneficial, insect-feeding spined soldier bug. A diagnostic clinic can help to tell the difference if needed.
Recordkeeping is like planting a tree. You may not be able to sit in the shade this year, but you will be glad for it on a hot day in August years from now. That’s the way it is with recordkeeping. Writing down and filing information takes time, often during very busy parts of the season, without any immediate value. However, the value of this information will be apparent when selecting resistant cultivars or pre-purchasing chemicals for future growing seasons.
For example, the bacterium that causes Goss’s wilt survives in infested corn residue. If it was a problem in a field last year, and corn is planned for that field again, it makes sense to purchase hybrids with resistance to Goss’s wilt. This is one of many examples of why recordkeeping is important.
Goss's wilt lesion shown. One way to manage Goss's wilt is to use hybrids that are resistant to this disease.
Each scouting effort should document the cultivar being grown, as well as the date and crop stage. Look at neighboring fields and in neighboring ditches and areas to see if similar problems are present. In addition, keep a field-by-field set of records that may be useful as background information for each field. If you are not using an app for recordkeeping, there are useful worksheets that can be printed and brought to the field.
Resistant varieties (left) are less susceptible to disease even when disease-causing inoculum is present.
After scouting a field and information has been collected, it can be used for immediate or in-season management decisions or for future management in upcoming crops. Pesticide application is one of the few available immediate management options. However, there are more choices available for managing pests before a crop is planted. This makes pre-season planning very important. It will be more difficult to make decisions without previous knowledge of the field gained through scouting. Scouting also helps us determine if an immediate option will be economically beneficial. Three main categories of pest management options for corn and soybean are cultural controls, resistance, and pesticide application.
Cultural Controls
Cultural controls are often decided before planting and are designed to maximize the ability of plants to grow well. Healthy plants are less likely to be infected and/or damaged by pests. Examples of cultural control include the following: selecting high yielding varieties or hybrids that grow well in your soil type and climate, spacing plants appropriately in the field, rotation to non-host crops, tillage to reduce disease inoculum, planting at an optimal time, and weed management.
Seed Selection
Choosing high-yielding cultivars is one of the most important decisions a farmer can make. Selecting the most appropriate cultivars for the growing region will promote successful establishment and encourage vigorous plant growth. Healthy plants are better able to withstand pests and other problems. Seed should have a high germination percentage. In addition, using high quality seed may prevent the introduction of new pathogens or weed seed into a field.
Various seed treatments are available for insects and diseases (including nematodes) in field crops.
Development of white mold, a soybean disease, can be favored by a dense canopy; narrow row width is a contributing factor to a dense canopy.
Crop Rotation
Rotating crops in a field each year can minimize persistent outbreaks in field crops by forcing insects to move to find food or by reducing disease inoculum. Corn rootworm is a good example of an insect pest that can be managed using crop rotation. Be aware that some pests attack both corn and soybean and may be able to survive for several years without a suitable host.
Many pathogens can only survive on infested crop residue for a limited period of time. The reason for this is that saprophytic (organisms which survive by feeding on dead organic matter) micro-organisms are better at competing for food, and some may even destroy or suppress a pathogen. With no suitable host, disease causing pathogens will die off.
Tillage
There are several reasons farmers use tillage in field crop production, particularly weed control and seedbed preparation. Tillage can disrupt weed vegetative growth and seed reproduction, simply by killing the emerging weeds.
While the negative environmental consequences of tillage (e.g., soil erosion) should be considered, disrupting soil also can be used for cultural control of insects and to bury disease harboring crop residue. When crop residue is buried, it minimizes a source of disease causing inoculum. Buried crop residue also decomposes quicker than residue left on the soil surface and thereby reduces survival of the pathogen. For example, many corn foliar diseases may be more severe if previously infected residue remains on the surface. Also, seedling diseases may be more problematic in fields that have minimal tillage, because soils remain cooler and wetter and thus more conducive for seedling disease development.
Certain insects, such as the black cutworm (adult moth shown), may be more problematic in fields planted late. However, there are insects favored by early planting as well.
Harvesting plants in a timely manner is another good management practice, especially for diseases that will affect harvest efficiency. For example, if a field has severe stalk rot, the risk of lodging increases and harvesting a field before lodging occurs will make it easier.
Scouting for stalk rots 40-60 days after pollination can help to determine at-risk fields. These fields should be considered for the earliest harvest possible so that lodging is minimized. Lodging increases the difficulty of harvest.
Weed Management
Although managing weeds is important to protect crops from the resource competition occurring between plants, it is also important in managing some insects and disease. For example, Desmodium weed species are hosts of bean pod mottle virus, which affects soybean. Therefore, these hosts enable the pathogen to produce more inoculum. Furthermore, weeds may improve the microclimate for spore production by increasing relative humidity and/or decreasing light intensity.
Weeds also impact insect behavior. Most insects are mobile as adults and can move out of fields to seek shelter, mate, and feed. By eliminating weeds and other alternative plants like grasses, insects have limited food choices, and egg laying and overwintering sites. For example, black cutworm moths are attracted to fields with winter annual weeds growing in them for egg laying.
Aspergillus ear rot can begin at points of insect injury to ears.
These lodged corn plants are the result of corn rootworm feeding. Proper insect management will help to prevent injury such as this.
Sanitation
Keep weed contaminated manure, hay, crop residues, and other sources of weed seed from being applied to cropland. Also, clean farm machinery between fields to avoid transport of weed seed, rhizomes, tubers, and rootstocks, as well as disease inoculum.
Good Agronomic Practices
Good agronomic practices include planting at the recommended plant population and managing fertility to help reduce disease by limiting stress on the host plant. If crops are planted at higher than the recommended plant populations, competition for light, water, and nutrients can increase susceptibility to disease (e.g., corn stalk rots). Some diseases and insect pests are associated with crops under nutrient stress. Any practice that provides optimum conditions for early and vigorous growth of crops helps give them a competitive edge over weeds, insects, and disease. Cover crops or closed canopies can be used to shade out weed species. However, cover crops may also be problematic if they increase risk of disease or insect injury to the primary crop. Controlling weeds in non-cropland areas, including fencerows, drainage ditch banks, and rights-of-way, can help prevent the spread to fields.
Host Plant Resistance
Before planting, host plant resistance is another management option that should be considered. The use of resistant plants can reduce or eliminate a pest’s ability to cause plant damage. A plant with resistance may still be injured, but it may have the ability to minimize or compensate for the injury. On the other hand, a resistant plant may hinder the ability of a disease-causing organism to infect the plant, or diminish an insect’s capacity to reproduce.
Selecting a plant with built in pest defense is a great choice as genetically resistant plants can be effective, targeting a specific pest(s), and relatively inexpensive compared to other methods. Farmers should seek out available host plant resistant cultivars whenever possible.
A dark brown lesion on the lower stem of soybean is a characteristic symptom of Phytophthora root rot. Soybean varieties are available that have resistance to specific pathotypes of this disease.
Examples of Host Plant Resistance
Transgenic corn with Bt proteins protecting the plant from corn rootworms, European corn borer, corn earworm, and others.
Soybean aphid resistant soybean cultivars utilizing naturally occurring genes to suppress aphid growth and reproduction.
Cultivars resistant to many diseases, including soybean cyst nematode on soybean or gray leaf spot on corn.
When the method of plant insect resistance kills or reduces the life expectancy and reproductive potential of insects that feed on these plants, it is called antibiosis resistance. Plants can also be bred to be less attractive to insects, which results in insects avoiding these plants. This is referred to as antixenosis resistance.
Corn hybrids with certain genetic traits are resistant to feeding by European corn borer larvae.
Adam Sisson
Disease resistance can also work in a number of ways. One way is by preventing colonization and subsequent disease development. For example, if a plant is resistant to soybean cyst nematode, the nematodes are unable to establish feeding sites in the soybean cyst nematode-resistant soybeans, and therefore the nematodes die. Some forms of resistance can slow or reduce the buildup of inoculum. Corn plants resistant to the pathogen that causes northern corn leaf blight can become infected and colonized, but the lesions are smaller, less distinct than on susceptible hybrids, and produce fewer spores.
Just like other management strategies, pathogens or pests can become resistant to the resistant plants, which is why integration of management strategies is always important.
Host Plant Resistance Video
Pesticide Application
Pesticide application is another tactic used to prevent or reduce pests. Pesticides are natural or synthetic chemicals that kill or suppress living organisms including insects, bacteria, fungi, nematodes, and weeds. Pesticides have developed into an important tool in modern agriculture, and there are many methods for application such as foliar, soil, and seed treatments.
Pesticides can be applied in many ways, including to the soil, plant canopy, or seed.
Daren Mueller
Fungicides can be used to manage disease. Fungicides are available as seed treatments and protect the germinating seed from infection by soil-borne pathogens. Almost all hybrid corn is treated with seed treatments. Recently, percentage of soybean seed treated each year has increased. Fungicides can also be applied to the leaves of crop plants. Depending on which chemical group the fungicide belongs to, it can stop germination of the spore and thus infection of the host, or it prevents growth of the fungus and consequently colonization of the host tissue. In general, fungicides are most effective when applied before disease is well established in a field.
Herbicides are used to suppress or kill weeds. There are many different herbicides available, and some knowledge about how particular herbicides work is necessary before applying them. Herbicide rate and application timing are always important factors to consider when using herbicides. It is important to select herbicides based on the weeds present in a field. Selective herbicides will control certain plant species and leave others (the crop) relatively unharmed. Nonselective herbicides will typically control all vegetation contacted by the herbicide application. A burn-down treatment that is used to control all vegetation prior to crop emergence is an example of nonselective herbicide use.
Insecticides are used for management of insects. Chemical control is a very effective method to suppress insects and is an important component of IPM. Insecticides can be applied as foliar and soil applications or seed treatments. Insecticides provide many advantages for yield protection, including quick control, ease of application, and being relatively inexpensive. But, there are also some disadvantages. Most available field crop insecticides have broad-spectrum activity and will kill beneficial and pollinating insects. In addition, there is always a risk of chemical exposure to the insecticide applicator and other people or animals. Insecticides can also be applied when they are not needed, resulting in a loss of money and efficacy.
A decision to apply a pesticide should be based on IPM principles. Some factors to consider before spraying include plant susceptibility to pests, pest pressure, past, current, and forecasted weather conditions, cropping history, economics, and the surrounding land such as other crops, houses, and bodies of water (lakes, ponds, rivers, and streams). Also, consider alternative management options for pests if available.
While economics may drive many pesticide use decisions, the environment and people must also be considered. Caution must be taken when using pesticides during mixing, application, clean-up, and other procedures. Safe practices are outlined on the label that comes with every pesticide. The label also indicates the right kind of equipment and clothing that will keep you safe during an application. Bottom line: Be safe and watch out for the well being of the environment, animals, and other people when using pesticides!
Pesticide resistance management is also an important consideration. Overuse or misuse of pesticide formulations can increase risk of pests developing resistance to a pesticide. This means the pesticide will no longer work as well, or at all, for managing a pest organism. Weeds, insects, and disease-causing organisms can all become resistant to pesticides. When pests become resistant to a pesticide, a valuable tool for management is lost. Ways to decrease risk of resistance development include only applying a pesticide when it is needed, following label instructions, rotating pesticide active ingredients, and using non-pesticide methods of crop protection.
Twospotted spider mites can be more of a problem in dry conditions. Death of beneficial organisms after most pyrethroid insecticide applications may actually result in increased problems with twospotted spider mites.
Adam Sisson
Pesticide labels contain a lot of information, including safety and application instructions. This example has been abbreviated: only the first page of the label is shown here.
Other aspects of management besides pesticide application can be based on incidence or severity of crop injury. For example, if it is found that 10-15 percent (incidence) of stalks or more are rotted in a field during scouting, this triggers the consideration to harvest this field as early as possible. Stalk rots can cause plants to fall over, or lodge, in a field.
Thresholds
Basing pesticide applications on profitable economic decisions is an important concept in IPM. The expectation of crop fields being pest free is unrealistic. This is because the mere presence of some pests in a field will not lead to economic loss. However, certain levels of disease and numbers of insects can signal when to apply a pesticide. Two measurements used for decision-making are the economic injury level and the economic (or action) threshold. The economic injury level is the lowest number of insects or disease that cause economic loss. The economic threshold indicates the insect or disease density that triggers management action to prevent economic loss from occurring. The economic threshold accounts for several factors, including the population of pest species, damage incurred from that population, cost of control, and value of the crop. Economic thresholds are typically set at 50-60 percent below the economic injury level, to prevent the population from reaching damaging levels. Spraying solely based on the calendar date or plant growth stage instead of pest presence or likelihood of occurrence could lead to unnecessary applications, which increases production costs, pesticides in the environment, and risk of insecticide resistance.
There are several helpful concepts for determining when thresholds are reached. One is the incidence of the disease or pest damage. This is simply the percentage of plants you examine that have the pest damage. For example, a field with 5 injured plants out of a sample of 100 plants indicates a 5 percent incidence of injury in that field. A second concept is the severity of disease or pest damage. This is the percent of plant tissue that is damaged, this may include the harvestable commodity (i.e., soybean seed) or other plant parts (i.e., soybean leaves). For example, 30 percent defoliated, 20 percent loss from a hailstorm, or 15 percent of the leaf with disease lesions. A third concept is estimating the population of insects present in the field. This can be done by making visual estimates or counting the number of insects in a sweep-net or drop-cloth sample. An example of this would be an average of 9 stink bugs collected in a set of 25 sweep-net samples. Thresholds generally vary for different diseases and insects and crop growth stages.
The Crop Protection Network provides a training tool to help crop scouts improve their ability to estimate insect defoliation and disease severity on corn and soybean.
An example of 20% eyespot severity on a corn leaf as shown by the online training tool developed by the Crop Protection Network.
Earn Certified Crop Advisor CEUs after reading this web book. Successfully complete a quiz for this chapter to earn 0.5 CCA CEUs at the Crop Protection Network CCA CEU page.
Understanding what is happening in a corn or soybean field is an important part of scouting and making informed management decisions. This includes being able to identify corn and soybean growth stages, determining stand counts, estimating yield potential, and obtaining background information on the field. This information helps to determine pesticide application timing, replant decisions, injury assessments to crops, and many other crop and pest management decisions.
Scout crop fields regularly and keep track of what you find to help make informed pest management decisions.
Iowa State University Integrated Pest Management
Insects, diseases, weeds, and other disorders are most likely to be more problematic during certain stages of corn plant development. Because of this, understanding the growth stage of a plant aids the farmer, agronomist, or crop scout in efficiently and effectively scouting their fields.
Vegetative growth stages are based upon the number of visible leaf collars. So the number of visible leaf collars is equal to a vegetative growth stage (V stages). Reproductive growth stages (R stages) are based on the development of the kernels. Growth stages may overlap in a field, and a growth stage for a field begins when at least 50 percent of the plants have reached or are beyond a certain stage.
Temperature drives corn development, and the accumulation of heat units known as growing degree days (GDD) can be measured and used to predict corn growth stages. This concept is discussed in more detail in section 2.3.
Vegetative Stages | Reproductive Stages |
|---|---|
VE — Emergence of the shoot from the soil. | R1 — Silk — Any silk is visible. |
V1 — Lowest leaf has a visible collar; this leaf has a rounded tip, unlike subsequent pointed leaves. | R2 — Blister — Kernels are small and white; the endosperm (kernel fluid) is clear. |
V2 — Two lowest leaves have a visible collar. | R3 — Milk — Kernels are yellow with milky white fluid. |
V(n) — “n” leaf collars present; there are 17-22 V stages before tassel emergence. | R4 — Dough — Kernel contents are pasty as starch accumulates. |
VT — Lowest branch of the tassel is visible. | R5 — Dent — Most kernels are dented due to starch hardening at the top of the kernel. As maturity progresses, the starch hardens and the milk line moves toward the cob. |
| R6 — Black layer or physiological maturity — The milk line is no longer visible; a black layer forms at the kernel’s attachment, which signifies the end of dry matter accumulation. |
Vegetative Stages
There are many vegetative stages of corn plants, beginning with emergence and ending with tasseling.
The stalk (A), leaf collar (B), and sheath (C) are identified here. The collar is where the leaf blade visually breaks away from the sheath and the stalk of the corn plant.
The first leaf of the corn plant has a rounded tip.
V1 corn plant
V6 corn plant
As lower leaves start to fall off the plant, split stalks to determine growth stage.
VT corn plant
Corn Growth Staging
Reproductive Stages
After VT, staging is no longer based on the vegetative appearance of the plant, but focuses only on the ear to determine the stage of the plant and field. Look at kernels in the middle of the ear when determining reproductive (R) developmental stages. Both number and names are used when referring to corn R stages. For example: R1 = Silking. There are six R stages of corn.
R1 stage corn
R2 stage corn
R3 stage corn
R4 stage corn
R5 stage corn
R6 stage corn
Corn Growth Stages
Iowa State University Integrated Pest Management
Certain aspects of soybean management must be considered at each growth stage, due to various problems associated with each stage that can interfere with growth and development. Problems that occur early in the season may contribute to yield loss experienced at harvest.
Understanding the different parts of the soybean plant is important for staging fields. The trifoliolate leaf is a compound leaf made up of three leaflets. Although the trifoliolate leaf appears to be three separate leaves, it is one leaf attached to the plant by a single petiole. The number of trifoliate leaflets is often used to determine the vegetative growth stage of soybean plants. Reproductive stages are based on flower, pod, and seed development. Because growth stages may overlap, field level growth stages are determined when at least 50 percent of the plants have reached or are beyond a certain stage in the upper four nodes of plants.
Soybean maturity groups grown in the U.S. range from 00 to IX. The maturity grouping reflects how quickly a soybean plant develops from emergence to maturity. Geographies with longer growing seasons can plant higher maturity group soybean plants. Soybeans with lower maturity group classifications are grown in the northern states while higher maturity group soybeans are grown in southern states.
Soybean varieties may also be grouped as determinate or indeterminate depending on their reproductive growth habit. Determinate soybeans complete their vegetative growth then switch to reproductive and initiate flowering across the entire plant at one time. Indeterminate soybeans begin reproductive development at the bottom of the plant while the top of the plant continues to grow vegetatively.
Soybeans have an interesting relationship with particular nitrogen fixing bacteria called Rhizobia. Soybeans gain nitrogen from these bacteria and in return, the bacteria obtain carbohydrates from the soybean. This kind of relationship where both organisms benefit is called a symbiotic relationship.
Vegetative Stages | Reproductive Stages |
VE — Emergence | R1 — Beginning bloom — Plants have at least one open flower at any node. |
VC — Unrolled unifoliolate leaves | R2 — Full bloom — Plants have an open flower at one of the two uppermost nodes on the main stem. |
V1 — First trifoliolate leaf | R3 — Beginning pod — Pods are ¼ inch long at one of the four uppermost nodes on the main stem with a fully developed leaf. |
V2 — Second unrolled trifoliolate leaf | R4 — Full pod — Pods are ¾ inch long at one of the four uppermost nodes on the main stem with a fully developed leaf. |
V(n) — “n” unrolled trifoliolate leaves | R5 — Beginning seed — Seeds are ⅛ inch long in the pod at one of the four uppermost nodes on the main stem. |
R6 — Full seed — Pods contain green seeds that fill the pod to capacity at one of the four uppermost nodes on the main stem. | |
R7 — Beginning maturity — One pod on the main stem has reached its mature color (tan or brown). | |
R8 — Full maturity — Ninety-five percent of the pods have reached mature color. |
Vegetative Stages
There are many vegetative stages of soybeans, beginning with plant emergence.
VE soybean plant
VC soybean plant
V1 soybean plant
V2 soybean plant
V3 soybean plant
Growth Stages of Soybean
Reproductive Stages
There are eight reproductive stages of soybean, beginning at flowering and ending when soybeans are fully mature.
R1 soybean plant
R2 soybean plant
R3 soybean plant
R4 soybean plant
R5 soybean plant
Early pod development
R6 soybean plant
Seed development of soybean
R7 soybean plant
R8 soybean plant
Late pod development
Images in this section courtesy University of Wisconsin-Madison
Corn development can be predicted by tracking growing degree days (GDD) which measure heat accumulation using daily air temperatures. For example, shoot emergence occurs when 90 to 120 GDD have accumulated after planting. Corn growth occurs when temperatures are above a certain minimum or base temperature. The base temperature for corn development is 50°F. Development of some insect species also may be predicted by using degree days, but the base temperature may differ. Insect degree days will be discussed in Chapter 3: Introduction to Insects.
The instructions for calculating corn GDD are as follows:
Collect the daily high and low air temperatures and adjust (if necessary) for the base (50°F) and maximum (86°F) temperatures. If the low is under 50°F, use 50°F to calculate GDD for that day. If the high exceeds 86°F, then use 86°F to calculate GDD.
The average of the adjusted high and low temperatures minus the base temperature equals the daily GDD accumulation.
Add GDD gained for each day to estimate the accumulated GDD over time.
Example of calculating growing degree days:
Day | Low | High |
1 | 55°F | 80°F |
2 | 40°F | 66°F |
3 | 72°F | 92°F |
Calculation for Day 1: | (80+55)/2 - 50 = 17.5 GDD | |
Calculation for Day 2: | (66+50)/2 - 50 = 8 GDD | |
Calculation for Day 3: | (86+72)/2 - 50 = 29 GDD |
Total for three days: 17.5 + 8 + 29 = 54.5. Always round the growing degree day numbers up, so the final is 55 GDD.
Online tools, such as the Iowa Environmental Mesonet and the National Phenology Network, can help to determine accumulated degree days. Both of these resources can help determine growing degree days across the U.S.
Example of accumulated growing degree days for locations throughout Kentucky, provided by the Iowa Environmental Mesonet.
Knowing what is going on both nationally and locally can give clues to what problems may be expected. Much of the information is available through university extension websites, agriculture media, social media from respected sources, or in local coffee shops. Information on emerging pests, weather conditions, and other potential problems can help by giving you a starting point for what to expect once you start scouting.
Field Background and Predicted Information
Before you even get to the field, there are some important things that can help you figure out what may be encountered while scouting. This includes knowing what occurred in the field in the past, such as a disease outbreak two years ago; what is going on presently, including planting date, herbicides applied, cultivar, and other information; and what is predicted, which can include anticipating disease or insects based on cultivar resistance, weather forecasts, and pest warning systems.
As previously mentioned, there are many Internet sources warning of pest and disease development such as the Corn ipmPIPE and the Soybean ipmPIPE. Other resources, such as the Forecast and Assessment of Cropping sysTemS (FACTS), can help with decision making by forecasting or providing soil, crop, and weather information.
Good background information to have includes:
Previous crop history, both in the field and in adjacent fields
A history of chemicals applied on or near the crop, including herbicides, insecticides, fungicides, and fertilizers. For each, indicate when they were applied, how they were applied, what rate was used, and the weather conditions that occurred during and after application.
Date of planting, planting depth, and seedbed conditions
Hybrid and variety characteristics including ratings for disease resistance or other varietal tolerances to field conditions [for example Bt corn, glyphosate resistance, iron chlorosis tolerance (soybean), and soybean aphid resistance]
Current soil test information (soil fertility status and pH)
What types of soils may be present in the field. Check out the NRCS Web Soil Survey
Estimates of soil moisture and notes about areas such as end rows or where fieldwork was performed in adverse conditions where compaction may be a concern
At the completion of the season (both just prior to and after harvest) note:
How well plants are standing
Condition of ears and pods
Stalk strength and health of the root system (late season lodging can be a serious problem and leave seed/volunteer crop plants for next year)
If yields were what you expected, or if there was a problem, ideas as to what caused the loss
If weed control was effective, and if not, what species are problems and what caused the problem.
Some states provide historical imagery online. Images for Iowa are available at the Iowa Geographic Map Server. Historical imagery can be used to help figure out current issues in the field. For example, the below aerial image from the 1960s shows a railroad running diagonally through the bottom right of the field outlined in black (left image). By 2011 the railroad track is gone, but it is still clearly affecting the portion of the field it once ran through (right image).
It is interesting to note the number of small, individual fields that existed in the 1960s and how many have been conglomerated into larger fields in the more recent image. However, some evidence still can be seen of their existence. The fence line running horizontally through the center of the image in the 1960s can be faintly detected in 2011, even though the smaller fields are now gone.
Pay attention to recent or forecasted weather where you are scouting. Humid, rainy, and/or stormy weather can be conducive to many crop diseases.
Also, consider recent weather that can contribute to plant stress. Abiotic disorders caused by adverse weather conditions can cause direct problems to any crop or may make plants more susceptible to disease or insect pests. Flooding, hail, and drought conditions all can lead to predictable problems. Be on the lookout for current and forecasted weather conditions that may be conducive to disease development (i.e., humid or wet weather) or for insect migration or growth (i.e., storms that blow in cutworm moths and rust spores or dry weather favoring spider mites).
Optimal corn seeding rates vary in the Midwest from 28,000 to 42,000 seeds per acre based on specific field conditions, genetics, and the environment. Generally, a seeding rate to obtain a final stand of 35,000 seeds per acre will maximize corn yield. For soybean, a stand of at least 100,000 healthy, uniformly spaced plants per acre should be the target population.
Actual per acre population of plants may differ from the planting rate for a variety of reasons including poor weather or soil conditions, planter issues, diseases, insects, or poor germination. This makes it important to know the actual plant population in the field. Knowing the plant population can help determine treatment thresholds and replant decisions, and is the first step in estimating yield.
In order to find the population of a corn or soybean field, you will need to know the row spacing and have a tape measure or another measuring device to calculate distance.
Then follow these instructions:
Measure 1/1,000 of an acre (See Table 1 as a guide for the length of row needed).
Count the number of plants in the measured area.
Count in at least 6 to 10 representative places across the field. Do not intentionally avoid plant gaps; include these in areas assessed.
Multiply the average number of plants by 1,000 to obtain the final plant population per acre.
Table 1. Row spacing and row length to measure for determining plant populations in 1/1,000 of an acre.
Row Spacing | Row Length to Measure |
|---|---|
7" | 74' 9" |
10" | 52' 3" |
15" | 34' 10" |
20" | 26' 2" |
30" | 17' 5" |
36" | 14' 6" |
38" | 13' 9" |
Counting the number of plants per 1/1,000 of an acre in soybean can be more difficult than in corn because of high soybean plant populations. There are alternate methods for determining soybean stand counts including counting plants in 1/10,000 of an acre or within the diameter of an arbitrarily-placed hula hoop.
Follow these instructions for determining stand count using 1/10,000 of an acre:
Measure 21 inches of row length (see Table 2 as a guide for the length of row needed as well as the number of rows needed).
Count the number of plants in the measured area.
Count in at least 6 to 10 representative places across the field. Do not intentionally avoid plant gaps; include these in areas assessed.
Multiply the average number of plants by 10,000 to obtain the final plant population per acre.
Table 2. Row spacing, the number of rows to count, and row length to measure for determining soybean plant populations in 1/10,000 of an acre.
Row Spacing | Number of Rows to Count | Row Length = 1/10,000 Acre |
7.5" | 4 | 21" |
15" | 2 | 21" |
30" | 1 | 21" |
Follow these instructions for determining stand count using the hula hoop method:
Arbitrarily place a hula hoop in the soybean field.
Count the number of plants within the hula hoop.
Count in at least 6 to 10 representative places across the field. Do not intentionally avoid plant gaps; include these in areas assessed.
Multiply the average number of plants by the multiplication factor that corresponds to the hula hoop diameter to obtain the final plant population per acre (See Table 3 as a guide for multiplication factor to use for each hoop diameter).
Table 3. Hoop diameter and corresponding multiplication factor for determining soybean plant populations using the hula hoop method.
Diameter of Hoop | Multiplication Factor |
18" | 24,662 |
21" | 18,119 |
24" | 13,872 |
27" | 10,961 |
30" | 8,878 |
33" | 6,165 |
How to Estimate Plant Population
Replanting may be necessary following seedling damage or loss caused by early season diseases, prolonged cold soils, frost, flooding, hail, or insect damage.
Corn
Two scenarios typically exist in fields with problematic stands:
Plant heights or developmental stages may differ as a result of non-uniform emergence.
Plant population may be significantly lower than desired.
Typically, replanting is beneficial only with reduced plant populations, not with uneven emergence. To decide whether to replant:
Estimate the remaining plant population. Do not count plants in the affected area that are severely injured.
Calculate expected yield from the existing stand using the table below.
Compare yield potential of the replanted crop with the potential yield of the existing crop.
Estimate replant costs. Replant costs include tillage, seed, fuel (for tillage and planting), additional pesticides, labor, etc.
The probability of fall frost damage to late planted corn increases. Consider shorter maturity hybrids in very late replant situations.
Planting Date¹ | |||||
|---|---|---|---|---|---|
Population (plants/acre) | April 20th to May 5th | May 5th to May 15th | May 15th to May 25th | May 25th to June 5th | June 5th to June 15 |
45000 | 97 | 93 | 85 | 68 | 52 |
40000 | 99 | 95 | 86 | 69 | 53 |
35000 | 100 | 96 | 87 | 70 | 54 |
30000 | 99 | 95 | 86 | 69 | 53 |
25000 | 95 | 91 | 83 | 67 | 51 |
20000 | 89 | 85 | 77 | 63 | 48 |
15000 | 81 | 78 | 71 | 57 | 44 |
10000 | 71 | 68 | 62 | 50 | 38 |
Percent Maximum Yield | |||||
¹This data is for Iowa only. Values based on preliminary Iowa research and modeling; 100% yield potential is estimated to occur with a 35,000 plant population and early planting date.
When flooding causes stand loss, replanting those spots may be an option.
Adam Sisson
Soybean plants have the ability to compensate for fluctuating stands.
This ear has approximately 40 kernels in each row.
The ear has 16 rows.
Estimating Corn Yield
Soybean yield can be estimated in the later reproductive stages (R5 and after) using pod number, seed number, and seed size. Follow these instructions to estimate yield:
Count the number of soybean pods per 1/10,000 acre (see Determining Plant Populations for soybean).
Determine an average number of seeds per pod.
Use Table 1 to determine an appropriate seed size factor.
Estimated yield (bushels per acre) = (number of soybean pods X number of seeds per pod) divided by seed size factor.
For more accurate estimations, repeat this process several times throughout a field.
Table 1. Seed size factor
Seeds per Pound | Seed Size Factor |
2,500 (large seed) | 15 |
2,666 | 16 |
2,833 | 17 |
3,000 (normal seed) | 18 |
3,166 | 19 |
3,333 | 20 |
3,500 (small seed) | 21 |
There are also apps and online tools available to help estimate soybean yields.
Maturity and Drydown
Grain continues to dry down after it reaches physiological maturity. If harvested too wet, farmers will have to dry the corn so it will store adequately or they will have to accept a reduced payment at the elevator based on the amount of moisture in the grain.
Field Harvest Losses
Adjust harvest equipment to minimize grain loss. A substantial amount of grain can be lost from incorrectly adjusted machinery. For example, two kernels per square foot left behind the combine represents one lost bushel per acre for corn. One full ear lost per 1/1,000 acre represents 6 to 7 bushels per acre left in the field. Losses can be considerable in lodged or hail-damaged cornfields.
Adjust harvest equipment to minimize grain loss. A substantial amount of grain can be lost from incorrectly adjusted machinery. For example, two kernels per square foot left behind the combine represents one lost bushel per acre for corn. One full ear lost per 1/1,000 acre represents 6 to 7 bushels per acre left in the field. Losses can be considerable in lodged or hail-damaged cornfields.
Earn Certified Crop Advisor CEUs after reading this web book. Successfully complete a quiz for this chapter to earn 0.5 CCA CEUs at the Crop Protection Network CCA CEU page.
Entomology is the study of insects, which are more abundant and diverse than any other animal. In fact, there are more than one million described insect species. It is interesting to note that one out of every five animals is a beetle!
Insects inhabit almost every niche on earth and provide many positive services for humans and the environment. They pollinate flowers and food crops, they consume and recycle dead plants and animals, they feed on pest insects, they serve as food for other animals, they aerate the soil – the list goes on and on. Insects are such a vital part of the Earth’s environments that life would be very different if we did not have them here performing so many useful services.
It is important to remember that although a great deal of attention is paid to insects that negatively impact humans, like insects that carry human diseases, live in our homes, or eat our food, pest insects are just a small handful of the insects in our environment. It is estimated that fewer than 2 percent of insects are actually pests to humans. Although insects can cause a great deal of damage to crops, life as we know it would not be possible without them.
Aphids are a common pest in field crops.
Adam Sisson
As immature insects develop to adults, they undergo a series of changes called metamorphosis. This process allows insects to take advantage of multiple habitats, because the immature stages can feed on different food sources while occupying the same space as adults. The two most common types of metamorphosis insects develop through are called complete and incomplete metamorphosis.
Complete Metamorphosis
Complete metamorphosis is a four-stage process involving an egg, larva, pupa, and adult. Approximately 85 percent of insects undergo complete metamorphosis. Eggs are laid singly or in clusters and hatch into larvae. Larvae feed and molt several times, eventually turning into pupae, which do not feed. After the pupal stage is complete, adult insects emerge. Immature and adult insects look very different from each other and often take advantage of totally different food resources. For example, certain insect larvae are predators whereas adults of the same insect species are herbivores. Depending on the species, insect larvae are commonly called grubs, maggots, or caterpillars. Butterflies, moths, lady beetles, and bees are examples of insects that undergo complete metamorphosis.
Lady beetles are an example of an insect that undergoes complete metamorphosis. The larvae do not look like adults, but both adults and larvae feed on other insects.
Iowa State University Integrated Pest Management
Incomplete Metamorphosis
Incomplete metamorphosis is a three-stage process involving an egg, nymph, and adult. Sometimes this is referred to as simple metamorphosis. Eggs or egg masses are laid and eventually hatch into nymphs. Nymphs are small, wingless, and generally resemble adults. Because nymphs do not have wings, small wing pads gradually develop with each molt before becoming functional wings in the adult stage.
Unlike in complete metamorphosis, nymphs and adults feed on similar food sources. For example, both are predatory, or both are herbivores. As nymphs move from one stage to the next, they shed their old exoskeleton and grow a new one. Aphids, stink bugs, and grasshoppers are examples of insects that undergo incomplete metamorphosis.
Stink bugs are an example of an insect that undergoes incomplete metamorphosis. Notice how the nymphs appear similar to the adult but lack wings.
Iowa State University Integrated Pest Management
Degree Days
Growth of an insect species through various stages of development can be predicted using heat units. Degree days (DD) represent the accumulation of heat units above a critical development temperature (i.e., base temperature). When the daily temperature is below the critical base temperature for an insect, it will not develop. The necessary base temperature and accumulated DD required to develop from egg to adult varies among species. Sometimes DD are related to important events in the life cycle and may trigger scouting or certain management practices.
This table shows examples of insects and corresponding use of DD.
Insect | Base | Use of Degree Days |
|---|---|---|
Seedcorn maggot | 39°F | Adults emerge at about 200, 600, and 1,000 DD. |
Stalk borer | 41°F | Start scouting whorls to determine if larvae are present when 1,300-1,400 DD have accumulated. |
Western bean cutworm | 50°F | Half of adult emergence and egg laying occurs after 1,422 DD have accumulated after May 1. |
Black cutworm | 51°F | Larvae start cutting at 300 DD after eggs are laid. |
Corn rootworm | 52°F | About half of eggs hatch between 684-767 DD (soil). |
Distinguishing harmful insects from beneficial or benign insects is important. Proper identification is a key step towards making future management decisions. Insects are typically identified based on wing and mouthpart types, however there are many other distinguishing characteristics that can help with identification.
The insect body can be divided into three main regions—the head, which contains the antennae, eyes, and mouthparts; the thorax, which has the legs and wings; and finally the abdomen, which contains a large portion of the digestive system and the reproductive parts.
Morphology of the northern corn rootworm beetle.
Iowa State University Integrated Pest Management
Morphology of the green cloverworm larva.
Adam Sisson
Wings are one of the main features that allow us to categorize different types of insects. Most adult insects have wings, while larvae and nymphs do not. Wings provide both a great deal of mobility to insects and a useful key for field scouts to distinguish species. The adult stage of caterpillars (moths) that injure corn and soybean can be identified by wing patterns and colors. Many beetles can also be identified by distinctive markings on their forewings.
Beetles - have four wings, but the first pair of wings is hardened in order to protect the second pair of wings folded beneath. It is the second pair of wings that are used for flight (forewing of a western corn rootworm).
True bugs - have wings that are folded across each other to make an X-shape (lygus bug).
Grasshoppers - have leathery front wings and more membranous hindwings.
Wasps and bees - have two pairs of wings that sometimes are hooked together so it appears they have only one pair. Their wings are often clear with darker veins.
Earwigs - have wings folded up underneath the small leathery forewings, but do not fly much.
Flies - actually have only one pair of wings, the hind pair of wings has been reduced (tachinid fly).
Insect wings
Insect mouthparts are important for two main reasons. First, they may be used to identify the insect species. Second, mouthparts provide information on how insects feed and what sort of damage they cause. For instance, insects with sucking mouthparts use them like a straw or hypodermic needle. All flies have sucking mouthparts, and mosquitoes are a great example of insects that use sucking mouthparts like a straw to ‘suck up’ blood. Aphids also use sucking mouthparts to withdraw fluid from plants. Other insects, such as the spined soldier bug, are predatory insects that use these mouthparts when feeding on prey.
Other groups of insects, such as wasps and bees, beetles, praying mantises, and grasshoppers have chewing mouthparts. These insects chew on their food in a similar manner as humans. Honey bees have a lapping, tongue-like mouthpart used to collect nectar.
Bees have chewing mouthparts and “tongues” that can lap up nectar from flowers.
Scarab Beetle-mandibles
Praying Mantis mouthparts are used to chew insect prey.
Grasshoppers can chew leaves, soybean pods and seeds, and corn kernels.
Insect chewing mouthparts
Corn Leaf Aphid - On plants, insects with sucking mouthparts suck up the plant sap and tend to leave discoloration on the leaves. Many of the more serious plant pests have sucking mouthparts. Aphids have sucking mouthparts, withdrawing plant fluids while feeding.
Spined Soldier Bug - This spined soldier beetle is feeding on a lady beetle larva. Notice the lady beetle larva has been pierced with spined soldier beetle mouthparts.
Caterpillar - Moths and butterflies are interesting because they change mouthpart types during metamorphosis––they start out with chewing mouthparts as caterpillars, leaving holes in leaves or cutting stems. Then as adults they have sucking (or siphoning) mouthparts and suck up nectar from flowers.
Insect sucking mouthparts
Antennae are important sensory organs for insects. The antennae of insects are used as “feelers” that have a role in everything from sensing movement or vibration to smell and taste. Antennae come in a wide variety of shapes and sizes that are specially adapted to help insects survive.
Aphid antennae
Luna Moth antennae
Scarab Beetle antennae
Weevil antennae
Insect antenna
Insect legs are important for mobility and other tasks such as feeding or digging. Legs can also help to identify insects.
Mole Cricket - Insects that live underground often have legs modified for digging, such as this mole cricket does.
Praying Mantis - gets its name from its front legs, which make it appear as though the insect is praying. The insects front legs are actually modified to grab and hold onto the prey that it eats.
Grasshopper - Most of us are familiar with jumping legs. Jumping is an effective way for insects, like the grasshopper, to escape predators and move quickly through their environment.
Insect legs are also very diverse and can be used to help determine a specific insect.
Crop scouting on a regular basis is an integral part of managing insects in corn and soybean. The primary goal of scouting is to estimate the population of pests in a field. Noting the insects that are present, or the damage they are causing, can help to make timely management decisions and to be aware of problems that may arise later in the season. When to scout can be dependent on several variables including time of year, crop growth stage, accumulated heat units (degree days), weather, and insects caught in traps. These things can be clues as to what pest insects we would most likely see in a field. A scout should always enter a field with a plan. Know what pest insects pose the greatest risk to the crop at the given scouting time and focus scouting efforts there while also observing for secondary pests. Only spend as much time in each field as is necessary to make a decision. When scouting several fields over a large area, time is of the essence.
Field characteristics can be clues about which insects may be present. These include low-lying, wet, or weedy areas, proximity to or rotation with certain plants or crops, past incidence or injury, field size and other conditions. For example, true armyworms can migrate to corn from maturing small grain crops, and grasshoppers are more likely to cause damage in weedy fields and fields with higher sand soil content. There are several methods to determine insect presence once you are in the field. These include capturing insects using a sweep net, shaking insects off a plant onto paper, a drop cloth or into a bucket, visually counting insects present on plants, and looking for injured plants.
Insect Scouting
Pheromone traps have a sticky substance on the inside and chemical lure that attracts black cutworm moths. Moth captures are used to determine when to scout for black cutworm larvae.
Walking through a field and counting plants that have cut by black cutworm larvae helps to determine if an insecticide application is warranted.
Scouting soybean aphid requires counting the number of aphids present on a plant.
Using a sweep net is one way to determine insect presence and population in a field.
Insect scouting
How to Use a Sweep Net
Feeding
Japanese beetles feeding on corn silk.
Spider mite symptoms on soybean.
Some insects migrate into a corn field from other plants, and may be more of a problem at the field edge.
Japanese beetles feeding on soybean leaves.
Insect feeding
Check the entire plant when scouting. Some insects are root feeders, such as corn rootworm larvae and true white grubs. Others feed inside of plant tissues such as stalk borer or European corn borer. Many insects feed on the leaves or silks, such as Japanese beetles, corn rootworm adults, and grasshoppers. Understanding pest behavior is also important. For example, soybean looper and velvetbean caterpillar are ferocious foliage feeders of soybeans in the south. While soybean loopers primarily feed in the bottom of the plant canopy and work their way to the top, velvetbean caterpillars do the opposite. This is important to understand because soybean looper is resistant to many generic insecticides that still control velvetbean caterpillar.
An understanding of the maturity group and growth habit is also important when scouting crops. We can use the maturity group as a calendar to know when the crop should be reaching key growth stages and what pests to be focused on when scouting. The growth habit (determinate vs. indeterminate) of soybeans may play a role in insect sampling efficiency. For example, early season infestations of stink bugs may be found lower in the canopy of indeterminate soybeans because this is the only part of the plant where pods are present. This makes sampling with sweep-nets less efficient as proper coverage is difficult low in the canopy.
Be sure to consider the condition of the entire field when scouting. Some insects, like the fall armyworm or stalk borer, can be found in patches or at the edge of the field but may not be located throughout. Looking at the whole field is important because a pest that is causing issues in a few localized spots may be managed using smaller spot sprays, while spraying the entire field may not be necessary. Even if an insect outbreak is found too late for treatments to be of benefit, knowing about the outbreak is valuable. This knowledge may inform management strategies in coming years.
Knowing common pest and beneficial insects in corn and soybean is important. As you gain experience scouting fields, it will get easier to identify some of these insects.
Even though these plants are already lodged, you can learn from this situation to help with future management decisions.
A keystone concept of IPM is the use of economic thresholds instead of calendar-based pesticide applications. As discussed in Chapter 1.4, an economic threshold is the cut-off point that tells you to spray before economic damage greater than the cost of an insecticide application occurs. Information gained from scouting is used to determine when thresholds are met, and insecticide applications are justified. Economic thresholds may be the number of insects per sweep or per plant or defoliation percentage. The Crop Protection Network developed an online tool that helps crop scouts train their eyes to estimate insect defoliation on corn and soybeans.
An example of using an economic threshold for insect management occurs with the soybean aphid. Insecticide applications are not considered economic until the aphid population reaches 250 per plant and is increasing. The video below shows a method of scouting for soybean aphid from the midwest called "Speed Scouting."
Speed Scouting
Different levels of defoliation help to determine whether an insecticide application may be beneficial. Train your eyes to properly estimate insect defoliation with this online tool from the Crop Protection Network.
Sometimes the application of an insecticide can actually result in an increase of pest organisms when beneficial insects are killed by the insecticide. This can be the case with the two-spotted spider mite (mites and damage shown) when pyrethroid insecticides are applied.
There are many management strategies that can be employed to combat insect pests. Pesticide application, mentioned previously, is useful for nearly immediate control of insects. However, there are a variety of other options for managing insect pests in fields. These include weed control, appropriate tillage, plant spacing, crop rotation, seed selection, and host plant resistance. Refer to Chapter 1 for more details on each management strategy.
Managing insects in field crops takes a lot of work. Knowledge of how plants and insects interact in a system is an important first step. Developing a field history can help make management decisions easier. Implement multiple, proactive IPM tactics to minimize insect pressure. Select high-yielding seed and scout regularly to improve the odds of a successful crop. Use economic thresholds and only spray when needed.
Application of an insecticide is one of several ways to manage insects pests.
Insects other than pests exist in crop fields. Some insects are beneficial to crop production such as predators, parasitoids, and pollinators. Beneficial insects provide a service for farmers as part of their normal life cycle. When present in crop fields, these insects work to suppress pest insect populations, helping to reduce the need to make insecticide applications by attacking pest species. Noting the presence of beneficial insects is important, as is adjusting management tactics accordingly. There also may be incidental insects—those who are neither friend nor foe—that may just be passing through the field or carrying out normal life processes. This makes scouting and correct identification very important.
One type of beneficial insect is a predator. Predators feed directly on prey, just like an eagle that catches and eats a fish. Many insects (and spiders) survive the same way, by catching and eating other insects. Lady beetles are a great example of an insect predator. Both the larvae and adult feed directly on aphids or other soft bodied insects. Parasitoid insects kill prey in an indirect way. Many parasitoids lay eggs within or on another insect’s body. These eggs hatch and the larvae feed on the living tissue of the host as they develop, eventually killing the host. An example of this is parasitoid wasps that seek out aphids in corn and soybean. When the wasp larva has completed feeding inside an aphid host, it will cut a hole in the side of the aphid and exit as a wasp. These "mummified" aphids are brown and stand out among healthy members of the colony.
Lady beetle larva eating a soybean aphid.
Lady beetle larva
Multicolored Asian lady beetle.
Nabid
Spider eating a bean leaf beetle.
Insect predators
Many insect species are pollinators of food crops. Some pollinators are social insects, such as honey bees, and others are solitary creatures. The European honey bee is the most well-known and effective pollinator species. These bees are easily managed by humans to pollinate crops and provide honey. However, other insects such as native bees, flies and butterflies provide pollination services as well. Although soybean and corn are not dependent on pollination by insects, pollinators are an important part of the ecosystem.
Ecosystem diversity is important for beneficial insect success. Providing diversity in an agricultural monocropping ecosystem can be difficult. Parasitoid wasp larvae may be able develop inside multiple insect hosts, but adults might need pollen or nectar to survive. Flower availability all summer as a nectar and pollen resource is a vital part of encouraging natural biological control. Thus, a variety of flowering plants increases natural biological control.
A mummified aphid is the result of a small wasp developing inside the aphid.
Parasitized host catepillar after parasitoid has emerged.
Tachinid fly egg on a green cloverworm.
The Tachinid fly lays eggs on host insects.
Insect parasitoids
Many insecticides used to control agricultural pests will also kill beneficial insects. By only applying insecticides when needed based on economic thresholds, we can help preserve beneficial insects and pollinators by reducing the number of applications made during the growing season. One example of beneficial insects that often go unnoticed are predatory mites. Many broad-spectrum insecticides kill these beneficial mites that feed on crop-damaging spider mites. Populations of crop-damaging spider mites may then increase rapidly, or "flare" in a crop field as the beneficial mites are no longer feeding on them.
Spider mite populations may increase rapidly, or "flare" in a crop field if beneficial mites are no longer feeding on them.
Adam Sisson
Earn Certified Crop Advisor CEUs after reading this web book. Successfully complete a quiz for this chapter to earn 0.5 CCA CEUs at the Crop Protection Network CCA CEU page.
Plant pathology is the study of plant disease. Understanding the basics of plant disease is beneficial for identification and management of these crop issues. Disease causing organisms are called pathogens. Pathogens utilize corn and soybean plants for food, competing with humans and other organisms for this resource.
Farmers face plant disease issues every year. Diseased crops can be obvious when visible symptoms occur, and other times crop losses can go unnoticed. Because individual diseases require differing conditions to develop, the problematic disease or diseases may change each growing season. Crop diseases can cause significant loss, yet at other times their impact is minimal. The Crop Protection Network has developed an online tool to help track the impact of individual corn and soybean diseases for many U.S. states and Ontario, Canada.
Gray leaf spot is a common yield-reducing foliar disease of corn.
A plant disease is any abnormal condition that alters the appearance or function of a plant. It is a physiological process that affects some or all plant functions. Disease may also reduce yield and quality of the harvested product. Disease is a process or a change that occurs over time. It does not occur instantly like hail injury, which is an example of a crop disorder.
The visible effects of disease on plants are called symptoms. Any detectable change in color, shape, and/or functions of the plant in response to a pathogen or disease-causing agent is a symptom. Leaf spots or blights, discoloration of plant tissue, stunting, and wilting are symptoms that may be evidence of disease.
Symptoms can occur throughout the plant, or they can be confined to localized tissues. Although certain symptoms are characteristic of a particular disease, a number of pathogens may produce the same or similar symptoms. Furthermore, symptoms often change over time, and their expression is influenced by environmental conditions, pathogen race, and/or crop cultivar.
Anthracnose stalk rot appears as a dead flag leaf on scattered plants in a corn field.
One of the symptoms of anthracnose stem blight in soybean is “shepherd’s crooking” caused by petiole infection.
Brown spot is a very common disease of soybean. Brown lesions form on infected leaves and eventually the leaf will yellow and fall from the plant.
Crazy top of corn can result in tissue deformation and proliferation, hence the name crazy top.
Damping off, or the death of a seedling, is a symptom of disease on the plant’s roots.
Eyespot lesions on a corn leaf appear as small spots with a yellowish halo surrounding lesions when held up to light.
Phytophthora infection results in a large stem lesion on soybean plants.
Soybean sudden death syndrome symptoms appear as yellow and dead tissue between leaf veins.
Plant disease symptoms
Signs of plant disease are physical evidence of the pathogen, for example, fungal fruiting bodies, bacterial ooze, or cyst nematode females. Signs can help with plant disease identification and can occur on both internal and external plant tissues
White mold signs on soybean include small, black structures called sclerotia, which help the fungus survive during the winter, and white mycelial growth on stems
Common smut appears as whitish, distorted, and swollen galls on plant tissue.
Blue fungal growth on soybean roots can be a sign of sudden death syndrome.
Common rust causes lesions on plant tissue which rupture to release reddish fungal spores.
In moist weather, downy mildew lesions can produce fuzzy, gray tufts of fungal growth on the leaf underside.
This greenish mold is a sign of Aspergillus ear rot of corn.
Powdery mildew causes white, powdery growth on soybean foliage.
Plant disease signs
To understand a plant disease and how to manage it to prevent or reduce its impact, it is important to know about the elements necessary for disease development, each of which make up a side of what is known as the disease triangle. The series of events that occur as a plant disease develops is called the disease cycle. Management actions are designed to disrupt certain stages of the disease cycle to prevent disease development.
The Disease Triangle
In order for a plant disease to become established, three things are required: a susceptible host plant, the disease-causing pathogen, and an environment conducive to disease development. These are the three sides of the plant disease triangle. Disease does not occur without all three components.
The plant invaded by the pathogen is called a host. Both corn and soybean are host to many pathogens, some of which infect both crops. The host serves as the food source for the invading organism. A host must be susceptible to infection by the pathogen. Corn and soybean cultivars are developed to have resistance to certain pathogens, and are assigned ratings based on how resistant or tolerant they are to infection by specific pathogens.
If a cultivar is resistant to a disease, that plant is not as favorable for pathogen growth as susceptible cultivars. Even in the presence of pathogens and a favorable environment, disease will not develop without a susceptible host plant.
Of all types of plant pathogens, fungi are the most common. These are spores of a pathogen that causes northern corn leaf spot on corn.
Temperature and moisture are the most important environmental factors that affect development of nearly all diseases. Both air or soil temperature and moisture can affect crops and/or the pathogen. If the temperature is unsuitable, a plant may grow poorly, and therefore become more susceptible to a particular disease. Temperature also may stimulate or reduce the growth of a pathogen resulting in a different level of disease. Excess or deficient moisture may also stress soybean plants, making them more prone to attack by some pathogens. Additionally, free moisture is important for the development of many fungal and bacterial pathogens.
Free moisture is a critical factor for infection for many diseases. Often, dry conditions are not conducive to disease development.
Relative humidity, soil pH, soil texture, light, and nutrient status may affect disease development as well. Moreover, such factors as compaction, tillage practices, planting depth, seedbed preparation, and residue management can have significant effects on disease development.
A susceptible host plant, a pathogen and a favorable environment are the three factors composing the plant disease triangle. All three factors are necessary for the development of a plant disease; thus, disease can be affected by altering any of these three factors. For example, the host plant can be changed by growing disease-resistant varieties. The pathogen can be removed by tilling residue, rotating crops so that pathogens do not survive year to year on the same crop, controlling insects that carry pathogens to plants, or using fungicides to kill the pathogen. Finally, the environment can be managed so that it is less favorable for disease, such as by changing row spacing or draining low areas.
The disease triangle is an important concept in plant pathology. Disease will only occur if these three factors interact simultaneously.
Like any disease, soybean rust requires a conducive environment, a susceptible host plant, and the soybean rust pathogen to develop.
The bacteria (inoculum) that cause Goss's wilt of corn are dispersed by splashing water, and infect plants through wounds caused by hail, wind, and blowing soil. The plant is then colonized by the bacteria and symptoms appear. The Goss's wilt bacterium survives the winter in corn residue in the field.
Knowing how a pathogen is spread can be valuable information when planning a disease management program. For example, it may be difficult to control the spread of pathogens by wind or water, but it is possible to control movement of diseased seed or infected plant material to restrict movement of pathogens on equipment or tools, and to reduce levels of insect vectors or shift planting dates to avoid high populations of insect vectors.
Infection occurs when a pathogen successfully enters a plant and then grows, reproduces, and spreads as it colonizes the plant. Pathogens enter a host through natural openings, wounds on plant surfaces, or by penetrating directly into the plant. Insect vectors may acquire pathogens from diseased plant tissue and then introduce the pathogen into healthy plant tissue as they feed.
Some pathogens attack and multiply only in leaf tissue. Others attack and multiply in stems, roots, fruits, or the conducting tissue of the plant. Another type may attack virtually the entire plant while some attack only seedlings or mature plants, and some have no preference.
The pathogens infecting corn and soybean each have a different life, or disease, cycle. This table shows the ways in which they survive during periods of harsh conditions, how they are spread, and how they infect a host.
White mold of soybean is an example of a disease that has a single disease cycle each year.
Gray leaf spot is a corn disease with a repeating disease cycle. Infection begins on lower leaves as spores are splashed onto them. Eventually the lesions formed on lower leaves will produce spores which can infect leaves higher on the plant as the season progresses.
Fungi are the largest and perhaps most well-known group of plant pathogens. The vast majority of fungal species, however, are beneficial. Many help decompose organic matter, releasing nutrients for other plants and organisms to use. Fungi and fungal products are used in food preparation; for instance, yeast is used in making bread and beer, in manufacturing processes, and in medicine (e.g., penicillin). However, several thousand types of fungi can cause plant disease, and a relatively small number of them cause disease in humans and livestock. Most plant pathogenic fungi are extremely small and, except for possible extensive growth on the surface of a plant, normally cannot be seen without a microscope.
Fungi lack chlorophyll, the green-colored compound that most plants use to complete photosynthesis to make food, so they must obtain their food from either dead or decaying organic matter or from living tissue.
Most fungi are composed of a growing structure of delicate, thread-like filaments called hyphae. Hyphae absorb both the water and nutrients needed for growth and reproduction of the fungi. They may also secrete enzymes, toxins, and other chemical substances that may be important factors in disease development and symptom expression. A mass of hyphae is referred to as mycelium. The “fuzzy” fungal growth that is sometimes visible on plant surfaces is mycelium. However, mycelium frequently develops completely or primarily within the host and is not visible on the plant surface.
Signs of powdery mildew on soybean are mycelium growing on the surface of the leaf.
Most fungi reproduce by forming spores. Spores are carried to plants primarily by wind and water. Some types of spores are produced inside structures called fruiting bodies that may be seen on or in plant tissues. Spores and fruiting bodies are often used to identify a fungus. Some spores and fruiting bodies are resistant to adverse environmental conditions and can survive in soil or decaying plant material for a long time.
This soybean seedling is dying due to Phytophthora infection. Spores of this oomycete have flagella, or “tails,” that are used to swim through water to infect new plants. Chemicals from soybean roots attract these spores.
Bacteria are primarily spread from plant to plant by wind-driven rain and gain entrance into plant tissues through natural plant openings. Wounds in plant tissues from insects, hail, wind, or other causes also provide entry points for bacteria.
Typical symptoms of bacterial diseases include leaf spots, water soaking, and soft rots of plant tissues.
Bacteria (small, dark spots) can be observed streaming from plant tissue (on right) while viewed under a microscope.
Bacteria are perhaps more familiar to us as the cause of important human and animal diseases such as tuberculosis and pneumonia. However, most bacteria are harmless and many are even beneficial such as the nitrogen-fixing bacteria present in soybean roots. Nonetheless, bacteria can also be destructive plant pathogens.
Bacteria are extremely small microorganisms. Individual bacterial cells require a microscope to be seen. They reproduce by individual cells splitting into nearly equal halves, each becoming a fully developed bacterium. A bacterial population may increase to very high numbers within a short period of time. For example, if a bacterium divided (reproduced) every 30 minutes, a single cell would produce more than 250 trillion descendants in 24 hours.
As bacteria divide, the cells tend to clump together in masses called colonies. Bacterial cells and colonies vary in size, shape, color, and growth habit. These characteristics are used to identify specific bacteria.
Like fungi, bacteria cannot make their own food; they must obtain it either from dead or decaying organic matter or living tissue. Nearly all bacteria have the ability to grow and develop on dead tissue. Most plant pathogenic bacteria populations are not adept at competing with other organisms in the soil, so their populations may decline rapidly in the absence of a host.
Bacterial blight of soybean can cause leaf spots, which are symptoms of infection by this disease.
Close-up of pustule on soybean leaf caused by bacterial pustule. Pustules will rupture, releasing additional bacteria.
Bacterial infections can result in soft rot of tissue. Shown here is the stalk of a corn plant that has been rotted.
Goss's wilt is a bacterial disease causing irregular-shaped foliar lesions.
Bacterial leaf streak lesions on corn.
Bacteria
Viruses are very small. This image was produced using a very high powered microscope and shows Maize dwarf mosaic virus.
Maize dwarf mosaic virus symptoms on corn.
Barley yellow dwarf virus symptoms in a corn field.
Alfalfa mosaic virus symptoms on soybean foliar tissue.
Bean pod mottle virus is spread by the by the bean leaf beetle during feeding.
Symptoms of Soybean vein necrosis virus may appear along leaf veins.
Pod symptoms of Tobacco streak virus on soybean.
Viral disease symptoms on corn and soybean
Soybean cyst nematode females on soybean roots.
Nematodes overwinter mainly in the egg stage. Most plant-parasitic nematodes live in the soil and feed in or on plant roots.
Most important plant-parasitic nematodes feed on plant roots and directly interfere with water and nutrient uptake by the plant. Root injury causes aboveground symptoms similar to those produced by other conditions that damage root systems. Plants frequently appear to be suffering from lack of moisture or nutrient deficiency, even when water and minerals are adequate. When nematodes occur in high population densities, stunting, yellowing, loss of vigor, general plant decline, and eventual death of plants are typical above-ground symptoms.
Soybean cyst nematode disease cycle showing the various nematode life stages and how the nematodes develop within the soybean root.
Iowa State University Integrated Pest Management
Scouting is an important part of disease management in corn and soybean. When and how to scout will change depending on the disease and weather. For example, some diseases show up near the beginning of the season, such as seedling blights. Others occur at the end of the season like stalk rots. Foliar diseases can be present mid-season. Diseases such as charcoal rot are favored by dry conditions while gray leaf spot is more problematic with high humidity. This illustrates that time of year and growing conditions are favorable for certain diseases. The good thing is this information can be used to help figure out when to look for specific diseases.
When looking for plant disease, you want to pay special attention to irregular areas in a field or to spots prone to disease conducive conditions. Side-hills, low spots, field entrances and edges, areas with flooding or poor drainage, yellow patches of foliage, and areas where disease was an issue in past years are good spots to scout. Be sure to check throughout the field as some diseases are caused by spores that simply blow into an area and land on leaves anywhere in a field. Also, management decisions to be used over the entire field should be considered in light of the condition of the entire field, not just a few particular spots.
Be sure to check the lower and upper canopy of plants. Brown spot of soybean starts in the lower canopy, as does anthracnose leaf blight of corn. Downy mildew can appear on leaves in the upper canopy of soybeans and common rust can generally be observed on mid to upper corn leaves. Root diseases can often be detected when foliar symptoms such as wilting or yellowing occur. Roots can be pulled and checked for symptoms or signs of root rots and nematodes. Stalks, stems, petioles, seeds, pods, ears, and other plant tissues can also be infected by disease, which makes it important to pay attention to the whole plant. Remember, disease severity (percent of damaged tissue) and incidence (percentage of plants diseased) are useful tools for quantifying plant disease.
Even if you miss a disease outbreak and are too late for treatments to work, there is still some value in the information you come across. Locating “hot spots” of disease may trigger management strategies to reduce inoculum in subsequent years. This information may help in discerning future decisions on tillage, crop cultivar, rotation, and even combine sanitation precautions for harvest.
Scouting for nematodes in both soybean and corn involves looking for spots in a field exhibiting symptoms of nematode damage. Aboveground symptoms of nematode feeding include stunting, chlorosis, uneven stands, and early maturity. Soybean cyst nematode (SCN) females can be found on soybean roots as soon as four to six weeks after a field is planted. When soil sampling for SCN, there are certain guidelines to follow that will help to obtain the best representative sample of an area. The more samples you take in a field and the smaller the area sampled, the better the idea you can get of the nematode population in the soil. Nematodes that feed on corn, for the most part, can be sampled until R3, or after harvest. Soil and root samples can be sent to a diagnostic clinic where accurate counts of nematodes can be obtained.
The Crop Protection Network has an encyclopedia of crop diseases that can be sorted by crop, time of season, and plant part impacted to help identify diseases found while scouting.
Scouting for Crop Diseases: Stalk Rots
Scouting for Crop Diseases: Ear Rots
Scouting for Crop Diseases: Soybean Cyst Nematode
Developing thresholds for plant diseases is time consuming and expensive. Thresholds need to be based on several things, including disease severity, incidence, progression of disease on plant tissues, regional disease development, host plant resistance, plant growth stage, past and predicted weather, previous crop planted, and price of grain and application costs. For this reason, they are of the most value in crops where multiple fungicides or other pesticide applications are necessary on an annual basis. Therefore, most disease thresholds focus on high-value commodities like turf and horticulture or specialty crops. There are very few models for predicting outbreaks or determining thresholds for corn or soybean diseases.
One threshold developed over 20 years ago, which illustrates how complicated developing these thresholds can be, is for gray leaf spot of corn. The threshold is when at least 50 percent of plants sampled before tasseling show lesions from the disease on or above the third leaf beneath the ear leaf on susceptible or moderately susceptible hybrids. This example is based on disease incidence (50 percent of plants infected), progression of disease on plant tissues (disease on the third leaf below the ear leaf), plant growth stage (prior to tasseling), and host plant resistance (susceptible hybrids planted). Other factors are added to the above threshold when the hybrid is moderately resistant to gray leaf spot. When this is the case, disease favoring circumstances such as infected corn residue or warm and humid weather must also be present before a fungicide could be considered on these hybrids. This adds “past and predicted weather” and “previous crop planted” to the already rather complex threshold for gray leaf spot. Further confounding this threshold is the emergence of new fungicides, continued development of hybrid resistance, and the fluctuating marketing price for corn. In general, a fungicide application is not recommended on hybrids that are resistant or moderately resistant to foliar disease.
Instead of focusing on thresholds, we suggest determining if in-season management decisions (most often foliar application of fungicides) are needed by following simple steps during the season:
This seed corn field has been severely blighted by gray leaf spot.
Choosing the appropriate disease resistant cultivars requires knowing what disease issues are likely to occur in a field. This can be accomplished by scouting and keeping records of diseases that were a problem in past years.
These corn plants are susceptible to Goss's wilt and blight, which has led to severe disease development.
Earn Certified Crop Advisor CEUs after reading this web book. Successfully complete a quiz for this chapter to earn 0.5 CCA CEUs at the Crop Protection Network CCA CEU page.
Weed management is vital for maximizing crop production and is an important component of IPM. In order to manage weeds you will need to identify the weed, consider the impact of the weed, and understand its life cycle. Many weed management techniques exploit the life cycles of weeds and use weed biology characteristics in the development of control strategies.
A simple definition of a weed is "a plant out of place." Even corn and soybean plants become weeds when they crop up as volunteer plants in fields where their presence is not welcome.
Weeds may be classified by their structure and form such as grasses, sedges, or broadleaf weeds. Weeds are characterized further within those larger groups by their life cycles and how they develop and reproduce.
Weed Identification Resources
It can be difficult to identify weeds, which makes it beneficial to have helpful resources to aid in the process. Depending on your region, a few recommended references include:
Weeds of the Midwestern United States and Central Canada, by Charles Bryson, Michael DeFelice, and Arlyn Evans;
Weeds of the Great Plains, by James Stubbendieck;
Weed Identification Field Guide, from Iowa State University Extension and Outreach;
An IPM Pocket Guide to for Weed Identification in Field Crops from Michigan State University;
Weed ID Guide from University of Missouri;
Common Weeds of Kentucky from the University of Kentucky;
Weeds of the Northeast, by Richard Uva, Joseph Neal, and Joseph DiTomaso;
Weeds of the West, by Larry Burrill et al.; and
iNaturalist plant identification app
These are great resources with images and other information to help identify weeds in your crop field. Remember, weed samples can also be sent to diagnostic laboratories for professional identification.
Weeds typically fall into one of three life cycle classifications: annuals, biennials, or perennials. Some weeds may be classified into more than one life cycle. Weeds are usually best adapted to survive in a crop with a similar life cycle, germination time, or growth habit. Weeds have many characteristics that make them able competitors. The most effective control methods are often based on the life cycle of a weed.
Annual weeds complete their life cycle in one year and reproduce by seeds. There are summer and winter annual weeds. Summer annual weeds germinate in the spring, then grow, flower, and produce seeds during one growing season. They are the most common type of weed in corn and soybean fields, due to their similar growing season. They also have a similar growing season to corn and soybeans. Winter annual weeds germinate in the late summer or fall, establish a root system and vegetative growth, overwinter, and then resume growing next spring. They usually flower and set seed in spring or early summer and then die. Winter annual weeds can pose problems in fall-seeded crops, early spring grains, pastures, and no-till fields. Annual weeds are most easily controlled in the seedling stage and become more difficult to control as they grow and mature.
Ivyleaf morningglory is an annual weed with vines that can climb around crop plants.
Common lambsquarters
Giant foxtail
Purslane
Summer annuals
Winter annuals
Biennials
Biennial weeds require two years to complete their life cycle and, like annual weeds, only reproduce by seeds. Seeds germinate in the spring or summer and produce root systems and rosettes of leaves during the first year. The following spring stems bolt (elongate) and plants flower, produce seeds, and die. Biennial weeds are typically a problem in no-till fields, pastures, and other undisturbed areas. Some biennial weeds can also behave as annuals, completing their life cycle in a single growing season. Chemical control of biennial weeds is most effective when applied to seedlings or during the rosette stage, before stems bolt.
Musk thistle
Wild carrot
Wild parsnip
Biennials
Perennials
Perennial weeds live multiple years. Perennial plants may be classified as "herbaceous" or "woody." Woody perennials typically have aboveground plant parts that can overwinter, whereas herbaceous perennials regrow each season from underground overwintering structures. They reproduce vegetatively and/or by seeds. Perennials typically inhabit no-till fields, pastures, roadsides, and, occasionally, tilled fields. Most perennial weeds found in row crops regrow annually from underground overwintering structures.
Herbaceous perennials can be grouped into two classes, simple and creeping. Simple perennials usually have taproots and reproduce by seed. Creeping perennials can reproduce by seed and vegetatively by rhizomes, tubers, stolons, budding roots, and bulbs. Tillage breaks vegetative structures into pieces that can regenerate into new plants, potentially spreading the infestation within or between fields.
Perennials may require either repeated efforts or a combination of management tactics to achieve adequate control. A well-timed systemic herbicide application may provide the most effective chemical control. Perennials are easiest to control as seedlings.
Weeds as Competitors
Weeds are very well suited for competition with crops, and seem to occur in an endless supply. Why are weeds so successful? A key reason is seed characteristics. Often weed seeds can remain dormant in the soil for many years (the seeds of velvetleaf have been reported to remain viable for over 100 years). Then, when the right conditions occur, they germinate and grow. Some weedy species produce seed for an extended period of time, and many weed species can produce thousands of seeds on a single plant. Weeds may also produce seeds under adverse environmental conditions. Finally, weeds have developed various effective ways of spreading their seeds, with different species using different techniques to spread seeds either short or long distances.
Weeds spreading through soil movement
Weed seed spread by sticking to clothing
Weed seed spread by sticking to a dog
Humans and animals spreading weed seed
A few methods of weed seed dispersal include wind-blown seeds (dandelions), animals or humans moving burs (cockleburs and burdock), thorns or stickers transporting parts of plants to new locations (Canada thistle and leafy spurge), and birds that ingest weed seeds and then excrete them in different places (mulberry trees in fencerows between fields). Humans also do a lot to aid in weed dispersal by moving soil or compost, moving weed seeds or plant parts with tillage or harvest equipment, introducing non-native plants that become invasive weeds, and planting desirable plant seeds that are contaminated with weed seeds.
Weeds that can reproduce vegetatively have an advantage in areas that are not tilled regularly and often pose serious long-term problems. Rhizomes are underground stems that root at nodes and allow the plant to reproduce and spread vegetatively. Canada thistle and quackgrass are two weed species that reproduce by growth of sturdy rhizomes. Stolons are aboveground creeping stems that can root at the nodes and produce new plants. Ground ivy, also known as creeping charlie, is an example of a weed that reproduces effectively by stolons.
Some weed species reproduce vegetatively by bulbs, bulblets, or tubers. These are underground structures that are actually leaf or modified stem tissue where food (carbohydrates) is stored for later growth, overwintering, and production of new plants. Wild garlic and yellow nutsedge are examples of plants that have underground reproductive leaf tissue structures. Some plants also have aboveground bulblets that can produce new plants. Wild onion and wild garlic are two examples of weedy plants that produce aboveground bulblets.
Some weeds can produce new plants from parts broken off the original plant. Purslane and Asiatic dayflower are examples of plants that can regenerate another plant just from a section of the original plant. These species can often survive on the soil surface for several days without water and will produce roots at the nodes to reestablish and reproduce.
Additional weed characteristics that allow them to grow and be competitive with desirable plant species include the ability to germinate and grow in many different environments, rapid seedling growth that allows them to be competitive quickly, ease of pollination, the ability to reproduce vegetatively, and the ability to tolerate adverse environmental conditions.
Morphology of Grasses
Stems
Weed stems, like leaves, are extremely variable. Yellow nutsedge has edges on the stem, giving rise to the rhyme "sedges have edges." Scouring rush has hollow, jointed stems that can be plucked apart. Yellow foxtail stems are flattened and hairless, but marestail stems are covered with stiff hairs. Goosegrass may appear prostrate, while many other grasses are erect and some broadleaf plants like dandelion have a rosette of leaves in place of the normal stem. Field bindweed or ivyleaf morningglory have climbing vines.
Ligules
Ligules are the thin membrane or ring of hairs at the junction of the leaf sheath and blade on the inside of the leaf collar on grasses. Broadleaf plants do not have ligules. Giant foxtail ligules are hairy, while membranous ligules can be found with quackgrass. Downy brome ligules can vary in appearance, while barnyardgrass does not exhibit any ligules. Ligules tend to be very small structures that may require a magnifying lens to clearly observe and identify.
Seedhead, flowers, or fruit
The seedhead is called an inflorescence and is where flowering occurs and seeds are formed. Three types of seedheads are panicles, spikes, and racemes. Panicles are seedheads with a main axis and subdivided branches. Foxtail species have cylindrical-shaped, bristly panicles. Fall panicum has large, spreading panicles, while witchgrass panicles are dense and funnel-shaped. Common waterhemp has narrow, compressed panicles containing either male or female flowers. Spikes are elongated, unbranched seedheads. Italian ryegrass has a spike seedhead with spikelets attached alternately and edgewise along the stem. Foxtail barley spikes are bristly and nodding, sometimes partially enclosed in upper sheaths. A raceme is an inflorescence with spikelets or flowers on stalks coming off a central axis. Longspine sandbur has a raceme with many round, spiny burrs attached by short stems to zigzag stalks. Shepherd’s-purse has very small, white flowers clustered in elongated racemes. Other types of flower structures include umbels (wild carrot, wild parsnip) and disk flowers (sunflower, Canada thistle). Fruit can also help to differentiate weeds. Eastern black nightshade berries are green when immature and shiny black at maturity. Smooth groundcherry has orange, red, or purple berries surrounded by a green, lantern-shaped, papery bladder.
Bristly, nodding foxtail barley spikes.
Cylindrical, bristly foxtail panicles.
Large, spreading fall panicum panicle.
Narrow, compressed common waterhemp panicles.
Seedheads
Importance of scouting fields for weeds
While there are a lot of potential pest threats to corn and soybean crops, many of them (insects, mites, and diseases) are sporadic. We may not see them every year; they can be very dependent on the weather, what host crop is in the field, and a multitude of other factors. Weeds, however, are something we contend with at some level on nearly every acre, every year, in both corn and soybean crops.
Weeds have been an issue since crop farming began; as weed management strategies evolved, so did the weeds themselves. Since it is unlikely that we will "eradicate" or eliminate weeds from cropping systems in the near future, "weed management" is how we will protect our crops from the threat of yield loss from weed competition. We have very good tools to manage weeds, but the key to finding the right combination of tools is to understand what we are facing. Knowing what weeds are in our fields (and field borders), what size and stage they are in prior to managing them, and how species and populations responds to control strategies each year and across successive seasons are keys to long term weed management success. This is why a consistent scouting program is important. The initial, and arguably most important step in weed management, is figuring out what weed we are dealing with.
Weeds impact crop management decisions for nearly every acre of corn and soybean each year.
A program where scouting occurs multiple times during a season:
Tells us what we are dealing with and guides our management plan
Allows us to evaluate how the plan is working throughout the season and gives us time to adjust the plan
Shows us how the plan worked after everything has been implemented
Guides us going forward, fine tuning future management strategies and the management plan for the next crop
Early season scouting
Defining early season scouting for weeds depends somewhat on your farming system and the weeds you fight. In conventional tillage systems, primary tillage often allows farmers to start with a clean slate, a weed free field; thus, early season scouting might mean beginning to scout a few weeks after tillage or preemergence herbicides have been applied. This timing will allow farmers to catch early season escapes and formulate an early postemergence treatment plan or timely cultivation pass. For minimum tillage or no tillage growers, early season means taking a look at the fields one or more times before a burndown herbicide application or light tillage pass. Knowing what early season weeds are out there (especially winter or early spring annuals) and adapting your management strategy to what you find often means the difference between failure and success in early season weed management. Early season scouting is critical in helping to answer questions like "Will a light tillage pass control those early season weeds?" in a minimum tillage system; or "How mature/large are those early season weeds in my no-till system? Do I need to change herbicide rates or products to get better control?"
Follow up scouting to assess the success of the initial management strategies is important as well. Evaluating the results will help guide your potential postemergence weed management strategies. Determining the effectiveness (or ineffectiveness) of early season management can also help guide strategies that could be implemented later that season or in preparation for next year.
Weeds in emerged crops compete for light, water, and nutrients.
Timely scouting is also crucial for guiding application of postemergence products or mechanical control. Knowing what weeds are competing with emerged crops, along with weed size and stage, are key to formulating a postemergence management plan. Weeds can begin to affect yields as early as just a few weeks after crop emergence, so it is more than just a matter of "Can I control these weeds?" It is important to control them effectively and early to protect yield.
The next round of scouting should begin within the first two weeks after crop emergence to guide postemergence weed control. While many herbicides have the ability to control large weeds, this is highly discouraged as application delays can result in reduced yield and profitability due to early-season competition, as well as increased selection pressure for herbicide resistance.
With the multitude of herbicide resistant weeds producers contend with, most of our postemergence management options have to be applied on relatively small weeds (often 4-6 inches or less) to be effective. Since some species such as waterhemp and palmer amaranth can grow at the rate of 1 inch per day or more, a consistent early season scouting program is imperative for guiding product choices, rates, and timing.
Scouting trips after the postemergence applications are just as important as the trips prior to them. Knowing how well the postemergence strategies worked is critical. If there are weed escapes:
Identify the species, size, and scope of the escapes
Investigate why there were escapes. Determining what went wrong is imperative in deciding your next steps. There are many reasons things can go wrong including:
Incorrect herbicide used
Product rate too low
Area of field skipped
Weeds are too large
Potential herbicide resistant weeds
Weed emergence after treatment
Rainfall too soon after application
Poor environment, i.e. too hot, too cold or too late in the day for good herbicide activity
Determine if another attempt at control is warranted this season or if it is something to keep record of for planning next year’s herbicide program
Late season scouting
Scouting for weed issues in the later stages of the crop season may sound like an exercise in futility since there are few, if any management options left in production fields and any yield impact to the crops has likely already occurred. However, there are several good reasons to consider scouting later in the season and again around harvest time. These include:
Late summer scouting can alert a farmer of weed escapes and help identify species and location to help guide management strategies that might be put into place prior to or within the next crop.
Identifying weed escapes can help prepare for future management decisions.
Some weeds may be worth finding and removing if the escapes are limited in scope (marestail and palmer amaranth are examples that farmers often mention). Removal of weeds like these (resistant to many herbicides and difficult to manage once established in larger areas) prior to setting and dropping seed can significantly reduce the pressure they can put on future crops if they become widespread in fields.
Taking a good look at border areas throughout the season can help farmer get ahead of weed problems as well. Sometimes weeds move via wildlife, winds, water movement, or other means and become established in areas with less competition from the crop and weed management strategies used in production areas. Finding these initial weed populations in field borders, buffer strips, terraces, waterways, and headlands and mowing, spraying, or pulling them out can reduce future challenges.
Weed species, density, and growth rates are critical factors influencing how long weeds can compete with the crop before yields are reduced. Treat fields with heavy infestations as soon as possible after weeds emerge. Fields that are kept free of weeds for the first several weeks after planting give the crop a competitive edge that allows the crop to shade out or out compete weeds that emerge later in the season.
The initial growth of weeds is relatively slow, but their growth rate increases rapidly as time progresses. Weeds as small as two inches tall can reduce corn yields if there are more than 10 per square foot. Crop yield loss per day increases due to increasing competition of larger weeds. If weeds are 2 to 4 inches, corn may experience a 0.5 percent yield loss per day. Daily yield loss increases to 1 percent or more when weeds are 6 inches tall.
Winter annual thresholds are not well defined. However, treatment decisions should consider species and distribution of weeds in a field. If a winter annual infestation is field-wide, consider a pre-plant burndown application. Patchy infestations may require only an adjustment to the burndown application at planting.
Even small weeds can cause yield loss if numerous enough.
Weed management is vital for maximizing crop production. Most successful farmers design control programs to obtain the best profit, not just to kill weeds. It is important to consider individual weed problems. Crops can tolerate certain numbers of weeds without suffering a yield reduction, but there are some weeds where 100 percent control may be desirable because they are particularly competitive, persistent, or difficult to control. These include some annual weeds such as waterhemp, Palmer amaranth, giant ragweed, common cocklebur, burcucumber, or shattercane, and several perennial weeds such as Canada thistle, bindweeds, quackgrass, and hemp dogbane.
There are four main systems of controlling weeds:
Preventive (not letting weeds become established),
Cultural (practices like adjusting the planting date to aid or deter weed development),
Mechanical (cultivation or hand pulling as examples), and
Chemical (herbicide application).
These methods are all most effective if applied at the correct time. For more information on management tactics refer to Chapter 1.
Tillage is one of the methods used to manage weeds in corn and soybean fields.
Earn Certified Crop Advisor CEUs after reading this web book. Successfully complete a quiz for this chapter to earn 0.5 CCA CEUs at the Crop Protection Network CCA CEU page.
Noninfectious disorders are caused by chemical or physical components in the environment that are harmful to plants. Noninfectious disorders do not reproduce or spread from plant to plant, and are not caused by living organisms. Each year, farmers must plan for and respond to noninfectious disorders in order to maximize yield and profit. Examples of noninfectious disorders are temperature and moisture extremes, hail, wind, lightning, unfavorable light, improper soil nutrient levels, toxic chemicals, and mechanical damage.
Cupped and crinkled soybean leaves can indicate herbicide injury.
Plants stressed by noninfectious disorders may be more prone to attack by infectious diseases. For example, soybean plants stressed by herbicide injury may be more prone to root rot diseases, or hail injury may provide an entry point for some pathogens to enter the plant.
Noninfectious disorders may produce symptoms such as wilting, stunting, yellowing, plant tissue deformation, or death of plant tissue. Symptoms of noninfectious disorders often resemble those caused by insects or diseases. For instance, nutrient deficiency symptoms may resemble symptoms of root rot diseases. Another example is herbicide injury to soybean leaves which may resemble virus-like symptoms.
Nutrient disorders in crops can produce fairly characteristic symptoms, such as this nitrogen deficiency on a corn leaf. Other nutrient deficiencies can be hard to determine based on symptoms.
Moisture extremes include both drought and flooding. This corn has curling leaves in response to a shortage of water.
Hail causes injury to corn and soybean plants by defoliating, bruising or breaking stems, and other damage.
Wind can cause damage by blowing plants over. Plants that have been blown over are more difficult to harvest.
Damage caused by weather conditions
Plants, like people and other organisms, require nutrients for proper development. There are the "macro" nutrients and the "micro" nutrients. Macronutrients are nitrogen, phosphorus, and potassium; plants need these in large amounts and they are the most significant. Micronutrients include iron, magnesium, sulfur, zinc, and others; these are also required by the plant but in smaller amounts.
Iron chlorosis can result when iron becomes unavailable to soybean plants in high pH soils.
When these nutrients are lacking or are unavailable to crops, development may be inhibited. Sometimes nutrients may be in the soil, but certain conditions exist so that plants are unable to take them out of the soil and use them. For example, in alkaline, high-pH soils, availability of iron is limited. In this situation, plants have difficulty obtaining iron from the soil even though it may be present.
Heavy rains can result in soil crusting, making it difficult for seedlings to emerge from the soil.
These three images show symptoms of paraquat herbicide injury on soybean, corn, and a weed. These symptoms could be confused with frogeye leaf spot of soybean or Holcus spot of corn.
These three images show symptoms of paraquat herbicide injury on soybean, corn, and a weed. These symptoms could be confused with frogeye leaf spot of soybean or Holcus spot of corn.
These three images show symptoms of paraquat herbicide injury on soybean, corn, and a weed. These symptoms could be confused with frogeye leaf spot of soybean or Holcus spot of corn.
Paraquat injury
The missing plants are the result of sprayer tires driving through the field at growth stage V2.
This injury pattern in the field begins where the crop sprayer likely entered the field and the injury fades as the driver progressed through the field.
When looking for symptoms of noninfectious disorders, there are some clues to look for in the field that may help to determine what kind of problem is present. These are:
Patterns in the field
Does the problem occur in a straight line or other shape? A sprayer may have overlapped herbicide application, causing plant damage. Or does it occur only in low spots? These spots may have been flooded for long periods of time, and plant roots may lack oxygen or a nutrient.
Glyphosate herbicide mistakenly applied to non-glyphosate resistant soybeans has resulted in a straight line of dead plants where spray coverage stopped.
Timing
Did symptoms show up after an herbicide application? Herbicide drift from another field may have damaged crops on the adjacent farm. Or, does it appear after certain weather events? Wind may have caused corn stalks to lodge or leaves may be tattered due to hail.
Other plants
How do surrounding plants appear? If surrounding plants appear similarly injured, and not in progressive states of injury, it may indicate a noninfectious disorder.
Soybean plants in a low area of the field injured as a result of flooding.
Some types of crop injury cannot be prevented, such as hail damage, but some, such as nitrogen deficiency, can be figured into management decisions for the coming years. Remember, herbicides, fungicides, and insecticides will not help when dealing with these disorders.
Good agronomic practices, such as planting at the appropriate time to avoid frost (both at the beginning and end of the season), drainage and erosion management, proper equipment calibration, and applying pesticides in the right weather conditions can help prevent noninfectious disorders from occurring. Another way to avoid nutrient deficiency and pH-related disorders is to sample soil and amend when necessary.
Even when a sudden, non-preventable disorder occurs such as hail, a field can be scouted to estimate potential yield loss. Depending on when hail injury occurs, corn and soybean plants have a remarkable ability to recover and continue development with minimal yield loss. Also, if unexpected problems such as flooding occur early enough in the season, replanting may be a viable option.
Cold temperatures early in the season can damage soybean seedlings.
A soil probe is used to obtain soil samples that can be tested for nutrient levels and other characteristics.
Earn Certified Crop Advisor CEUs after reading this web book. Successfully complete a quiz for this chapter to earn 0.5 CCA CEUs at the Crop Protection Network CCA CEU page.
With the creation of this book, we have endeavored to bring about a learning experience that increases crop scouting knowledge and abilities. It is our sincere hope that this has been a useful and interesting experience. As the reader, you are the reason that this text was written and assembled. Thank you.
Authors
Adam J. Sisson, Iowa State University; Daren S. Mueller, Iowa State University; Shawn P. Conley, University of Wisconsin-Madison; Corey K. Gerber, Purdue University; Scott H. Graham, Auburn University; Erin W. Hodgson, Iowa State University; Travis R. Legleiter, University of Kentucky; Paul P. Price, Louisiana State University; Kristine J. Schaefer, Iowa State University; Ed J. Sikora, Auburn University; Tessie H. Wilkerson, Mississippi State University; and Kenneth L. Wise, Cornell University.
Citation:
Sisson, A.J., Mueller, D.S., Conley, S.P., Gerber, C.K., Graham, S.H., Hodgson, E.W., Legleiter, T.R., Price, P.P., Schaefer, K.J., Sikora, E.J., Wilkerson, T.H., and Wise, K.L. 2021. Crop Scouting Basics for Corn and Soybean. Crop Protection Network. CPN 4007. Doi.org/10.31274/cpn-20201214-0.
Special thanks to
Lori Abendroth, Tim Berkland, Nathan Bestor, Kay Connelly, Andrew J. Geer, Laura Iles, Mark Johnson, Clarke McGrath, Andrew Penney, Richard Pope, Alison Robertson, and Jay Staker for their ideas, reviews, or early work in developing this book.
Images
Preface: Illustration by Emily Poss; Chapter 1-Scouting Corn and Soybean as Part of Integrated Pest Management: Daren Mueller and Brandon Kleinke; 1.1-Basic Guidelines for Scouting: Iowa Soybean Association On-Farm Network, Daren Mueller, Mark Licht, Rich Pope, Gary Munkvold, John Sawyer, and Adam Sisson. Illustrations by Renée Tesdall and Adam Sisson; 1.2-Tools for Scouting: Brandon Kleinke, Iowa State University Plant and Insect Diagnostic Clinic, and Daren Mueller. Illustrations by Emily Poss; 1.3-Recordkeeping and Scouting Information: Adam Sisson; 1.4-Use of Scouting Information: Adam Sisson, Daren Mueller, and Gary Munkvold; Chapter 2-Know Your Field: Iowa State University Integrated Pest Management; 2.1-Corn Growth Stages: Adam Sisson, Daren Mueller, and Iowa State University Extension and Outreach. Illustration by Renée Tesdall; 2.2-Soybean Growth Stages: Shawn Conley and University of Wisconsin-Madison; 2.3-Growing Degree Days: Iowa Environmental Mesonet; 2.4-Field Background and Predicted Information: Iowa Soybean Association On-Farm Network and Daren Mueller; 2.6-Replant Decisions: Adam Sisson and Daren Mueller; 2.7-Estimating Yield Potential in Corn: Adam Sisson and Daren Mueller; 2.8-Maturity, Drydown, and Field Harvest Losses: Daren Mueller and Adam Sisson; Chapter 3-Introduction to Insects: Adam Sisson; 3.1-Development of Insects: Adam Sisson and Daren Mueller. Illustrations by Renée Tesdall; 3.2-Insect Morphology: Adam Sisson, Laura Iles, Daren Mueller, and Iowa State University Integrated Pest Management; 3.3-Insect Scouting: Clarke McGrath, Adam Sisson, Daren Mueller, Gary Munkvold, and Iowa State University Integrated Pest Management; 3.4-Insect Thresholds: Daren Mueller and Adam Sisson; 3.5-Insect Management: Daren Mueller; 3.6-Beneficial Insects: Daren Mueller, Adam Sisson, and Iowa State University Integrated Pest Management; Chapter 4-Plant Disease: Adam Sisson; 4.1-Principles of Plant Disease: Alison Robertson, Daren Mueller, Adam Sisson, Gary Munkvold, Tristan Mueller, and Craig Grau. Illustrations by Renée Tesdall; 4.2-Fungi: Daren Mueller, Adam Sisson, and Craig Grau; 4.3-Bacteria: Ed Zaworski, Daren Mueller, Alison Robertson, and Adam Sisson; 4.4-Viruses: John Hill, Craig Grau, Adam Sisson, and Daren Mueller; 4.5-Nematodes: Craig Grau. Illustration by Renée Tesdall; 4.7-Disease Thresholds: Gary Munkvold; 4.8-Disease Management: Adam Sisson and Iowa State University Integrated Pest Management; Chapter 5-Introduction to Weeds: Brandon Kleinke; 5.1-Weed Development: Adam Sisson and Kristine Schaefer; 5.2-Morphology of Weeds: Kristine Schaefer. Illustrations by Kaitlin Lindsay; 5.3-Weed Scouting: Adam Sisson; 5.4-Weed Thresholds; Brandon Kleinke; 5.5-Weed Management; Brandon Kleinke; Chapter 6-Crop Disorders: Daren Mueller; 6.1-Introduction to Noninfectious Disorders: Gary Munkvold, Adam Sisson, Daren Mueller, and Tristan Mueller; 6.2-Scouting for Disorders: Adam Sisson and Daren Mueller; 6.3-Preventing and Responding to Noninfectious Disorders: Alison Robertson and Iowa State University.
Videos
Embedded videos developed by Brandon Kleinke with contributions by Daniel Ossian and Kent Johnson.
Sponsors
This educational resource was made possible by contributions from the National Corn Growers Association, Iowa State University Extension and Outreach; Iowa State University Integrated Pest Management; and the United States Department of Agriculture - National Institute of Food and Agriculture (USDA-NIFA).
This information in this publication is only a guide, and the authors assume no liability for practices implemented based on this information. Reference to products in this publication is not intended to be an endorsement to the exclusion of others that may be similar. Individuals using such products assume responsibility for their use in accordance with current directions of the manufacturer.
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