Soybean Aphid in Soybean

Crop Damage: Soybean aphids (Aphis glycines) are found primarily in the Midwest, but distribution extends to the Great Plains, northeastern coastal states, and several southern states. Soybean aphids can also be found in the southeastern provinces of Canada. Both nymphs and adults feed on plant sap using piercing-sucking mouthparts. Soybean aphids prefer to feed on the undersides of new leaves, but all aboveground plant parts can be fed on. Aphids excrete honeydew, a sugary liquid that promotes mold growth. Heavily infested plants become discolored and wilted. Prolonged feeding can negatively impact flowering and pod development, potentially lowering yields by 40%.

Pest Description: Soybean aphid adults are 1/16 inch long and have pear-shaped, yellow-green bodies with dark eyes and black cornicles (tailpipes) on the end of the abdomen. Wingless adults have a light green head and pale antennae, while nymphs appear similar but smaller. Winged aphids have a dark head and thorax, along with two pairs of clear wings that extend past the end of the abdomen. Young winged aphid nymphs are green and gradually turn darker and develop wing pads as they transition into adults.

Scouting: In the northern U.S. states and southern Canadian provinces, aphids may appear at soybean emergence (VE). In other U.S. states, aphids may not appear until late vegetative growth stages. Factors that increase risk include late-planted fields as aphids prefer new soybean foliage, plants under moisture stress, and fields close to buckthorn, which is the overwintering site for soybean aphids.

Scouting should begin based on local recommendations from local or state extension specialists. Check for aphids in five to 10 locations throughout the field, especially on the undersides of leaves and new foliage. As soybeans develop into late vegetative stages and early reproductive stages, aphids can be found on all parts of the plant. If aphids are present, count them on 40 plants for every 50 acres of soybean and estimate populations throughout the field. There is also a concept know as Speed Scouting for soybean aphid. More information on this process can be found at this site.

Before deciding to treat, consider that natural enemies can affect aphid populations. Predators include lady beetles (both larvae and adults), damsel bugs, and green lacewing larvae. Parasitoid wasps use aphids as hosts for their developing young. Look for brown or black puffy aphids, known as mummies, when scouting. Fungal pathogens may impact aphid populations. Check for “fuzzy” aphids, as these been infected by fungi. Also consider that heavy or multiple days of rain can reduce aphid populations. Scout after rain to assess aphid populations.

Management: The economic threshold for soybean aphids in the late vegetative through R5 growth stages is 250 aphids per plant, with 80% of the plants infested and populations increasing. Depending on economic conditions, this allows about five to seven days to schedule treatment before populations reach economically damaging levels. Treatment after growth stage R6 does not impact yields and is not justified. Many foliar insecticides are labeled for soybean aphids; however, resistance has developed with certain pyrethroid insecticides. Therefore, it is important to implement effective management strategies to prevent additional resistance. Some soybean varieties have been discovered to be resistant to soybean aphids. These varieties slow the growth of aphids and reduce their reproductive potential. However, fields with resistant varieties may still have aphids, making it important to scout these fields. Seed treatments are labeled for early-season soybean aphids, but the duration of activity may not protect soybeans from mid- or late-season infestations.

Developed and reviewed by Carol Pilcher, Iowa State University and Robert Wright, University of Nebraska-Lincoln, with support from the United Soybean Board.