Cabbage and Soybean Loopers in Soybean

Cabbage loopers (Trichoplusia ni) are commonly found throughout the U.S. and the southern provinces of Canada. Soybean loopers (Chrysodeixis includens) occur in the eastern coastal states and the southern tier states of the U.S. They can migrate to the Midwest, where they are considered an occasional soybean pest. Both cabbage loopers and soybean loopers have similar physical characteristics, cause similar types of damage, and can both be present in a soybean at the same time.

Crop Injury

Loopers begin feeding low in the soybean canopy. Young larvae feed on the underside of the leaves, leaving the upper surface and leaf veins intact, which gives the leaves a skeletonized appearance. As larvae develop, they move upwards and outwards to feed on the top leaves. Older larvae are voracious feeders, causing irregularly shaped holes in the leaves and severe defoliation.

Pest Description

Full-grown cabbage loopers and soybean loopers are 1 1/2 inches long and have two thin white lines along the back and a white stripe on each side. The bodies taper towards the head and broaden towards the midsection, where two pairs of prolegs are located. One additional pair of prolegs is located at the very end of the larvae. This arrangement allows the larvae to form a distinctive hump or loop as they move. In some cases, soybean loopers have true legs that are black, but this is not a reliable identification characteristic, as these black legs are not always present. Adult cabbage loopers have mottled dark gray and brown forewings with a silver “figure 8” pattern and a smaller spot in the middle of each forewing. Adult soybean loopers have mottled brown forewings with two irregular-shaped silver spots located near the center of each forewing.

Scouting and Management

States have different scouting protocols and management thresholds based on whether loopers are persistent or sporadic pests. High risk areas include late planted soybean fields, later maturing soybean varieties, soybeans in a double-cropping system, and soybeans planted near cotton fields. In some areas, cabbage loopers arrive during the soybean vegetative stages while soybean loopers infest fields later in the season. This assumption is not always true, as both cabbage and soybean loopers may both be present in the same field. Scouting and management information focuses on soybean loopers. 

Sampling methods include sweep nets, drop cloths, and defoliation estimates on randomly selected plants. Some states use more than one sampling method to make management decisions. When using a sweep net, take 25 sweeps in four or more locations. Make sure each sweep collects larvae deep in the canopy, as this is where young loopers feed. Count the number and estimate the approximate size of the larvae at each location. Treatment thresholds range from 19 to 38 larvae per 25 sweeps and some states indicate larvae should generally be 1/2 inch or greater in size. 

Drop cloth sampling should occur at 10 or more locations in the field. Place a 3-foot long white sheet between rows and shake the larvae off all plants along the sheet. Count the average number of larvae that fall on the sheet and estimate the approximate size of the larvae. Treatment thresholds range from three to eight larvae per row foot and some states indicate larvae should generally be 1/2 inch or greater in size. Thresholds are reduced in seed production fields. Row width variation will impact threshold numbers. 

When estimating defoliation, randomly select plants throughout the field and estimate defoliation over the whole plant. The standard soybean defoliation treatment thresholds are 30% in vegetative stages and 20% in reproductive stages. Some states suggest a threshold at 15%-20% defoliation in the most sensitive reproductive stages (R3 to R5). Loopers treatments are not advised after growth stage R6. Use the Crop Protection Network’s Insect Defoliation Tool to help you correctly assess the level of defoliation. If additional defoliators are present, assess their impact before making management decisions. 

Some management recommendations use multiple sampling methods to inform management decisions. For example, before bloom, treat when defoliation is 40%, and loopers are present; after bloom, treat when defoliation is 25%, and there are six to eight larvae per row foot. Predators, parasitic wasps, and fungal pathogens may reduce looper populations.  If natural enemies are present, assess their impact before making management decisions. Carefully consider your insecticide, as broad-spectrum insecticides will kill natural enemies and, in some cases, will cause a “flare” of loopers. Soybean loopers have developed resistance to several different classes of insecticides.

Please contact your local or state extension specialist for information on the most current sampling and management options.

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