Soybean Aphid

Figure 1

Figure 1. Wingless aphids (left) and winged aphids (right).

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The soybean aphid is small, pale yellow to green in color, with distinct black cornicles near the end of the abdomen (Figure 1). Aphids remove plant sap with piercing-sucking mouthparts and are often found feeding on the underside of new growth. There are both winged and wingless forms; the presence of the winged form often indicates migration. Aphids initially colonize the underside of young leaves; as the season progresses they move down to the middle of the plant and feed on stems and pods. Sooty mold can grow on aphid honeydew excreted on leaves. This can give soybean leaves a black appearance and interferes with photosynthesis.

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Damage occurs to soybean when large numbers of aphids remove water and nutrients from leaves and stems during feeding. This can cause leaf puckering, stunting, reduced pod and/or seed counts, and smaller seeds. Under heavy infestations, leaf edges may turn yellow, which may appear similar to potassium deficiency.

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Scouting for soybean aphid on soybean should begin in mid- to late June and continue until R6.5 (pods and leaves begin to yellow). Speed scouting is a quick sampling method developed by the University of Minnesota that can be used to improve scouting efficiency.1 Speed scouting uses infested versus non-infested plants rather than the average number of aphids/plant to determine if treatment is needed.2 If a plant has less than 40 aphids it is non-infested.

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Sample at least 11 plants/site in different parts of a field. Speed scouting worksheet for treatment decisions is available www.soybeans.umn.edu.

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Through stage R5, treatment is justified when an average of 250 aphids per plant is found on over 80% of the plants in the field and populations appear to be increasing.2,3,4 Soybeans benefit most from insecticide applications made during the R1 to R5 stage of growth, primarily to protect pod and seed development.