Soybean Aphid Management

KEY POINTS

  • Aphids can begin colonizing soybeans as early as May but outbreaks usually occur from flowering through early pod set.
  • Heavy aphid infestations can result in severe yield losses; controlling aphids is most critical during the early reproductive growth stages (R1 through R4).
  • Routine scouting is necessary to estimate aphid populations and to determine if insecticides applications are necessary.

Aphid Biology and Life Cycle

Soybean aphid is native to China and southeast Asia. It was first detected in North America (Wisconsin) in 2000, and by the end of the summer it spread throughout the north central states.1

The soybean aphid is small, pale yellow to green, with distinct black cornicles near the end of the abdomen. There are both winged and wingless forms. Aphids remove plant sap with piercing-sucking mouthparts and are often found feeding on the underside of new growth.

The soybean aphid life cycle is complex. Soybean aphids overwinter on a small woody shrub called buckthorn. They lay eggs on buckthorn in the fall from which wingless aphids hatch in spring. The aphids will complete 2 to 3 generations on buckthorn. In early June, winged females will develop and move into soybeans where many generations of wingless aphids will occur. During this time, aphid populations can grow exponentially. Populations can double in 2 to 3 days when conditions are favorable.2 Towards late summer or early fall, winged aphids will develop and return to buckthorn where they mate and lay eggs to overwinter. The presence of winged aphids can also indicate migration to other fields.

Damage to Soybean

Damage occurs to soybean when large numbers of aphids remove water and nutrients from leaves and stems during feeding (Figures 1 and 2). 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. Aphids initially colonize young leaves and branches and, as the season progresses, they move down to the middle of the plant and feed on stems and pods. The aphids excrete a substance called honeydew on which sooty mold can grow. This can give soybean leaves a black appearance and interferes with photosynthesis. Aphids can also transmit viruses such as soybean mosaic virus. Moisture stress during an aphid infestation can increase the risk for yield loss.

Scouting and Thresholds

Effective management of soybean aphid requires consistent scouting from seedling stage through pod-fill to track aphid populations. Begin scouting more intensively during the late vegetative stages and continue regularly at least once a week from flowering through seed fill. The presence of lady beetles (beneficial insects that feed on aphids) and ants (feed on the honeydew) on soybean often indicates that there is an aphid infestation. Determine the average number of aphids per plant on 20 to 30 plants throughout the field. Edges of fields can be hotspots for aphid infestations, resulting in overestimation of populations, so make sure to scout all areas of the field. Through the R5 growth stage, 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.2,3,4,5

Common insect predators include several species of lady beetles and their juveniles, adult and juvenile insidious flower bugs (also known as minute pirate bugs), green lacewing larvae, and parasitoids (tiny parasitic wasps).6 The presence of aphid mummies is an indication that natural enemies are present. Natural enemies may help regulate low to moderate aphid populations throughout the season; however, biological control is less effective when aphid populations have colonized the majority of plants in a field (greater than 80% of plants). Conserve natural predators when possible by using insecticides only when economic thresholds of the damaging insect are met.


aphids

Figure 1. Heavy infestation of aphids on the underside of soybean leaves.

Management

Many foliar insecticides are labeled for control of soybean aphids. The early reproductive growth stages (R1 to R4) are the most sensitive to stress from aphid feeding and subsequent yield loss. Therefore, protecting plants during the flowering through pod development stages will have the greatest impact on soybean yield potential. Applications made prior to flowering may not provide an economic benefit and can generally reduce beneficial insect populations, which may result in a resurgence of aphids. Applications made at or beyond the full seed stage of growth (R6) have not been found to improve yield potential unless plants are under additional stress such as drought.2,3 Good insecticide coverage at high water volumes and pressures is important for reaching aphids within the soybean canopy.


aphids

Figure 2. Aphid feeding on soybean stem.