Scouting for soybean aphid

Identifying aphids

underside of green leaf with many yellow insects and white discarded insect skins
Figure 1: Soybean aphids, including winged adults and cast nymph skins.

Soybean aphids are small (one-sixteenths of an inch or less), soft-bodied insects that use piercing-sucking mouthparts to remove plant sap.

Color and characteristics

Both winged and wingless forms can occur in the field. Their bodies are yellowish-green and pear-shaped, with a pair of dark cornicles (“tailpipes”) at the end of the abdomen. Winged aphids have a dark head and thorax (Figure 1).

During soybean pod fill, a small, pale-colored aphid form (referred to as white dwarves) may occur on leaves in the lower and middle canopy.

Presence of lady beetles and ants

The presence of lady beetles or ants on soybeans often indicates aphid infestation and can be particularly useful in finding small, isolated early-season aphid colonies.

Sticky waste

While feeding, aphids excrete unneeded plant sugars as waste. Accumulation of this sticky waste, known as honeydew, can make leaves appear shiny.

It also encourages the growth of sooty mold fungus, which creates a black, powdery coating on leaves. As the season progresses, the presence of honeydew and cast aphid skins indicate high soybean aphid populations.

Dead and diseased aphids

Some infestations can cause potassium deficiency symptoms on upper leaves. While inspecting plants for soybean aphids, be on the lookout for dead aphids, which may be indicators of beneficial organisms working to suppress aphid populations.

Aphid mummies are an aphid body’s dried, brown or black remains after tiny parasitic wasps have fed on it from the inside. Diseased aphids are discolored and fuzzy-looking (see Figures 22 and 23).

When to scout

close up view of green soybean plants with dwarfed leaves.
Figure 2: Severe soybean aphid damage. Note the distorted leaves and sooty mold.

In Minnesota, soybean colonization by aphids can occur as soon as soybean emerges, and aphid populations can persist until soybean leaf drop. Begin scouting for soybean aphid on soybean in mid- to late-June and continue until R6.5 (pods and leaves begin to yellow), regardless of calendar date.

If you stop scouting too early, late-season infestations can build to economically damaging levels. Yield loss can occur into early R6 (pod cavity filled by seed). Because aphid populations can rapidly grow, scout infested fields on a weekly basis.

At-risk fields

Yield-damaging soybean aphid populations can occur in any field. However, some fields tend to have more consistent problems with aphid infestations each year. Several factors increase the likelihood of aphid problems.

Early in the season

In the spring, aphids are often first found in geographic areas with abundant buckthorn. Smaller fields with wooded borders are often the first to develop high populations. In addition, the following fields often develop aphid populations sooner:

  • Early-planted fields.

  • Fields with more coarsely textured soil.

  • Fields that test lower in potassium.

Later in the season

Later in the season, full-maturity soybean or late-planted soybean, such as beans following peas, are often reported to have higher populations.

Rigorous and possibly more frequent scouting of high-risk indicator fields can provide valuable information on when an area’s aphid populations are beginning to increase. Increases should trigger scouting of other fields you’re managing.

Scouting methods and thresholds

Scouting requires entering the fields and inspecting plants. By the time you can detect soybean aphid populations from the road, significant yield loss has likely already occurred.

Through timely and thorough field-scouting, you can identify and treat significant soybean aphid populations before economic yield loss occurs.

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Managing aphids and other considerations

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Robert Koch, Extension entomologist and Bruce Potter, integrated pest management specialist, Southwest Research and Outreach Center

Reviewed in 2018

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