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Time to Scout for Soybean Aphids

Bruce Potter, University of Minnesota Integrated Pest Management Specialist, reports that soybean aphid populations continue to slowly grow. We are now looking at doubling times of about one week. Bruce recommends that it is time to start to scout for soybean aphids. Listed below are some of the main concepts to consider when scouting for soybean aphids in Minnesotain 2017, which can also be accessed in detail in a University of Minnesota Extension publication on Soybean Scouting at

What fields are most at risk?

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. In spring, aphids are often found first in geographic areas with abundant buckthorn. Smaller fields with wooded borders are often the first to develop high populations. In addition, early-planted fields and fields with coarser textured or lower potassium testing soils often have aphid populations develop sooner. Later in the season, full-maturity soybean or late-planted soybean, such as beans following peas, are often reported to have higher populations.

Scouting methods and thresholds

Soybean aphids are small (1/16 inch or less), soft-bodied insects that use piercing-sucking mouthparts to remove plant sap. Both winged and wingless forms can occur in the field. Their bodies are yellowish-green, pear-shaped and have a pair of dark cornicles (“tailpipes”) at the end of the abdomen. Winged aphids have a dark colored head and thorax. Scouting requires entry into the fields and inspection of plants. By the time soybean aphid populations can be detected from the road, significant yield loss has already likely occurred. Through timely and thorough field scouting, significant soybean aphid populations can be identified and treated before economic yield loss occurs.

The number of aphids per plant should be estimated from a representative number of plants. Plants should be selected from throughout the area being scouted. The average number of aphids per plant should then be calculated. Through R5 (seeds developing, but pod cavity not filled), use the economic threshold of an average of 250 aphids/plant AND more than 80% of plants having aphids AND aphid populations increasing.

Count both adults and nymphs, including any white dwarves. However, avoid counting soybean aphid look-a- likes, such as potato leafhoppers and whiteflies, as well as cast soybean aphid skins and dead aphids.

The above-mentioned economic threshold is based on the average of multiple plants selected from throughout the field and not just hotspots or field borders. Avoid making treatment decisions based on field borders. Small aphid hotspots often collapse from predation, parasitism or emigration.

Economic Thresholds and Injury Levels

The economic threshold of 250 aphids per plant remains valid despite changes in soybean and input prices. These changes in prices adjust the economic injury level (EIL), which is the point at which losses are sufficient to justify insecticide application. However, the economic threshold of 250 aphids per plant is still sufficiently below the EIL to allow several days to make an insecticide application before losses occur. A lower economic threshold for soybean aphid is not justified at this time.


David Nicolai
Extension Educator, Crops & Institute for Ag Prof Coord
(651) 480-7706
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