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Soybean Aphids Are Becoming More Prevalent

July 6, 2001
Survey teams coordinated by David Onstad, Department of Natural Resources and Environmental Sciences, continue to "discover" soybean aphids in soybean fields in northern and east-central Illinois. As of July 2, the soybean aphid had been found in the following counties: Boone, Bureau, Champaign, Cook, DeKalb, Grundy, Kane, Kendall, Lake, LaSalle, McHenry, Peoria, Stephenson, Vermilion, and Will. Entomologists in adjacent states are reporting similar findings of soybean aphids in an increasing number of counties.

Densities of soybean aphids in some fields also seem to be increasing. In one field in Kendall County sampled on July 1, the survey team found aphids on 49 of 50 plants sampled. The surveyors found as many as 400 aphids per plant on a few plants and 50 to more than 200 aphids per plant on the rest of the infested plants. They also noticed a fair number of lady beetle adults and larvae munching on the aphids.

Now that we are finding soybean aphids in soybean fields, we will focus on some other research objectives. Entomologists will continue to survey for soybean aphids in soybeans, with some efforts to determine whether the aphids are present in soybean fields south and west of Champaign. However, a new focus will be to monitor aphid population dynamics weekly in selected fields to determine whether densities are increasing or decreasing and to assess the impact of natural mortality factors, including predators, parasitoids, and pathogens. Plans are in place to conduct insecticide efficacy and timing trials in Champaign and DeKalb counties to assess the effects of insecticides on beneficial insects and to determine the effect of aphid injury on soybean growth and yields. In addition, plant pathologists are examining the potential for soybean aphids to transmit viruses in the field and searching for potential sources of resistance among soybean cultivars. Entomologists and plant pathologists in other states also will be conducting research as soybean aphids become more prevalent. We will try to keep you apprised of as many developments as possible.

We still believe it is too early to consider insecticide applications to control soybean aphids. However, it's not too early to start looking for them. The relatively large numbers of aphids found in some fields suggest that threatening densities could occur soon.

Remember what to look for when you search for soybean aphids. The soybean aphid is a small, yellow to yellow-green aphid with distinct black cornicles ("tailpipes" on the tip of the abdomen). They can be found on stems and young leaves of growing soybean plants and on the undersides of leaves of mature plants. Because there are no other aphid species that develop colonies on soybean in North America, it is safe to assume that colonies of tiny yellow aphids on soybean are soybean aphids. Colonies will consist of different size aphids (nymphs and adults), and white cast skins will be obvious in larger colonies. Plants infested with aphids may begin to show symptoms of injury. Heavily infested plants may be stunted, and leaves may become yellow and distorted.

Excellent photographs of wingless and winged soybean aphids, nymphs and adults, and eggs on Rhamnus (buckthorn) are located at The photographs were taken by David Voegtlin, aphid specialist in the Center for Economic Entomology at the Illinois Natural History Survey.

If you believe you have encountered soybean aphids as you scout soybean fields, let us know. We are very interested in keeping track of where and when the aphids appear throughout the season. Likewise, we'll keep you informed about the aphid's activities.--Kevin Steffey

Author: Kevin Steffey

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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