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Watch for Increasing Densities of Soybean Aphids During the Next Month

June 28, 2002
Findings of soybean aphids have continued throughout the upper Midwest during the past week. Scouting crews from the University of Illinois and Illinois Natural History Survey have found soybean aphids in Cook, Kendall, Lake, McHenry, and Winnebago counties. Although few aphids were found in any one field, the presence of small colonies on some plants indicates that population growth is under way. On June 19, Ron Estes, research specialist in agriculture in the Department of Natural Resources and Environmental Sciences, found aphids on 7 of 30 plants in one field in McHenry County, with as many as 6 aphids per plant. The soybeans were in the V2-V3 growth stages. The numbers of counties in which soybean aphids were found in surrounding states increased during the past week, but no one has reported finding significant numbers yet.

During the next month, everyone should keep their eyes on the growth of populations of soybean aphids. It's very important to monitor increasing numbers of soybean aphids, but it's equally important that we avoid overreaction. Although soybean aphids are colonizing small soybeans in Illinois this year, control measures typically are not warranted until aphid densities reach threatening levels on late- vegetative- or early-reproductive-stage soybeans. Between now and then, many factors can suppress populations of soybean aphids. As we have learned during the past 2 years, many natural enemies, especially the multicolored Asian lady beetle Harmonia axyridis, can reduce the number of aphids in a field. In addition, weather conditions during the next month may have a significant effect on soybean aphid populations. Asian literature indicates that soybean aphids reproduce faster when temperatures are below 81 deg F, so population growth may be slower if temperatures are high this summer. Heavy rainfall kills many aphids and may stop population buildup in some instances.

When you start looking for soybean aphids, make certain you identify them accurately. Other small insects, such as potato leafhopper nymphs, may also be found on soybean plants at this time of year. Soybean aphids are small (~1/16 inch), yellow-green insects with distinct black cornicles ("tailpipes") on their abdomens. Check out Dave Voegtlin's (Center for Economic Entomology, Illinois Natural History Survey) photographs at It is rare to find any other species colonizing soybeans in North America, so it is safe to assume that colonies of tiny, yellow-green aphids on soybeans are the soybean aphid.

We recommend the following steps for monitoring soybean aphid populations in Illinois:

· Sample leaflets from at least 25 randomly selected plants in the field. The plants should be separated by at least 25 paces.

· Pull off a fully expanded trifoliate leaf near the top of the plant and count the aphids on the middle leaflet. (Later in the summer you can sample leaves from the middle of the plant, too.)

· Calculate the average number of aphids per leaflet.

· If the average density of soybean aphids is 25 or more aphids per leaflet, return to the field in 5 days and resample the same general area to determine if the aphid population is increasing.

We have learned that the presence of alatoid nymphs (nymphs with "shoulder pads" that will develop into alates--winged adults) may be a sign that a soybean aphid population in a field is about to "crash," (i.e., the density will decline dramatically). Stresses on the crop and crowding by the aphids cause a generation of winged adults to form. Therefore, a high percentage of alatoid nymphs within a field indicates the forthcoming occurrence of winged adults that will leave in search of other fields.

Once again, I remind you that treating an infestation of soybean aphids too early is not wise. We need to allow time for natural enemies and weather to exert their influences. Applying an insecticide too early will kill natural enemies, as well as aphids, after which populations of soybean aphids will resurge because of their incredible reproductive capacity. A resurgence of soybean aphids in a field might require another, costly insecticide application. So be patient. Watch the aphids for a few weeks, and be ready to act if necessary.--Kevin Steffey

Author: Kevin Steffey

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

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