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Numbers of Soybean Aphids Keep Increasing

July 10, 2003

The title of this article is almost becoming a theme--the numbers of soybean aphids have been increasing in fields in northern Illinois over the past few weeks, with no sign yet of slowing down. Reports from states north of Illinois (e.g., Michigan, Minnesota, Wisconsin) are similar. Although the great majority of fields in northern Illinois have relatively low densities of soybean aphids, certain "hot spots" have been noted by several people. For example, Steve Doench, an agronomist with Pioneer Hi-Bred International in Woodhull, has reported that, although the numbers of aphids are relatively low in most fields in his territory, he has found fairly heavy infestations in Bureau, Henry, Lee, and Whiteside counties. He also has noted that as densities within a field increase, the aphids are distributing themselves deeper into the plant canopy on stems and leaves. This "redistribution" from the top leaflets to stems and leaflets deeper in the canopy has been recognized by others in past years.

We have received one report that a field of soybeans has been sprayed with an insecticide to control soybean aphids. Apparently the folks who made that decision thought that the symptoms of injury and the numbers of aphids justified the application. However, I remind everyone that some of the symptoms of soybean aphid in-jury can be confused with other problems (e.g., nutrient deficiency). I reemphasize that it's probably too early to spray insecticides for control of soybean aphids. The potential for soybean aphid populations to resurge in fields in which natural enemies have been killed is high. Multiple insecticide applications for control of soybean aphids make little economic sense.

We keep writing articles about soybean aphids in the Bulletin to keep people on their toes, not to fan the flames of concern. As soybeans develop toward reproductive stages, it's very important that people make themselves aware of the fields in which threatening densities of soybean aphids are being observed. Insecticide applications to prevent yield losses are recommended only during late vegetative and early reproductive stages of soybean growth.

Keep monitoring soybean aphid populations, and let's hope that natural enemies (which have not been prevalent in most fields to date) and hot, humid weather will contribute to the demise of the aphids. Also remember that, as densities of soybean aphids increase within a field, winged aphids are produced, and these aphids leave the field to reinfest other, less crowded fields. Consequently, noting the presence of alatoid nymphs (nymphs with "shoulder pads," indicating the eventual emergence of winged aphids) is crucial when making control decisions. If more than 50% of the aphids are alatoid nymphs, an insecticide application may not be necessary.--Kevin Steffey

Author: Kevin Steffey

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

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