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Soybean Aphids Build Impressive Fall Numbers

In late September, reports of "swarms" of aphids became common across much of central and southern Illinois. At the University of Illinois, it was common to see students walking to class swatting at clouds of "gnats," as they referred to them.

University of Illinois Mike Gray says treatment decisions for producers were made difficult, as many soybean fields were in late reproductive stages of development.

As temperatures decline and day length shortens, Gray says winged soybean aphid females are abandoning maturing soybean fields and flying to buckthorn. That's where they feed and begin producing nymphs that develop into oviparae.

Late in the growing season, winged soybean aphid males also are produced on soybean plants. The males leave soybean fields and attempt to find buckthorn plants and begin mating with the oviparae. The oviparae lay eggs that overwinter on buckthorn.

This annual fall dispersal of soybean aphids to their primary host has been described as a biological bottleneck," Gray says. "This so-called bottleneck could be readily observed across many areas of central and southern Illinois in late September. Aphid densities on buckthorn leaves were as high as many observers had ever witnessed."

Accurately predicting soybean aphid infestations for the upcoming growing season has proven to be challenging, Gray says.

"Certainly the stage has been set for abundant egg-laying on buckthorn plants this fall. Next spring, producers would be well advised to scout their soybean fields for aphids," he says. "If overwintering survival is good, natural enemy densities are low and the growing season is relatively mild, we could see significant management issues develop with this pest in 2010."

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