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The Search for Soybean Aphids Intensifies

June 29, 2001
After learning that entomologists at the University of Minnesota and Michigan State University had found soybean aphids in soybeans in their respective states, we intensified our survey efforts to determine whether this invasive pest had begun to colonize soybean fields in Illinois. Survey teams coordinated by David Onstad, Department of Natural Resources and Environmental Sciences, and David Voegtlin, Illinois Natural History Survey, examined soybean fields in northern Illinois during the weeks of June 18 and June 25. And their efforts have borne fruit. One survey team found soybean aphids in soybean fields in seven counties on June 20: Cook, Grundy, Kane, Kendall, Lake, McHenry, and Will. The soybean plants were in the V2 to V4 stages of development. The numbers of aphids found within a field ranged from 1 to 20 per 30 plants, and some of the aphids were alates (winged forms). The surveyors used a survey technique that has been agreed on by entomologists throughout the Midwest--examining 30 plants, each for only 45 seconds, and recording the numbers of aphids found.

More recently, entomologists at the University of Illinois found soybean aphids in a small field on the South Farm on June 25 and 26. This finding in Champaign County represents the most southern occurrence of the aphids in Illinois thus far.

So what does our finding soybean aphids in soybean fields mean thus far? Finding soybean aphids right now tells us that they survived the winter successfully on buckthorn (Rhamnus species). Finding the aphids in soybean fields means that they are leaving their primary overwintering host (buckthorn) to colonize their secondary host (soybean). But that's about all we know at this point.

Because of the seemingly sudden appearance of soybean aphids in the Midwest last year, some individuals might be inclined to overreact to their appearance in soybeans again this year. However, at this point, it's way too early to try to do anything about the pests. After all, the numbers of aphids found have been very low. And remember, we are looking specifically for soybean aphids, not carrying out typical scouting efforts. Most people trying to look for soybean aphids right now won't find them. In addition, the surveyors were in numerous fields in which they were unable to find soybean aphids.

If you decide to begin looking for soybean aphids, please make certain to accurately identify any insects you find. While looking at soybean plants on the South Farm, we found many potato leafhopper nymphs. First-instar potato leafhoppers are about the same size as a soybean aphid, and because we were in the field early in the morning, the leafhoppers were not very active. My eyesight isn't what it used to be, so I was fooled initially. However, a good hand lens revealed that some of the critters were potato leafhopper nymphs, not soybean aphids. When we prodded the leafhoppers, they characteristically curled their abdomens over their backs (sort of like a scorpion) or moved rapidly sideways or backwards.

We are not certain whether soybean aphid populations will build to economic levels this year because so many extenuating circumstances exist. For example, we witnessed significant natural control of aphids last year by a host of natural enemies, not the least of which was the multicolored Asian lady beetle, Harmonia axyridis. There is every reason to believe that predators, parasitoids, and pathogens will have some impact on soybean aphid populations this year. We also are not certain what environmental conditions in the Midwest will favor a buildup of soybean aphid populations. If developments this year mirror what happened in 2000, we probably won't see threatening levels of soybean aphids until mid- to late July.

Our recommendation right now is to sit tight and keep current with what we and other entomologists in the Midwest are observing. There are a lot of entomologists planning to conduct research on the soybean aphid in 2001, and there's a lot of chatter among everyone involved. We will continue our surveys, focusing first in northern and central Illinois and then extending into southern counties. We will distribute our findings in articles in the Bulletin, in news releases, in radio interviews, and through several venues on the Web (for example, the Soybean Aphid Watch discussed in issue no. 13, June 22, of the Bulletin).--Kevin Steffey

Author: Kevin Steffey

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

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