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Issue No. 21, Article 6/August 12, 2005

Western Corn Rootworm Adults Abundant in Some Soybean Fields

This morning, on my way to campus, I made a quick dash with my sweep net into a soybean field in Champaign County. There was nothing unique about this field; it was just located in a convenient stopping place. After 25 to 30 sweeps, I opened thenet and was greeted by a great diversity of insects. I also was impressed with the numbers of western corn rootworm adults that emerged from the bottom of the net. Many of these adults were gravid females, no doubt making egg-laying forays to the soil in this soybean field.

Western corn rootworm adults collected with sweep net from soybean field, Champaign County, August 10, 2005.

Multiple species of insects collected with sweep net from soybean field, Champaign County, August 10, 2005.

Now would be an excellent time to look in your soybean fields to determine the densities of western corn rootworm adults. This is especially true for producers in more southern and western regions of the state in which the variant is now colonizing. In addition to western corn rootworm adults, you're likely to find Japanese beetles, green cloverworms, bean leaf beetles, and other insects that may be causing significant defoliation. For much of this summer, we've focused on soybean aphids and twospotted spider mites in soybean fields. We shouldn't forget that a complex of other soybean insects can still cause economic losses due to defoliation and pod feeding. Please refer to Kelly Cook's article in this issue of the Bulletin for more detailed discussion.

If you haven't deployed Pherocon AM traps (yellow sticky traps) in soybean fields to monitor western corn rootworm adults, don't delay. Although western corn rootworm adults have inhabited cornfields and soybean fields for many weeks already, you may wish to quantify these numbers as we progress through the peak egg-laying period of variant western corn rootworm adults. Based on our research from 1999 through 2001 in producers' soybean fields, 90% of the oviposition by variant western corn rootworm adults had been completed by August 15, 12, and 26, respectively.

In southern and western counties of Illinois, producers are encouraged to at least deploy a few (4) Pherocon AM traps in their soybean fields. When captures begin to equal at least one beetle per trap per day, this signifies that colonization by the variant is under way. Please note, this does not signify that economic levels have been reached. Typically, five beetles caught per trap per day are needed to potentially justify the use of a soil insecticide in rotated corn next spring.

Let us know what your trapping efforts reveal.--Mike Gray

Mike Gray

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