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Issue No. 18, Article 3/July 25, 2008

Soybean Aphids Are Slowly Making Their Presence Known in Some Soybean Fields

Although the numbers of soybean aphids in the vast majority of soybean fields in Illinois are very low (in fact, aphids have been absent in many fields, too), the numbers on individual plants are beginning to increase in some fields. We initiated our regular sampling of 24 commercial soybean fields during the week of June 23 (the survey is supported by the Illinois Soybean Association), and the average numbers of soybean aphids per plant were mostly zeros during the first four weeks of the survey. But numbers have begun to creep up in some fields. For example, on July 22, our surveyor recorded an average of 75.5 aphids per R2-stage plant in one field in Stephenson County, an increase from 6.15 aphids per plant on July 15. Of the 20-plant sample in this field, one plant supported 298 aphids, and a nearby plant supported 402. Four other plants had triple-digit numbers of aphids, but six plants had 10 or fewer, or none. Results from these surveys should be accessible on our Web site by next week.

An infestation of soybean aphids obviously begins when individual winged females alight on a soybean plant and give birth to living young. For a time, if environmental conditions are favorable, the numbers of aphids on individual plants increase rather rapidly, and eventually more plants become infested. So finding a significant number of aphids on a few plants in the field should qualify that field for more intense scrutiny. The increase in numbers of soybean aphids in soybean fields this year is occurring later than usual. (But then, what hasn't occurred later than usual this year?)

It is likely that we will have to stay vigilant for soybean aphids well into August. I recall from our survey in 2006 that numbers of soybean aphids increased significantly from late August to mid-September in a few of our fields. The increase in numbers during that year, however, occurred when soybean fields were well beyond being affected by soybean aphids. We may not be able to count on that this year. We will keep you apprised of the development of soybean aphid populations in Illinois and elsewhere in the Midwest.--Kevin Steffey

Kevin Steffey

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