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Issue No. 16, Article 4/July 14, 2006

Soybean Aphids Not Causing Too Many Concerns in Illinois

Recent reports from Minnesota indicate that densities of soybean aphids are increasing in some areas of the state. Ian MacRae, entomologist at the University of Minnesota, reported that densities in the central Red River Valley increased rapidly during the first week of July and that fields were being treated with insecticides. Mark Abrahamson, Minnesota Department of Agriculture, reported in the July 7, 2006, issue of Minnesota Pest Report that soybean aphids had become more prevalent across central Minnesota. You can read the full report online (Adobe PDF).

In Illinois, although our weekly surveys reveal that soybean aphids are present, the numbers of aphids in any given field do not seem to be increasing much from week to week. For the sampling dates of June 26-27 and July 5-6, the surveyors found no aphids in most of the 26 fields sampled (10 fields in Woodford County, 10 fields in Stephenson County, and 6 fields in between the two counties). The highest average densities were 37.9 aphids per plant in one field in Woodford County and 3.7 aphids per plant in Stephenson County, sampled on June 26 and 27, respectively. More than 200 aphids were found on each of three plants in the field in Woodford County, and 74 aphids were found on one plant in Stephenson County. Since then, the average densities of aphids in those two fields have declined to fewer than 10 aphids per plant.

It is worth noting that the average densities of soybean aphids, although very low, increased noticeably from July 7 to July 11 in Stephenson County and in the six "transect" counties (Marshall, Putnam, Bureau, Lee, Whiteside, and Ogle). However, the largest density observed among those 16 fields was 2.95 aphids per plant (Stephenson County). All of these fields were in the late V stages or early R stages on July 10 and 11.

Our survey efforts will continue, and we encourage continued monitoring in all soybean fields, especially in northern counties. A return to hot weather will slow or stop aphid population growth, whereas an onset of cooler temperatures will encourage it. And remember, aphid populations can go from "0 to 60" very quickly. Thus far, the threat from soybean aphids has been nonexistent in Illinois.--Kevin Steffey

Kevin Steffey

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