No. 13 Article 4/June 17, 2005

Keeping Eyes on Soybean Aphids Throughout the Midwest

Reports of slowly but steadily climbing numbers of soybean aphids in soybean fields throughout the Midwest and beyond are the order of the day. You can add New York; Ohio; and Ontario, Canada, to the list of states (and provinces) in which soybean aphids have been observed in soybeans in 2005. The common denominator in a lot of the reports was that 20 to 30 soybean aphids had been found on isolated plants in most fields, a clear indication of colonies developing. Average numbers of aphids per plant were usually in single digits or the teens.

In Illinois, we continued our surveillance of soybean aphids, this time in northeastern Illinois, on June 13 and 14. Soybean aphids were found in almost all soybean fields visited in Boone, DeKalb, Ford, Grundy, Kane, and McHenry counties. In general, approximately 33% of the plants sampled were "infested" (at least one soybean aphid), although a couple of fields were 60-67% infested. The average density of aphids per plant was in the teens. Similar to other reports from around the Midwest, pockets of average densities of 20 to 30 aphids per plant were noted. The surveyors found between 175 and 200 aphids on one plant in McHenry County.

Following are some reports from other people scouting for soybean aphids in Illinois:

Eileen Cullen, extension entomologist at the University of Wisconsin, reported (June 14) the following information for the soybean aphid situation in Wisconsin:

Please keep in mind that I am reporting extremes, in some instances. Although aphids are being found regularly in many soybean fields, the densities still are relatively small. Nevertheless, there is ample evidence of populations increasing.

One of the primary concerns last week was the appearance of colonies of soybean aphids on small soybean plants, especially on newly unfolding trifoliolates (where most of our guys found the aphids this week). The collective wisdom among entomologists in the Midwest is that application of insecticides to control soybean aphids on young soybean plants will not pay for itself. According to Dr. David Ragsdale, research entomologist at the University of Minnesota, a density of 50 to 100 soybean aphids per plant can be held in check by natural enemies. Although few natural enemies have been found in soybean fields thus far this year, several predators will be attracted to soybeans as soybean aphid colonies build. As David indicated, applying insecticides for control of soybean aphids in early-stage soybeans will eliminate natural enemies, enabling soybean aphids to re-infest the field in "an enemy-free zone."

Recent storms undoubtedly helped the soybeans, but they probably had little overall effect on soybean aphids. Consequently, their numbers will continue to increase over the next couple of weeks, with more widespread infestations likely (i.e., higher percentages of infested plants per field). As infestations in individual fields develop, numbers could become critical relatively soon. The widely accepted economic threshold of 250 aphids per plant during stages R1 through R5 is still our yardstick. However, we may have to adjust if the lack of moisture continues.--Kevin Steffey

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