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Issue No. 21, Article 2/August 18, 2006

Soybean Aphid Densities Have Increased Noticeably in Some Fields in Illinois

Results from our weekly survey of 26 fields from Woodford County (10 fields), Stephenson County (10 fields), and six counties in between (one field in each county) revealed significant increases in densities of soybean aphids in some fields (Table 3). The increase in densities of soybean aphids was most notable in one field in Woodford County (Woodford 3, from 11.75 to 64.9 aphids per plant) and three fields in Stephenson County (Stephenson 1, from 26.95 to 82.25; Stephenson 5, from 102.5 to 170.55; and Stephenson 7, from 44.75 to 191.95). It is likely that the economic threshold (250 aphids per plant, 80% of the plants infested) will be exceeded in at least two of these fields. Interestingly, however, the numbers of soybean aphids in most of the fields sampled on August 10 or 11 declined or stayed about the same as the numbers found on August 1 or 2. So the impact of natural enemies and weather conditions should be assessed before making a decision to control soybean aphids with insecticides. We strongly encourage sampling fields every two to three days if average numbers of aphids are still below the economic threshold.

We also have obtained some data from an insecticide efficacy trial and a resistant variety trial, both in Whiteside County, that demonstrate the variability in numbers of soybean aphids from one area of a field to another, and even from one plant to another. In the variety trial, three plants were sampled in each plot. Although we cannot publish the results at this time (we have not completed our evaluations), following are the ranges of numbers of aphids per plant found within some of the plots: 8 to 818, 2 to 229, 7 to 1,362, 10 to 288, 2 to 1,403. You get the picture. Pretreatment counts in our insecticide efficacy trial reveal the same pattern. Five plants were sampled in each of the four reps, and the numbers of soybean aphids per plant on August 15 were 4 to 414 in Rep 1, 10 to 51 in Rep 2, 1 to 248 in Rep 3, and 10 to 258 in Rep 4.

I share this information to drive home the point that sampling in several areas of a soybean field is crucial for making decisions regarding control of soybean aphids. As our numbers verify, colonies of soybean aphids build up on individual plants rather dramatically in some instances. Until winged aphids are produced, the infestation may remain relatively confined within a field. However, as densities of soybean aphids increase, winged aphids become more common, and winged aphids fly from one area of the field to another or from one field to another. In fact, David Voegtlin, entomologist with the Illinois Natural History Survey, reported that the numbers of winged soybean aphids captured in the suction trap at the Northern Illinois Agronomy Research Center near DeKalb increased noticeably from the week ending July 28 to the week ending August 4.

Will soybean aphids threaten soybean yields in some fields in northern Illinois this year? It is possible, depending on the stage of development of the soybean plants in the fields. Remember that after soybean plants reach stage R6, it is questionable whether yield benefits would be realized from an insecticide application. However, we will have an opportunity this year to obtain yield data from soybeans treated at stage R6, and we'll let you know what we find. In the meantime, keep checking soybean fields for soybean aphids, and hope that predators and the weather keep them at noneconomic levels.--Kevin Steffey

Author:
Kevin Steffey

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