No. 19 Article 1/July 29, 2005

All Soybean Producers in Northern and Central Illinois Should Be Alert for Soybean Aphids

Our research efforts associated with corn rootworms forced us to delay our weekly survey for soybean aphids, but we should have updated information by the end of this week (July 28-29). In the meantime, the reports we have received from northern Illinois indicate that numbers of soybean aphids continue to increase past the economic threshold of 250 aphids per plant. In a soybean field in Whiteside County in which we have established some soybean insect research trials, numbers of soybean aphids have exploded recently. Our summer crew found as many as 1,400 aphids on one plant, with average numbers in the plots well above the threshold level.

The suction traps that are operating in Illinois reveal that winged aphids are being captured in the traps in northern Illinois. During the week ending July 15, the following numbers of aphids had been captured in four northern locations: 4 in Monmouth, 9 in DeKalb, 64 in Freeport, and 2 in Joliet. The numbers of soybean aphids captured in traps in DeKalb and Freeport in 2005 are lower than the numbers captured at these locations at about this same time of year in 2003. However, captures of noticeable numbers of soybean aphids in these traps indicates winged aphids are leaving heavily infested fields to establish colonies in other soybean fields.

In an effort directed by Dr. David Voegtlin, entomologist at the Illinois Natural History Survey, suction traps were erected at several sites in Indiana, Michigan, Minnesota, and Wisconsin earlier this year. The traps have been operating this summer, and the data gathered will soon be available on a regional Web site (still under construction). However, the numbers of soybean aphids captured in three suction traps in Michigan have been very high (853, 582, and 302), and numbers captured in Indiana, Minnesota, and Wisconsin also have been noticeable (e.g., 42 in one trap in Indiana, 97 in one trap in Minnesota, and 316 in one trap in Wisconsin).

Spraying insecticides to control soybean aphids is ongoing in Michigan, Ohio, and Wisconsin, as well as in scattered areas in northern Illinois and northern Indiana. If population densities continue to increase and winged aphids continue to colonize additional soybean fields, we can expect an outbreak that may be similar to what occurred in 2003. However, as we have stated many times over, several factors can suppress soybean aphid populations, so scouting and use of established thresholds should still rule the day.

Producers in central Illinois should not be caught unaware. At about this same time of the growing season in 2003, soybean aphid populations began to increase in size in central counties. The first two weeks of August will be an important period for assessing the potential impact of soybean aphids. Yield losses attributable to soybean aphids have been documented for R1- through R5-stage soybeans.

I recently read an article about insecticides and preharvest intervals written by Kevin Black, entomologist with Growmark, for his newsletter to FS member companies. His article reminded me that precautions are in order regarding application of insecticides to soybeans late in the season. The preharvest intervals for insecticides that might be used to control soybean aphids and/or twospotted spider mites are listed in Table 1. If an insecticide application to soybeans is necessary, please understand this information, and other precautions, before deciding which insecticide to apply. Spraying some products too close to harvest time could result in illegal residues of the active ingredients.--Kevin Steffey

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