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Issue No. 20, Article 2/August 6, 2004

The Slow Creep of Soybean Aphids

Although it never seems as exciting to write about insect problems that are not happening, the non-occurrence of an insect outbreak should be greeted with cheers, especially after the tough time we had with soybean aphids in 2003. Reports throughout the Midwest continue to confirm that densities of soybean aphids are for the most part far below threshold levels and will require no insecticide applications. However, we still have the rest of August to keep up the scrutiny.

This having been said, it is important to point out that many soybean fields are maturing rapidly past the growth stages of most concern. In its report for the week ending August 1, the Illinois Department of Agriculture indicated that "pods were setting on 64 percent of the soybeans, still well ahead of last year's 30 percent and 51 percent five-year average." As we have indicated previously, yield losses caused by soybean aphids are most likely when pest densities reach economic levels between reproductive stages R1 (beginning bloom) and R4 (full pod). For the most part, entomologists still are not certain whether soybean aphids cause much yield loss if infestations occur during the R5 (beginning seed) and R6 (full seed) stages, although all of us agree that the threat to yield during these stages is considerably reduced.

Drs. David Onstad (University of Illinois Department of Natural Resources and Environmental Sciences) and David Voegtlin (Illinois Natural History Survey) continued surveying soybean fields for soybean aphids in selected townships in Champaign, Kendall, Tazewell, and Woodford counties during the week of August 2. They found soybean aphids in every field they surveyed, but the densities were very low. For example, they found a range of 4 to 200 soybean aphids per 50 plants (0.08 to 4 aphids per plant) in fields in Champaign County. They observed 102 aphids on one plant in one of the fields in Champaign County, the largest number they have counted on one plant this summer. One of the fields they surveyed in Tazewell County was 100% infested, with 70 aphids per plant as the highest number they observed in that field. The soybeans were in stages R5 and R6.

"Hot spots" within fields also continue to show up. Matt Montgomery, Extension educator in Menard and Sangamon counties, "stumbled across" some 3-foot-diameter hot spots in a soybean field near Greenview. The densities of aphids in these hot spots were well above threshold. Matt also observed aphid mummies, evidence that parasitoids were at work in the field. The globular, copper-colored mummies are the remains of aphids whose insides have been consumed by a parasitoid larva. Often, a hole from which the adult parasitoid emerged can be observed.

Soybean aphids on soybean stem, Menard County, 2004. (Photo courtesy of Matt Montgomery)

Soybean aphid “mummies,” Menard County, 2004. (Photo courtesy of Matt Montgomery)

We applaud your vigilance with this aggravating pest this year. As we have said, we usually don't receive quite as much scouting information about any other pest of corn and soybean in Illinois, with the exception of corn rootworms, of course. As many of you have learned, continuous focus on the soybean aphid is building a foundation for understanding its population dynamics. After all is said and done for 2004, we will likely begin looking toward 2005. It will be interesting to learn what the suction trap network reveals this fall.--Kevin Steffey

Kevin Steffey

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