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Issue No. 21, Article 3/August 13, 2004

Soybean Aphids Mostly No-Shows in 2004

For the first time in 2004, David Voegtlin (Illinois Natural History Survey) found soybean aphids in some of the suction traps established in Illinois. Recall that these suction traps were designed to sample the flying aphid populations in nine different locations in the state. You can review both the current year's data and past years' data on our IPM Web site.

During the week ending July 30, 2004, one soybean aphid was captured in suction traps at each of the following locations: Freeport, Joliet, Monmouth, and Orr. During the same week, four soybean aphids were captured in the suction trap near Eureka. These numbers are extremely low compared with the numbers being captured in the suction traps in 2003, reflecting the very low counts of soybean aphids that have been observed in soybean fields this year.

For a bit of "ground truthing" near Eureka, David Voegtlin and David Onstad (Department of Natural Resources and Environmental Sciences) have sampled soybean fields within a few miles of the suction trap. On August 2, 2004, they observed an average of three aphids per plant (averaged over 50 plants sampled in each of four fields). The soybeans in these fields were in the pod-filling stages (R5 and R6) of development.

During the first week of August, David Voegtlin also made a trip to southern Illinois to determine how far afield the soybean aphid was in 2004. He found very low numbers of soybean aphids in Fayette, Franklin, and Pope counties in soybean fields ranging in maturity from V6 to R4. He noted that many soybean fields in southern counties were in the vegetative stages of development and suggested that in some of these fields, the population densities of soybean aphids could approach the threshold (250 aphids per plant) during the first 3 weeks of August.

There's not much more we can add to the soybean aphid story for 2004, at leastfor now. Most soybean fields are at or beyond the developmental stages during which soybean aphids cause the greatest concern (R1 through R4). According to the Illinois Crop and Weather report (Illinois Agricultural Statistics Service) for the week ending August 8, 2004, 96% of the soybeans in Illinois had bloomed and 82% were setting pods. However, keep scouting soybean fields for soybean aphids until you are certain that the pests pose little to no threat. David Voegtlin will continue to examine the contents from the suction traps into October, and we will inform you if he detects a significant fall flight. Remember that in 2002, the fall flight of soybean aphids, as reflected by captures in the suction traps, foretold at least in part the outbreak that occurred in 2003. We still do not have enough data to rely on captures of soybean aphids in suction traps as a fool-proof prediction tool, but continued data-gathering may eventually shed more light on soybean aphid dynamics.--Kevin Steffey

Kevin Steffey

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