No. 12 Article 4/June 15, 2007

Soybean Aphid Numbers Small in Illinois, Unusually Large (This Early) Elsewhere

While reports of small numbers of soybean aphids in soybean fields in northern Illinois are starting to trickle in, events farther north and east are boiling over. Our first indication of this was a June 8 report from Tracey Baute, field crop entomologist with the Ontario Ministry of Agriculture, Food and Rural Affairs. She observed unusually large numbers of soybean aphids on relatively young soybean plants--as many as 316 on V2-stage soybean plants in small plots, with an average of 111 aphids per plant. In a nearby commercial soybean field, she found an average of more than 50 aphids per plant on stage V1 and V2 soybeans. She did not find many predators in the fields. Shortly after receiving this report, Chris DiFonzo, extension entomologist at Michigan State University, found some even larger numbers on V2- and V3-stage soybeans near Saginaw. On June 8, the density in the field was about 70 aphids per plant. On June 11, the density had increased to 255 aphids per plant, with more than 600 per plant along one edge of the field.

V2-stage soybean plant infested with soybean aphids--June 8, 2007, Ontario, Canada (photo courtesy of Tracey Baute, Ontario Ministry of Agriculture, Food and Rural Affairs).

V3-stage soybean plant infested with soybean aphids--June 8, 2007, Gera, Michigan (photo courtesy of Chris DiFonzo, Michigan State University).

These reports of such large numbers of soybean aphids on small soybean plants illustrate that we still have a lot to learn about this important pest. The large numbers this early in the season also raise some significant questions. The economic threshold for soybean aphids is 250 or more aphids per plant, but this threshold is associated with R-stage soybeans, possibly late V stages, not with early V stages. So what does this mean? We entomologists have written repeatedly that early-season insecticide sprays will kill early-season predators (e.g., Orius species, which are known to be present in soybean fields right now), and we have warned against early-season applications. However, we have not encountered the situation that is occurring right now. Although we do not have a handle on potential yield losses attributable to early-season infestations of soybean aphids, it is difficult to argue against an insecticide application when densities of soybean aphids already exceed the published threshold. This is particularly true considering the ability of soybean aphid populations to double in a matter of days. The shortage of soil moisture in many areas doesn't help much.

The situation in Illinois is a shadow of what is occurring to the northeast. Several people have found very small numbers of soybean aphids in soybean fields in northern Illinois, and predators are relatively prevalent at this time. In addition, our recent and projected temperatures have not been and will not be conducive for rapid development of soybean aphid populations. Remember, the developmental rate of soybean aphids declines quickly when temperatures reach the high 80s and low 90s.

Recommendation? Everyone associated with soybean production in northern Illinois should be examining soybean fields right now for soybean aphids, among other pests. We would appreciate receiving any reports of observations of soybean aphids, in either the presence or absence of predators, so we can get the word out. We initiated our weekly survey for soybean aphids in Bureau, Lee, Ogle, Stephenson, and Whiteside counties on June 11 and 12 and in Marshall, Putnam, and Woodford counties on June 13. Thus far, the densities of soybean aphids in surveyed fields are very low (0 in many fields). But their presence bears watching.--Kevin Steffey

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