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Issue No. 11, Article 2/June 8, 2007

Soybean Aphids Make Early Appearances in Soybean Fields in the Midwest

In last week's issue of the Bulletin (No. 10, June 1, 2007), we noted the first report of soybean aphids in soybean fields in the Midwest in 2007 in east-central Minnesota. In the few days thereafter, we received several reports of entomologists and consultants finding soybean aphids in soybean fields throughout the Midwest. The first report of soybean aphids in a soybean field in Illinois came in from Ryan Stoffregen of Advanced Crop Care in Kingston, Illinois. He reported that one of his agronomists found a soybean aphid on May 30 on the underside of a leaf in an early-planted soybean field near Kirkland (DeKalb County). Our crew will examine some soybean fields for soybean aphids in several north-central and northwestern counties this week, and we'll let you know what we find. We also have received reports of soybean aphids in soybean fields in Indiana, Michigan, Ohio, and Wisconsin as well as in Ontario and Québec, Canada.

Eileen Cullen, Extension entomologist at the University of Wisconsin, wrote a particularly good article about soybean aphids in Wisconsin in the June 5 issue of Wisconsin Crop Manager. Some of the points she makes are worth repeating here. You can read about other findings of soybean aphids in the following newsletter articles:

To be honest, these reports of soybean aphids appearing in soybean fields are not unusually early. I glanced through previous issues of the Bulletin and learned that the earliest report of soybean aphids in soybean fields in Illinois occurred on May 27, 2005, in Ford County. Reports of soybean aphids in soybean fields throughout the Midwest during the last week of May and first week of June have been rather commonplace since the pest's discovery in August 2000. The soybean aphids that are colonizing soybean fields "as we speak" originated from winged aphids that flew from their overwintering host, buckthorn. In some instances, winged aphids have been in soybean fields, probably shortly after they arrived.

In several previous issues of the Bulletin, we have speculated about the potential for a soybean aphid outbreak in 2007. That potential was moderated a bit by the freezing temperatures in early April, but many other factors will determine whether soybean aphids become a widespread and economic problem this summer. Probably the two most important factors that affect development of soybean aphid populations are temperatures and natural enemies:

  • Temperatures in Illinois in 2007 may have the biggest impact on the potential for buildup of large colonies of soybean aphids. As many will remember, we anticipated a widespread outbreak of soybean aphids in Illinois in 2005, but temperatures were too hot for the aphids, which typically do not develop well when temperatures approach or exceed 90°F. For a good review of the effect of temperatures on population development of soybean aphids, visit the University of Minnesota "Soybean Aphid Growth Estimator: The Sage Model".
  • Natural enemies always play a role in regulating soybean aphid populations. Early-season predators such as Orius insidiosus, the insidious flower bug, often are present in soybeans as soybean aphids arrive, and they can suppress early-season buildup of populations. Later in the year, the familiar multicolored Asian lady beetle will begin to show up, possibly in large numbers, depending on the extent of soybean aphid populations.

Orius nymph feeding on soybean aphid (photo courtesy of John Obermeyer, Purdue University).

Mulitcolored Asian lady beetle adult feeding on soybean aphids (photo courtesy of Marlin Rice, Iowa State University).

It's probably worth pointing out that some people have observed relatively large numbers of lady beetle larvae feeding on aphids in wheat. We frequently focus our attention on the relationship between multicolored Asian lady beetles and soybean aphids and fail to mention that lady beetles are generalist predators that will feed on any smaller insect they can get their mandibles on. So the occurrence of multicolored Asian lady beetles in wheat now could bode well if we have a soybean aphid outbreak later. Multicolored Asian lady beetles overwinter as adults, and in the spring they lay eggs near food (e.g., wheat fields infested with aphids). The larvae that hatch are voracious predators. Assuming that survival of these lady beetles in wheat is good, the aphids in wheat should sustain the lady beetles until adults develop and seek out other food sources (e.g., aphids in soybeans).

Multicolored Asian lady beetle larva (photo courtesy of Jeffrey Hahn, University of Minnesota Extension).

So it's time to start looking for soybean aphids in soybeans throughout northern Illinois and keeping track of their densities. It's far too early to consider control measures; insecticides are warranted only when densities reach 250 aphids per plant on 80% of the plants. As we have stated numerous times, spraying insecticides too early in the season likely will exacerbate the issue by killing early-season predators and allowing later infestations of soybean aphids to grow unchecked.--Kevin Steffey

Kevin Steffey

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