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Issue No. 13, Article 3/June 20, 2008

First Soybean Aphids Reported in Illinois in 2008

Russ Higgins, University of Illinois Extension IPM educator in Matteson, found soybean aphids on volunteer soybean plants in a cornfield in Grundy County on June 13. He found nine wingless aphids on one V3-stage plant and three on another, and he observed some evidence of predation, although lady beetles were not present in the subject field. In an adjacent soybean field, Russ found no soybean aphids, but he observed lady beetles, probably feeding on other small insects (e.g., thrips) already present on the soybean plants.

Soybean aphids on V3-stage soybean plant, Grundy County, June 13, 2008. Note evidence of predation on aphid in upper left. (Photo courtesy of Russ Higgins, University of Illinois Extension)

Not surprisingly, this first report of soybean aphids in Illinois occurred a little later in 2008 than in previous years. Its first appearance during most years since its discovery in North America in 2000 has been during the last week of May and the first week of June. The earliest report of soybean aphids on soybean plants in Illinois occurred on May 27, 2005, in Ford County. The weather and cropping patterns in 2008 have been everything but typical, so it's not surprising that the first appearances and some behaviors of insects have been affected, too.

Over the past several years, we have relied on the numbers of winged soybean aphids captured in suction traps during the previous fall to forecast the potential for an outbreak. The numbers of aphids captured during the fall months of 2002, 2004, and 2006 were relatively large, and widespread economic infestations of soybean aphids occurred during the following years (2003, 2005, and 2007). For Illinois, you can view the captures of winged soybean aphids in suction traps on-line. You can see for yourself that the numbers captured during the fall months of 2007 were very low (mostly zeroes), suggesting that the potential for an outbreak in 2008 is very low.

It is important to note that there have been regionalized exceptions to the "every-other-year rule" for outbreaks of soybean aphids. For example, although soybean aphids got a particularly early start in Michigan in 2007, widespread outbreaks failed to materialize in that state. However, a widespread outbreak of soybean aphids occurred in parts of Illinois, Iowa, Minnesota, and Wisconsin during the same year. Environmental conditions at the time of colony establishment and the few weeks following establishment profoundly influence the growth and development of populations of soybean aphids. Although we anticipated an outbreak of soybean aphids in northern Illinois in 2005, the prevailing high temperatures that summer retarded soybean aphid population growth in most counties (with the exception of some "hot spots").

Since the arrival of the soybean aphid in North America, we have not had the type of weather and growing conditions we have experienced thus far in 2008, so we are in uncharted territory with respect to soybean aphids. Consequently, we strongly encourage people to begin looking for soybean aphids as soon as possible. The soybean aphids that are colonizing soybean fields right now originated from winged aphids that flew from their overwintering host, buckthorn. Because there are fewer soybean fields at this time in 2008 (73% planted, 58% emerged as on June 16) than usually are available to soybean aphids during most years at this time, infestations could become relatively large in scattered fields. The temperatures this past week have been ideal for soybean aphid development.

The optimal temperatures for soybean aphid development are between 77 and 86°F, with the most favorable estimated to be 82°F. At temperatures between 68 and 86°F, the pre-reproductive period takes 5 to 7 days before aphids start giving birth to nymphs. At 68°F, soybean aphid populations can double in less than 2 days; at 77°F, in 1.5 days. At 86°F, populations double in 2 days. A more detailed explanation about the relationship between temperatures and soybean aphid development is provided in the University of Minnesota "Soybean Aphid Growth Estimator: The SAGE Model."

Dr. David Ragsdale and his associates at the University of Minnesota have conducted extensive research related to the biology, ecology, and management of soybean aphids. Dr. Ragsdale has collaborated with staff of the Plant Management Network to develop two audio-synchronized PowerPoint presentations that provide excellent refreshers and some new information. "Soybean Aphid Biology in North America" and "Soybean Aphid Economic Threshold and Economic Injury Level" are available on-line. Both presentations are well worth review and offer solid preparation for the potential occurrence of soybean infestations in 2008.

Be aware that many small arthropods (insects and their relatives) inhabit soybean fields from very early in the season through August. As you begin soybean-scouting activities in earnest, make sure you have a good grasp of the sorts of arthropods you might encounter. A couple of good references are Identification of Soybean Aphid and Look-alike Species, published by the North Central IPM Center, and Identifying Natural Enemies in Field Crops, published by Michigan State University. With one or both publications in hand, you should have enough information and comparative photos to distinguish among soybean aphids, thrips, insidious flower bugs, spider mites, leafhoppers, and so on. You also will be able to determine whether early-season predators are having any impact on early-season soybean aphid populations.

As always, the bottom line is that you should be aware of the presence or absence of soybean aphids and manage accordingly. Virtually all research findings have indicated that early-season applications of insecticides for soybean aphids create more harm than good, so we advocate patience.

We will initiate regular surveys of soybean aphids in commercial soybean fields during the week of June 20. As we have done in the last two years, we will survey 10 fields in Stephenson County, 10 fields in Woodford County, and 1 field in each of the following counties--Bureau, Lee, Marshall, Ogle, Putnam, and Whiteside. This effort, part of a cooperative project with entomologists at Purdue University, is funded by the Illinois Soybean Association. We will make the information from this regular survey readily accessible on our Web site, so watch for future information.--Kevin Steffey

Kevin Steffey

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