No. 12 Article 4/June 16, 2006

Soybean Aphids in Soybean Fields in Illinois

With reports of soybean aphids beginning to colonize soybean fields in the upper Midwest, it was only a matter of time before someone found soybean aphids in soybean fields in Illinois. Our first 2006 report came from Ryan Stoffregen, Advanced Crop Care, on June 8. Initially he reported very low numbers of soybean aphids in soybean fields in Boone and DeKalb counties. On June 12, Ryan reported finding soybean aphids on about 20% of the plants in a soybean field near the McHenry-Kane county line. There were three to four aphids per infested plant, and ants were climbing plants to harvest the aphids’ honeydew. A field in Boone County had about an 8% infestation, with three to four aphids per plant.

Beginning the week of June 12, we began weekly monitoring of 26 fields from Woodford County to Stephenson County. Our surveyors will monitor 10 fields at various distances from the suction trap near Metamora (Woodford County), 10 fields at various distances from the suction trap near Freeport (Stephenson County), and six fields between the two locations (in Marshall, Putnam, Bureau, Lee, Whiteside, and Ogle counties). The soybean plants were quite small (VC to V3) on June 12 and 13 during the first monitoring trip, but the surveyors already have observed soybean aphids. One soybean aphid was found on one V1-stage soybean plant in Woodford County on June 12, but aphids were slightly more common in the more northern fields. As many as three aphids were found on individual plants in Marshall, Putnam, and Bureau counties on June 12, and a high of eight aphids was found on one plant in a field in Stephenson County on June 13. At least one soybean aphid was found in four of the 10 fields sampled in Stephenson County.

I report these very low numbers only to give a baseline indication that soybean aphids are beginning to colonize soybean fields in northern Illinois, an indication that should intensify scouting efforts. With the predicted high temperatures, the development of soybean aphid populations will slow down, at the very least, and possibly stop if temperatures exceed 90°F. However, it’s far better to be aware of early-season infestations of soybean aphids than to be surprised by large colonies later in the summer. As I indicated in last week’s issue of the Bulletin (issue no. 11, June 9, 2006), insecticide applications at this time are neither warranted nor beneficial.

Our two surveyors observed insidious flower bug nymphs in one field in Woodford County, suggesting that these early-season predators could be waiting for soybean aphids when they arrive. (Well, actually they’re not "waiting"; they’re feeding on other insects already present in the field.) Research conducted by entomologists at Purdue University suggests that these predators may be responsible for suppressing soybean aphid populations early in the growing season, leaving the cleanup for multicolored Asian lady beetles later in the summer.

We will keep you apprised of our findings weekly throughout the summer, and we invite you to do the same with us. The more in-field observations of soybean aphids we have, the better we can assess the extent of a soybean aphid outbreak, or lack thereof.
--Kevin Steffey

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