No. 8 Article 4/May 18, 2007

More Information About Soybean Aphids

In last week's issue of the Bulletin (no. 7, May 11, 2007), Mike Gray shared some information about soybean aphids that we had received from David Voegtlin, research entomologist at the Illinois Natural History Survey. He and Bob O'Neil, research entomologist at Purdue University, had embarked on a trip through northern Indiana, northwestern Ohio, southern Michigan, and northern Illinois looking for soybean aphids on buckthorn. Bob recently elaborated on their findings:

"Aphid numbers in all sites except those in the Quad City area were relatively low. In the Quad Cities, large colonies were easily found with many plants having distorted leaves. Ants were observed tending colonies and predators (Harmonia) were present, particularly at one site. Comparing the number of aphids we saw this spring to the number of eggs found in fall 2006, there is clear evidence of a massive winter kill--probably associated with sub-freezing temperatures in early April (e.g., after the aphids had begun emerging late March, near bud-break). My guess is that the cold killed the plants' leaves and thus . . . the aphids. Surviving aphids were either in protected locations (e.g., S facing slopes), emerged from late hatching eggs, or lived on earlier maturing plants, that would have more leaf survival post-freeze. So relative to the fall populationa big drop-off.

"But relative to other years, the numbers were relatively 'high.' In 2006 [we] sampled the [northeast Indiana-Ohio-Michigan] route. We found 2 [soybean aphid] colonies, total. In 2005, we sampled the Quad City area and [northern Illinois], to the [western suburbs] of Chicago. [Soybean aphid] colonies were easily found, although ca. 50% of sites had no colonies."

It seems as if the threat of a widespread, intense soybean aphid outbreak in 2007 has been reduced, but the surviving aphids still suggest that economic infestations are possible, at least in pockets. If the weather conditions this summer are conducive to soybean aphid population growth (i.e., temperatures mostly in the low to mid-80s), pockets of economic infestations, especially if natural enemies are scarce, may blossom into more widespread infestations.

We will begin weekly sampling of 26 commercial soybean fields (from Woodford County to Stephenson County) in mid-June, and we are establishing soybean aphid research trials in both Whiteside and Champaign counties this spring. These activities are supported by funding provided by the Illinois Soybean Association. Through articles in the Bulletin, through other media, and through educational programs, we will keep you apprised of developments throughout the summer.--Kevin Steffey and Mike Gray

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