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Aphid alert

Agriculture.com Staff 07/27/2006 @ 8:24pm

Will there be soybean aphids this summer? "Yes," deadpanned Matt O'Neal, an Iowa State University (ISU) soybean entomologist at last winter's ISU Crop Management Conference. "Will there be economically damaging aphid thresholds?" he continued. "Maybe."

That's the dilemma regarding soybean aphids. Since they first infested U.S. soybean fields in 2000, soybean aphids have made it apparent they're here to stay. Yet, knowing when and how much they'll damage soybeans is murky.

So far, soybean aphids have followed a pattern of severe infestations in odd years and mild ones in even years. A watershed year for soybean aphids was 2003. "Yield losses in 2003 were astounding, and that's what got everyone's attention," says Dave Ragsdale, a University of Minnesota entomologist. That year, properly timed treatments boosted yields by 13 to 18 bushels an acre more than untreated fields, he adds.

Even in odd years, aphid infestations can be fickle. "We had a few around last year, but not many," says Chuck Myers, a Lyons, Nebraska, farmer, and a High Yield Team member. In some areas, aphids haven't returned with a vengeance since 2003.

"In 2003 they were bad around here," says Ron Heck, a Perry, Iowa, farmer and High Yield Team member. However, subsequent infestations have been lighter, and threshold levels were not reached in 2005.

The biannual pattern is rare among insects, says Christian Krupke, Purdue University Extension entomologist. "One hypothesis on this trend is that the aphid is so new to the Midwest that the pest is still coming to equilibrium, or finding its place," he says. "Its eventual equilibrium will most likely be somewhere between the peaks and valleys we're seeing in alternate years." Logic dictates that aphids should be sparse in 2006, given it's an even year. Still, you should be scouting your soybean fields now. One ominous indicator in Indiana was a lack of aphid predators, such as the Asian lady beetle, going into last winter. These voracious predators can devour up to 150 aphids daily. "In a big aphid year, we get a lot of Asian lady beetles, and a lot of them go into overwintering," says Krupke.

The following spring they are there to feed on aphids and lower the aphid numbers. Their cycle sort of follows the aphids' cycle, except that it is delayed by one year." With fewer natural predators, aphid populations could rapidly build, doubling in two days.

Just when you think aphids will devastate a field, though, hot and dry weather can halt their spread. In 2005, predicted high numbers of soybean aphids in Illinois did not materialize due to 90-degree-F.-plus temperatures. "Weather will always be a wild card," says Kevin Steffey, University of Illinois Extension entomologist. "We can't predict well, but we can prepare well. There's no substitute for scouting."

Will there be soybean aphids this summer? "Yes," deadpanned Matt O'Neal, an Iowa State University (ISU) soybean entomologist at last winter's ISU Crop Management Conference. "Will there be economically damaging aphid thresholds?" he continued. "Maybe."

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