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Delayed Soybean Planting: Prospects For Insect Injury

The stormy spring weather across much of the nation’s mid-section that continues to cause planting delays will also affect this season’s insect activity, said Mike Gray, a professor of entomology at the University of Illinois.

On May 28, the National Agricultural Statistics Service reported that planting estimates indicate approximately 40 percent of Illinois soybean acres have been planted, and roughly 12 percent of the soybean crop has emerged across the state. Gray said these percentages are well below the five-year averages for Illinois by this date—53 percent planted and 28 percent emerged.

As overwintering bean leaf beetles break dormancy and begin to seek out soybean fields, those fields that are first to emerge will be most susceptible to early season feeding, Gray explained.  “Overwintering adults typically become active in April and initially may spend most of their time feeding within alfalfa or clover. As soybean plants become available in May and June, they become a preferred host. Fields most at risk this spring would include those that were planted first within an area and are now serving as a very attractive trap crop. These fields should be scouted for signs of defoliation,” he said.

Fortunately, a rescue treatment for seedling soybeans is most often not justified because densities of 16 beetles per foot of row (early seedling stage) or 39 adults per foot of row at the V2+ stage of development are necessary for economic injury, Gray added.

Continuing delays in soybean planting could dim the prospects for soybean aphid establishment this season. According to Gray, infestations of this insect pest have become less predictable and more sporadic the last several years in many areas of the Midwest.

Soybean aphids first detected in North America (Wisconsin) in July 2000, quickly spread to 10 North Central states by September of that same year. At the conclusion of the 2003 summer, they could be found in 21 states and three Canadian provinces. Gray explained that entomologists have learned a great deal about this aphid species during the past 13 years and have developed some very sound economic thresholds that can be used in the effective management of this insect pest.

“Currently, alate (winged) viviparous (give birth to living young, nymphs) females are flying from their primary and overwintering host (common buckthorn) to their secondary host (soybean plants). These spring migrants may have more challenges this year locating soybean fields that are ready to receive them,” he said.

Gray added that it is too early to offer any kind of firm predictions for soybean aphids this year. More moderate summer temperatures would benefit soybean aphids. “For now, it appears they may have some establishment hurdles to clear this spring,” he said.

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