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A bean leaf beetle sitting on a plant leaf Rachel Boone, Bugwood.org
EARLY THREAT: A bean leaf beetle feeds on a plant leaf. In some areas of South Dakota, a large number of these soybean-eating beetles may have survived the winter. The first generation of them will feed on soybean seedlings.

Bean leaf beetle poses early threat to soybeans

Large numbers of bean leaf beetles may have survived the winter in South Dakota.

Be prepared to scout for bean leaf beetle soon after soybeans emerge this year.

Adam Varenhorst, South Dakota State University Extension field crop entomologist, says that in some areas a large number of beetles may have survived the winter.

His warning is based on a mortality model that uses the accumulation of sub-freezing temperatures (14 degree F or colder) and the known response of overwintering bean leaf beetles in both exposed (under crop residue in a field) and protected (residue in wooded areas) environments to such temperatures.

In the winter of 2018-2019, nearly 100% mortality was predicted for bean leaf beetles in South Dakota. But this year, mortality is as low as 61% in some areas of the state. The lowest mortality rates are predicted from Chamberlain, S.D., east to the Iowa border, and south of Interstate 90. There were low mortality rates in some places in north central South Dakota, too.

Once soybeans begin emerging, the bean leaf beetles that are still alive will move into soybean fields and begin feeding on the seedlings, Varenhorst wrote in a recent SDSU Crop and Pest newsletter.

“This overwintering generation is capable of causing serious defoliation injury, especially when large populations successfully overwinter,” he wrote.

Been leaf beetles can be controlled with insecticide seed treatments or foliar insecticide applications. See the “2020 South Dakota Pest Management Guide — Soybeans” for a list of labeled products.

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