University of Illinois

No. 9/May 23, 1997

It's Not Too Early to Watch for Bean Leaf Beetles

Bean leaf beetle adults that overwintered in protected areas are coming out of "hibernation" now, so early planted soybeans should be checked for the presence of these pests and symptoms of feeding injury. Some astute observers have seen bean leaf beetles in alfalfa, around houses, and even in corn already this spring, so their presence in soybeans is imminent.

Bean leaf beetles, color variations.

When bean leaf beetles become active in the spring, they move first to alfalfa and clover fields because soybeans usually are not available. Their feeding in alfalfa and clover is not economic. However, as soon as soybeans emerge, bean leaf beetles abandon the forage fields and colonize the soybeans. In fact, the earlier a soybean field is planted, the more susceptible it is to infestations of bean leaf beetles in the spring.

Bean leaf beetle injury to soybeans in the spring is most severe before emergence of the first trifoliate leaf. Feeding by bean leaf beetles on the hypocotyls (the portion of the stem below the cotyledons) can kill the plants and significantly reduce the stand. Feeding on the cotyledons before emergence of the first unifoliate leaf weakens the plants. If the cotyledons are destroyed before the unifoliates are fully unrolled, yield losses can be significant. Leaf feeding (typically small, rounded holes) is less damaging than plant cutting or feeding on the cotyledons.

Look for bean leaf beetles and symptoms of their feeding injury as soon as soybeans emerge from the soil. Rather recent research from Nebraska suggests that economic damage does not occur until the density of bean leaf beetles exceeds 16 per foot of row early in the seedling stage of development, and 39 per foot of row at stage V2+. Previously published thresholds were too low. Consequently, insecticide applications for control of bean leaf beetles attacking seedling soybeans rarely is economically justified. However, the price of soybeans may alter your opinion about economic thresholds. Use your best judgement, and try to make a wise decision.

More specific scouting guidelines and insecticides suggested for control of bean leaf beetles will be discussed in a future issue of this Bulletin.

Kevin Steffey, and Mike Gray, Extension Entomology, (217)333-6652