No. 8 Article 5/May 18, 2007

Bean Leaf Beetle Activity Should Be Monitored

Excellent conditions over the past two to three weeks in most areas of Illinois allowed a lot of growers to finish planting corn and turn their attention to planting soybeans. The acres of soybeans planted are well ahead of the 5-year average for mid-May, and soybeans are emerging throughout the state. Emerging soybeans will be attractive to bean leaf beetles, which have been active for a while this spring after emerging from their hibernation quarters. Remember, the bean leaf beetles we observe this spring are the same ones we saw late last summer before they flew to their overwintering habitats. The adults of the first generation of bean leaf beetles will show up in the summer after the current batch of beetles lays its eggs and the insects develop through larval and pupal stages.

Overwintered bean leaf beetles usually show up first in the earliest planted soybean fields, where they feed on cotyledons, unifoliolate leaves, and the early trifoliolate leaves. However, with so many fields of soybeans available for their feeding pleasure, populations of bean leaf beetles may be diluted among the acres, resulting in reduced levels of feeding injury in lots of fields. And as all of us have learned, early-season soybeans are really tolerant of insect-feeding injury. The static thresholds for bean leaf beetles in the spring are 16 per foot of row in the early seedling stage or 39 per foot of row at stage V2+ (University of Nebraska). These thresholds are not applicable, of course, if the beetles happen to be transmitting bean pod mottle virus. However, Illinois soybean growers have not experienced much incidence of this disease over the past few years, so the threat seems to be relatively low. Nonetheless, careful observations are called for. A history of occurrence of bean pod mottle virus might warrant insecticide application when densities of bean leaf beetles are much lower than the aforementioned static thresholds.

Bean leaf beetle feeding on unifoliolate leaf of a soybean seedling (photo courtesy of Kevin Black, Growmark, Inc.

Several insecticides are suggested for control of bean leaf beetles feeding on soybeans in Illinois (see Table 1). All of these products are effective against bean leaf beetles under most circumstances. Please read and follow all label directions and precautions.--Kevin Steffey

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