2005 Year in Review

From the Fields – Bean Leaf Beetles

As of May 22, 42% of the soybeans has emerged in the Northwest Crop Reporting District (12 northwestern counties) of the Illinois Agricultural Statistics Service. Bean leaf beetles are typically the first insect pests and the earliest emerging soybean fields commonly attract the largest populations of this insect.

The beetle, about ¼-inch long, can be green, yellow, tan, or red and always has a black triangle behind the head on the wing covers. The beetles that are observed now are the same ones seen last fall, those that ended the second generation. In other words, they over winter as adults.

Early feeding injury includes damage to the cotyledons and shot holes between leaf veins in the unifoliate and trifoliate leaves. When growing conditions are favorable for soybean, defoliation injury usually does not result in economic losses later in the season.

Treatment threshold for damage from leaf feeding at the early seedling stage is 16 beetles per foot of row and 39 per foot of row at V2 stage or later. Economic damage to soybean seedlings requires a large number of beetles and rarely is treatment justified for leaf feeding at this stage. However, if the injury exceeds the above-mentioned threshold and if the crop is growing slowly, an insecticide application may be warranted.

However, if the beetles transmit bean pod mottle virus (BPMV) to the seedlings, few beetles are needed to spread the virus from plant to plant. Management guidelines for the beetle and the virus have not been established because the timing of the transmission of the virus is still unknown. Insecticide application to control early season bean leaf beetles to manage BPMV is generally not warranted in Illinois.

The good news is that based on the low densities of bean leaf beetles that occurred late last summer, the potential for large populations this spring (the same beetles that over wintered from last fall) is low.

Once soybean are past the early vegetative stages, scouting for this insect should resume when beetles (first generation) are feeding on the leaves in mid-summer. A second generation emerges in September and feeds on both leaves and pods.

Further information is available at Extension offices and from agriculture dealers.

by  Editor, theCity1.com
June 6, 2005