Bean Leaf Beetle
Description: The adult is ¼ inch in length and varies in color from red to orange to tan and always has a black triangle at the base of the forewings. Wing covers usually have four rectangular spots but these spots may be two in number or absent.
Damage Symptoms: Early planted soybeans and especially the first fields to emerge in an area may exhibit seedling injury due to feeding by overwintering adult populations. Foliar injury due to adult feeding will again appear in late June and continue until fall as a succession of 1st and 2nd generation adults emerge and feed on the crop.
Feeding injury by the larval stage on the root nodules may be detected, but such injury is not regarded as economic.
When pod set occurs, adults will feed on both the foliage and the succulent pods – especially the pods in the upper canopy. As soybean foliage begins to turn dry and discolor in late August or early September, pod feeding may intensify as the pods become more succulent than the foliage.
Pod injury caused by bean leaf beetle enables infection of the developing bean seeds by micro-organisms leading to moldy beans that (1) may exhibit incomplete development, (2) stick to the pod and are lost during harvest, or (3) appear in the final harvest causing dockage at the time of sale.
Recognition of stages of pod injury is necessary to determine if pod feeding has just begun or peaked. Fresh pod injury initially appears green. After a couple days the scars appear white. Old scars on the pod appear dry and brown. The relative number of fresh vs. old scars provides an indication of whether pod feeding activity is currently an increasing problem.
As the level of pod injury increases, the probability for multiple feeding scars increases and the potential for infection also increases.
Scouting: Scout for adult beetles as soon as seedlings emerge and then during the pod-filling stages to determine whether an insecticide treatment is needed.
Management: Population activity tends to be heavier in areas of concentrated soybean production. Early planting of soybeans enables establishment and development of the 1st generation which in turn generates the 2nd generation. In contrast, late planted soybeans supports limited development of 1st generation since the stand may emerge after overwintering adults have concluded egg laying. Therefore, the relative proportion of soybeans planted early in an area may have a significant impact on the establishment and development of populations.
Sources:
The Ohio State University
Iowa State University Extension, Soybean Disease and Pest Management Field Guide. 2008: 42.
