The bean leaf beetle (Cerotoma trifurcata) is a common soybean pest that can damage soybean by feeding on cotyledons, leaves and pods. Populations of bean leaf beetles have been increasing in the North Central region. Back-to-back mild winters have favored the survival of overwintering adults.
Colors and spot patterns of bean leaf beetles can range considerably. Therefore, careful attention is needed to distinguish these beetles from other important beetles found in soybean, such as the lady beetle.
Read more on beetle identification (link to ISU newsletter)»
Adult beetles overwinter under leaf debris near soybean fields. Once they become active in the spring they will feed on wild legumes, alfalfa and clover. Once soybeans begin to emerge, the beetles quickly leave these alternate hosts and concentrate on soybean.
The overwintered bean leaf beetle population will give rise to another round of first generation adults who in turn lay their eggs in the soil. These larvae give rise to second generation adults that can be found in soybean from late-August to mid-September where they feed on leaves and pods. This describes a two generation per year bean leaf beetle, and this is probably what we have in southern Wisconsin.
In addition to the physical damage from feeding on soybean, the bean leaf beetle is also a vector of bean pod mottle virus (BPMV). The insect feeds on infected plants and transmits the virus particles to the next plant on which it feeds. Virus transmission can occur at any growth stage, however early infection poses the greatest risk of yield loss.
Early soybean planting often coincides with high populations of overwintered bean leaf beetle adults moving into soybeans to feed and lay eggs. This increases the chance of BPMV transmission to soybean. Concentrate initial scouting activities in early-planted fields.
Insecticides are available for control. Treatment thresholds, for defoliation only, varies according to crop value and treatment cost. For VC to V2 soybeans the threshold varies between 2 to 10 beetles per plant. Synthetic pyrethroid insecticides tend to provide the most consistent bean leaf beetle control.
Treatment thresholds for prevention of bean pod mottle virus are not yet available. However, consider these criteria before treating:
- Were yields last year lower than expected and unexplainable? Were virus symptoms (leaf mottling, discolored seed, green stem) present ? If so, then BPMV is likely a concern for the coming season.
- Soybean varieties differ in susceptibility to BPMV. If a variety was planted last year which had significant bean leaf beetle feeding yet few, if any, virus symptoms, that same variety may be considered (somewhat) tolerant to BPMV.
- To prevent transmission of BPMV, an insecticide application must be made in the very early stages (V-V2) of soybean development.
- A second application may be necessary during the emergence of first generation beetles (late June or early July).
Questions? Please contact Eileen Cullen, UW Extension entomologist at firstname.lastname@example.org