Crop Injury: Bean leaf beetle (Cerotoma trifurcata) is a pest of soybean that causes damage throughout the soybean plant’s life cycle. The adult is primarily a defoliating insect but feeding on pods can also directly cause a decrease in seed quality and yield. When feeding on pods, it leaves a transparent inner skin that allows light and pathogens to damage the seeds. Bean leaf beetles can harbor bean pod mottle virus (BPMV) and other pathogens and may transmit these pathogens to soybeans. Virus transmission is generally most damaging to soybeans when it occurs during soybean vegetative stages.
Pest Description: Adult beetles are ¼ inch long and can vary in color; they are generally orange, tan, gray, and red. All adults, regardless of their primary color, display a distinguishing black triangle located on the back between the head and thorax. Some insects of all colors also have four black spots in the middle of the forewings, while other insects do not.
Bean leaf beetle feeding on soybean foliar tissue. Image: Iowa State University Integrated Pest Management Program.
Bean leaf beetle injury to soybean pods. Image: A. Sisson.
Life Cycle: One (northern United States) to three generations (southern United States) of bean leaf beetle occur every year. Adults from the previous season overwinter in leaf litter and emerge early in the spring, but numbers may be reduced from a harsh winter. This overwintering generation feeds on perennial legumes such as alfalfa before moving to soybean fields to feed and lay eggs. Eggs are laid in the soil near soybean plants. Larvae feed on soybean nodules and roots but are not believed to cause economic damage.
Bean leaf beetle life cycle in the Midwest United States. Illustration: Iowa State Univeristy Integrated Pest Management Program.
Scouting: Scouting for bean leaf beetle occurs throughout the growing season as they can be an economic pest from seed emergence until R7. Use a visual count on very small soybeans or a sweep net or ground cloth sampling method on larger plants to estimate the level of infestation and use thresholds developed by your state extension service to inform management decisions.
Management: Later planting dates normally have reduced early-season bean leaf beetle pressure because bean leaf beetles move to early-planted fields. Insecticide seed treatments control early-season bean leaf beetle and may be warranted to suppress populations in fields with consistent BPMV or bean leaf beetle problems. Foliar insecticides should only be used if beetles are present above local threshold levels.
Developed and reviewed by Fred Musser, Mississippi State University; Erin Hodgson, Iowa State University; and the Iowa State University Integrated Pest Management Program.