In March, it was predicted that bean leaf beetles would have low overwintering mortality based on our mild winter. You may have noticed adults became active in alfalfa starting in April. They are strongly attracted to soybean and will slowly move as plants emerge this month. The adults fly short distances (less than 167 feet on average) and infestations can be highly aggregated. Some research plots around southern and central Iowa have decent numbers feeding on unifoliates.
Bean leaf beetle is easily disturbed and will drop from plants to seek shelter in soil cracks or under debris. Sampling early in the season requires you to be sneaky to estimate actual densities. In some cases, you may just see leaf defoliation and not the beetles (Photo 1). Although overwintering beetles rarely cause economic damage, their presence may be an indicator of building first and second generations later in the season.
Maybe more important than defoliation, bean leaf beetle can vector bean pod mottle virus in soybean. There can be a reduction in yield in bean pod mottle virus-infected plants resulting from reduced seed size and pod set. This effect on soybean yield is most severe when soybeans are infected as seedlings. Bean pod mottle virus can cause a bleeding hilum or seed coat discoloration, and so food grade beans are at a higher risk for grading penalties (Photo 2). Fields with a history of persistent infestations and bean pod mottle virus should consider an insecticidal seed treatment.
Overwintering beetles should be suppressed by the high adoption rate of insecticidal seed treatments in Iowa. But if first and second generation beetles become an issue, updated treatment thresholds for higher market values will be provided. For more information about bean leaf beetle biology, visit this ISU Soybean Insects Guide website.