Figure 2. Overwintering bean leaf beetles can defoliate young soybean plants and vector bean pod mottle virus.
Figure 2. Overwintering bean leaf beetles can defoliate young soybean plants and vector bean pod mottle virus.

Most Iowa winters are harsh on overwintering bean leaf beetles. Typical statewide overwintering mortality ranges from 60-99 percent (see a 20-year historical record of predicted bean leaf beetle mortality in central Iowa.) A combination of cold winter temperatures and the high adoption of insecticidal seed treatments have drastically curbed bean leaf beetle populations throughout much of the state.

Bean leaf beetle adults are susceptible to cold weather and will die when the temperature is below -10 degrees C, but they have adapted to winter by protecting themselves in leaf litter and insulating snow cover. An overwintering survival model was developed by Lam and Pedigo from Iowa State University in 2000, and is helpful for predicting winter mortality based on accumulated subfreezing temperatures. Figure 1 is a map of predicted mortality in Iowa for the 2011-2012 winter.

Mild winter for bean leaf beetleIn general, Iowa experienced a warm winter, with predicted mortality ranging from 30-53 percent. The predicted mortality estimates are the lowest since the overwintering model was developed. Many areas in the state had less than normal accumulated snow cover, which could increase actual adult mortality. A recent ICM News article discusses the implications for warm winters, lack of snow cover and insect survival. 

Overwintering adults are strongly attracted to soybean and will move into fields with newly emerging plants (Fig. 2). Early-planted fields should be monitored closely this year, given the predicted likelihood of adult overwintering survival. Other fields of concern include food-grade soybean and seed fields where reductions in yield and seed quality can be significant. Information about managing bean leaf beetle and bean pod mottle virus is available on the Iowa State entomology website. 

Bean leaf beetle is easily disturbed and will drop from plants and seek shelter in soil cracks or under debris. Sampling early in the season requires you to be sneaky to estimate actual densities. Although overwintering beetles rarely cause economic damage, their presence may be an indicator of building first and second generations later in the season.