Bean leaf beetle adults are susceptible to cold weather and most will die when the air temperature falls below -10°C. However, they have adapted to winter by protecting themselves under plant debris and loose soil. 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 accumulating subfreezing temperatures. Predicted mortality rates in Iowa were extremely high during the 2013-2014 winter and ranged from 89-99 percent (Fig. 1). Not surprisingly, central and northern Iowa experienced colder temperatures, and most of the bean leaf beetle adults are not expected to survive.
The statewide-predicted mortality from the 2013-2014 winter is the highest since Marlin Rice started tracking these data in 1989. It is important to remember insulating snow cover and tillage residue can help protect bean leaf beetle from harsh air temperatures. Recent fluctuating temperatures can also negatively influence spring populations. The average mortality rate in central Iowa over the last 25 years is 72 percent, but >99 percent of adults were predicted to have been killed last winter around Ames (Fig. 2).
Although overwintering beetle populations are expected to be low this year, consider scouting soybean fields, especially in southern Iowa, if:
- Soybean is planted near alfalfa fields or if the field has the first-emerging plants in the area. Overwintering adults are strongly attracted to soybean and will move into fields with emerging plants (Fig. 3).
- Fields have food-grade soybean or are seed fields where reductions in yield and seed quality can be significant.
Fields have a history of bean pod mottle virus.
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.