By Erin Hodgson and Nick Schmidt, Department of Entomology
Bean leaf beetles have adapted to Iowa's winters by protecting themselves in leaf litter, but they are still susceptible to cold temperatures. Harsh winters can cause significant mortality. In general, bean leaf beetle adults will die if temperatures fall below -10C. An overwintering 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. Figure 1 is a map of predicted mortality in Iowa from the 2009-2010 winter. In general, Iowa experienced very low temperatures, with predicted mortality ranging from 82-99 percent. These numbers are similar to the 2008-2009 winter.
Figure 1. Predicted overwintering mortality of bean leaf beetle based on accumulating subfreezing temperatures from October 2009 — February 2010.
As with all insects, growth and development is highly regulated by temperature. In other words, warmer temperatures will shorten the time it takes to become adults. The overwintering adults become active in late April or May, and begin looking for food. Often soybean isn't emerged at this time, so the adults may be feeding on alfalfa or other wild legumes. As soybean emerges, the adults will move into fields. The first generation becomes active in July and the second generation emerges in late August or early September. Usually the second generation is much more abundant and has the potential to cause economic damage.
Overwintering adults are strongly attracted to soybean and will move into newly emerging fields. 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.
Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. Nick Schmidt is a graduate student in the Department of Entomology.