Bean leaf beetles may have suffered high mortality rates this winter throughout much of the Midwest, particularly in the western Corn Belt.
While bean leaf beetles have adapted to winter by protecting themselves in leaf litter, they are still susceptible to cold temperatures. Harsh winters can cause significant mortality, say Iowa State entomologists Erin Hodgson and Nick Schmidt.
In general, they say bean leaf beetle adults will die if temperatures fall below 14 F. For example, they expect that throughout most of Iowa, 99% of bean leaf populations have died, with the exception of the southeastern part of the state where 82% or more have perished. These numbers are similar to the 2008-09 winter.
"As with all insects, growth and development is highly regulated by temperature," Hodgson says. "In other words, warmer temperatures will shorten the time it takes to become adults."
Overwintering adults will 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 soybeans emerge, 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 soybeans 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.
Although overwintering beetles rarely cause economic damage, their presence may be an indicator of building first and second generations later in the season.