Mild Winter Favors Bean Leaf Beetle Survival

By Erin Hodgson and Ashley Dean

Bean leaf beetle adults (Photo 1) are susceptible to cold weather and most die when air temperatures fall below 14°F (-10°C). However, they have adapted to winter by protecting themselves under plant debris and loose soil. Each spring, adult beetles emerge from their overwintering habitat and migrate to available hosts, such as alfalfa, tick trefoil, and various clovers. As the season progresses, bean leaf beetles move to soybean and other hosts. While adult activity can begin before soybean emergence, peak abundance often coincides with early-vegetative soybean.

An overwintering survival model developed by Lam and Pedigo from Iowa State University in 2000 is helpful for predicting winter mortality based on accumulated subfreezing temperatures. Predicted mortality rates in Iowa are variable for the 2019-2020 winter, ranging from range from 42-81% (Figure 1). Mortality was highest in northern Iowa (69-81%); the average mortality rate across Iowa is 59% for the 2019-2020 winter. Field agronomist Rebecca Vittetoe noted active adult bean leaf beetles in Washington county this week!

Marlin Rice started reporting bean leaf beetle mortality predictions in 1989. Last winter, the predicted mortality of bean leaf beetles in central Iowa was 61%, which is about ten percent lower than the 30-year average of 71% (Figure 2). It is important to remember insulating snow cover and crop residue can protect bean leaf beetles from harsh air temperatures. However, fluctuating temperatures can reduce spring populations.

Overwintering beetle populations are expected to be high this year. Consider scouting soybean fields, especially in southern Iowa, if:

1. Soybean is planted near alfalfa fields or if the field has the first-emerging soybean plants in the area. Overwintering adults are strongly attracted to soybean and will move into fields with emerging plants.
2. Fields are planted to food-grade soybean production or are seed fields where reductions in yield and seed quality can be significant.
3. Fields have a history of bean pod mottle virus.

Bean leaf beetles are easily disturbed and will drop from plants and seek shelter in soil cracks or under debris during scouting. 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 higher first and second generations later in the season. More detailed information about bean leaf beetle and bean pod mottle virus are available.