Bean leaf beetle adults are susceptible to cold weather, and most will die if exposed to air temperatures below 14°F. However, they avoid harsh temperatures by burrowing 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 preferred hosts, like soybean. While initial adult activity can begin before soybean emergence, peak abundance often coincides with early-vegetative soybean.
We can estimate winter mortality based on accumulated subfreezing temperatures using an overwintering survival model developed by Lam and Pedigo from Iowa State University in 2000. Predicted mortality rates in Iowa are variable for the 2021-2022 winter, ranging from 58-97% (Figure 1). Mortality was highest in northern and central Iowa (78-97%); the average mortality rate across Iowa was 73% for the 2021-2022 winter.
These mortality predictions have been tracked since 1989 with Marlin Rice, affiliate professor in the Department of Entomology at Iowa State University. The predicted mortality of bean leaf beetle in central Iowa this winter was 78%, about 6% higher than the 30-year average of 71.7% (Figure 2).
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It is important to remember that insulating snow cover and crop residue can help protect bean leaf beetle from harsh air temperatures, and variable snow and residue cover is not accounted for by the model. Fluctuating temperatures can negatively influence spring populations. Lack of snow cover combined with fluctuating temperatures over the winter likely resulted in more mortality of bean leaf beetle than predicted by the model.
Although overwintering beetle populations are expected to be lower than last year throughout much of the state, it is important to scout for this pest. Scout soybean fields, especially if:
- Soybean is planted near alfalfa fields or if the field has the first-emerging soybean in the area. Overwintering adults are strongly attracted to soybean and will move into fields with emerging plants.
- Fields are planted to food-grade soybean production or are seed fields where reductions in seed quality can be significant.
- Fields have a history of bean pod mottle virus.
Sampling early in the season requires you to be “sneaky” to accurately estimate densities as bean leaf beetles are 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.