Bean leaf beetle adults are cold sensitive and most will die if exposed to temperatures below 14°F. However, they avoid temperature extremes 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 clover, and various clovers. As the season progresses, bean beetles move to preferred hosts, such as soybeans. Although initial adult activity may begin before soybeans emerge, the peak in abundance often coincides with the start of soybean vegetation.
We can estimate winter mortality based on accumulated below-freezing temperatures using a winter survival model developed by Lam and Pedigo of Iowa State University in 2000. Predicted mortality rates in the Iowa are variable for the winter of 2021-2022, ranging from 58 to 97% (Figure 1). Mortality was highest in northern and central Iowa (78-97%); the average death rate in Iowa was 73% for the winter of 2021-2022.
These mortality predictions have been tracked since 1989 with Marlin Rice, an affiliate professor in the Department of Entomology at Iowa State University. Predicted bean leaf beetle mortality in central Iowa this winter was 78%, about 6% higher than the 30-year average of 71.7% (Figure 2). It is important to remember that isolating snow cover and crop residue can help protect the bean leaf beetle from harsh air temperatures, and that variable snow and residue cover is not taken into account by the model. Fluctuating temperatures can have a negative influence on spring populations. The lack of snow cover combined with fluctuating temperatures during the winter likely resulted in greater bean leaf beetle mortality than predicted by the model.
Although overwintering beetle populations are expected to be lower than last year across most of the state, it is important to look out for this pest. Inspect soybean fields, especially if:
- Soybeans are planted near alfalfa fields or if the field contains the first soybeans that emerge in the area. Overwintering adults are strongly attracted to soybeans and will move into fields with emergent plants.
- Fields are planted for 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 because bean leaf beetles are easily disturbed and will fall off plants and seek shelter in cracks in the ground or under debris. Although overwintering beetles rarely cause economic damage, their presence can be an indicator of first and second generation building later in the season. More detailed information on bean leaf beetle and bean pod mottle virus is available online.
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