Near-Average Bean Leaf Beetle Mortality

Bean leaf beetles overwinter as adults (Figure 1), which are susceptible to cold weather, and most will die if exposed to air temperatures below 14°F. They avoid harsh temperatures by burrowing under plant debris and loose soil. In early spring, the adult beetles migrate to available hosts, such as alfalfa, tick trefoil, and various clovers. They are strongly attracted to soybean. Adults move locally in the landscape to soybean fields with emerging seedlings. While initial adult activity can begin before soybean emergence, peak abundance often coincides when the early-planted soybean fields start to emerge, leaving those fields prone to defoliation.

To get a better handle on bean leaf beetle winter survival, we estimate winter mortality based on accumulated sub-freezing temperatures using an overwintering survival model developed by Lam and Pedigo from Iowa State University in 2000. 

Using this model, predicted mortality rates across Iowa for the 2024-2025 winter, range from 59-88% (Figure 2). Mortality was the lowest in east-central, south-central and southeast Iowa (59-62%); the average mortality rate across Iowa was 72% for the 2024-2025 winter.

These mortality predictions have been tracked since 1989. This winter, mortality across the state was about average. The predicted mortality of bean leaf beetle in central Iowa this winter was 74%, which is just slightly above the 37-year average of 71% (Figure 3).

It is important to note that insulating snow cover and crop residue can 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.

Based on this model, we expect higher overwintering bean leaf beetle populations in east-central, south-central, and southeast Iowa compared to other regions of the state this year.

Because these populations are highly variable, it is important to scout for this pest, especially if: 

1. Fields are planted near alfalfa fields or fields are the first-emerging soybean 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 seed quality can be significant.
3. 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. For early season scouting, slowly walk down 15-20 feet of row and count beetles and plants. Repeat this in four more areas of the field and calculate beetles/plant. Economic thresholds range from 2 to 8 beetles/plant depending on crop value and control costs. 

Although overwintering beetles rarely cause economic damage, their presence may be an indicator of increasing first- and second-generation densities later in the season. Find an economic threshold table and more information on scouting for bean leaf beetle in this encyclopedia article, and learn more about bean pod mottle virus from the Crop Protection Network.

A special thanks to Adam Sisson with the ISU Integrated Pest Management Program and Crop Protection Network for helping to run the model on the predicated overwintering mortality of bean leaf beetles.