2026 Bean Leaf Beetle Overwintering Mortality

Bean leaf beetle adults (Photo 1) 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, adult beetles migrate to available hosts, such as alfalfa, tick trefoil, and various clovers. They are strongly attracted to soybean, and adults move throughout the landscape to find soybean fields with emerging seedlings. While initial adult activity can begin before soybean emergence, peak abundance often coincides with the emergence and development of early-planted soybean fields, leaving those fields prone to defoliation.

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

Mortality is expected to be greater than 50% for most of Iowa, ranging from 49% in southwest Iowa to 92% in the northeast part of the state (Figure 1). The average mortality rate across Iowa was 66% for the 2025-2026 winter.

These mortality predictions have been tracked since 1989. The predicted mortality of bean leaf beetle in central Iowa this winter was 66%, which is below the 38-year average of 71% (Figure 2). 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.

You can explore historical bean leaf beetle mortality data for your region with a new spreadsheet in the bean leaf beetle encyclopedia article.

Based on this model, we expect overwintering bean leaf beetle populations to be higher this year across most of the state. Because these populations are highly variable, it is important to scout for this pest. Scout soybean fields, especially if:

1. 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.
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.

Even though planting is delayed across most of the state this spring, bean leaf beetle can survive on other hosts, like alfalfa, until soybean emerges. Therefore, it will still be important to scout for bean leaf beetle once soybean fields emerge. 

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 the number of beetles per plant. Economic thresholds range from two to eight beetles per 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 in the bean leaf beetle encyclopedia article, and learn more about bean pod mottle virus from the Crop Protection Network.