By Erin Hodgson, Department of Entomology, and Adam Sisson, Department of Agronomy
Bean leaf beetle adults are susceptible to cold weather and most will die when the air temperature falls below 14°F. However, they have adapted to winter by protecting themselves under plant debris and loose soil. Each spring, adult beetles emerge from overwintering habitat and migrate to available host plants, such as alfalfa, tick trefoil and various clovers. As the season progresses, bean leaf beetles move to more preferred hosts, like soybean. Note that while initial adult activity can begin well before soybean emergence, peak abundance can coincide with soybean emergence.
Photo 1. Adult bean leaf beetle. Photo by Winston Beck.
An overwintering survival model was developed by Lam and Pedigo from Iowa State University in 2000, and is helpful for predicting winter mortality based on accumulating subfreezing temperatures. Predicted mortality rates in Iowa were highly variable during the 2014-2015 winter, and ranged from 68-99 percent (Fig. 1). Northern Iowa experienced colder temperatures, and most of the bean leaf beetle adults are not expected to survive there.
Figure 1. Predicted overwintering mortality of bean leaf beetle based on accumulated subfreezing temperatures during the winter
(1 October 2014 – 31 March 2015).
The statewide-predicted mortality from the previous winter (2013-2014) was the highest since Marlin Rice started tracking these data in 1989. The average mortality rate in central Iowa over the last 25 years is 73 percent, and approximately 84 percent of adults were predicted to die last winter (Fig. 2). It is important to remember insulating snow cover and tillage residue can help protect bean leaf beetle from harsh air temperatures. Recent fluctuating temperatures can also negatively influence spring populations.
Figure 2. Predicted bean leaf beetle mortality by year for central Iowa; the red line indicates the average mortality rate.
Although overwintering beetle populations are expected to be low 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 plants in the area. Overwintering adults are strongly attracted to soybean and will move into fields with emerging plants.
2.Fields have food-grade soybean 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 beetle is easily disturbed and will drop from plants and seek shelter in soil cracks or under debris. 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 building first and second generations later in the season. To learn more about managing bean leaf beetle and bean pod mottle virus, click here.
Erin Hodgson is an associate professor of entomology with extension and research responsibilities; contact her at firstname.lastname@example.org or by calling 515-294-2847. Adam Sisson is an Integrated Pest Management extension specialist; contact him at email@example.com or by calling 515-294-5899.