Mild Winter Favors Bean Leaf Beetle Survival

April 24, 2020
ICM News

Bean leaf beetle adults (Photo 1) are susceptible to cold weather and most die when air temperatures fall below 14°F (-10°C). However, they have adapted to winter by protecting themselves 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 soybean and other hosts. While adult activity can begin before soybean emergence, peak abundance often coincides with early-vegetative soybean.

bean leaf beetle on green leaf
Photo 1. Adult bean leaf beetle. Photo by Winston Beck.

An overwintering survival model developed by Lam and Pedigo from Iowa State University in 2000 is helpful for predicting winter mortality based on accumulated subfreezing temperatures. Predicted mortality rates in Iowa are variable for the 2019-2020 winter, ranging from range from 42-81% (Figure 1). Mortality was highest in northern Iowa (69-81%); the average mortality rate across Iowa is 59% for the 2019-2020 winter. Field agronomist Rebecca Vittetoe noted active adult bean leaf beetles in Washington county this week!

predicted mortality map of Iowa
Figure 1. Predicted overwintering mortality of bean leaf beetle based on accumulated subfreezing temperatures during the winter (1 October 2019 – 15 April 2020).

Marlin Rice started reporting bean leaf beetle mortality predictions in 1989. Last winter, the predicted mortality of bean leaf beetles in central Iowa was 61%, which is about ten percent lower than the 30-year average of 71% (Figure 2). It is important to remember insulating snow cover and crop residue can protect bean leaf beetles from harsh air temperatures. However, fluctuating temperatures can reduce spring populations.

graph of bean leaf beetle mortality by year in Iowa
Figure 2. Predicted bean leaf beetle mortality by year for central Iowa; the red line indicates the average mortality rate (71%).

Overwintering beetle populations are expected to be high 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 soybean plants 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 yield and seed quality can be significant.
  3. Fields have a history of bean pod mottle virus.

Bean leaf beetles are easily disturbed and will drop from plants and seek shelter in soil cracks or under debris during scouting. 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 higher first and second generations later in the season. More detailed information about bean leaf beetle and bean pod mottle virus are available.

 

Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on April 24, 2020. The information contained within may not be the most current and accurate depending on when it is accessed.

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Erin Hodgson Professor

Dr. Erin Hodgson started working in the Department of Entomology at Iowa State University in 2009. She is an associate professor with extension and research responsibilities in corn and soybeans. She has a general background in integrated pest management (IPM) for field crops. Dr. Hodgson's curre...

Ashley Dean Extension Program Specialist III

Ashley is an extension program specialist for field crop entomology at Iowa State University. She coordinates the Iowa Moth Trapping Network, develops educational resources for field crop pests in Iowa, and aids in the research efforts of the