Mild Winter Favors Bean Leaf Beetle

Mild Winter Favors Bean Leaf Beetle

Most Iowa winters are harsh on overwintering bean leaf beetles. But this winter was significantly warmer than normal, so beware of bean leaf beetles this year.

The mild winter Iowa has just experienced may bring a pest-filled spring. Bean leaf beetles are one of the insect pests you will want to be on the lookout for, says Erin Hodgson, an Iowa State University Extension entomologist. She, along with ISU Integrated Pest Management specialist Adam Sisson, provides the following update.

Figure 2. Overwintering bean leaf beetles can defoliate young soybean plants and vector bean pod mottle virus.

Most Iowa winters are harsh on overwintering bean leaf beetles. Typically, for bean leaf beetles, the statewide overwintering mortality ranges from 60% to 99%. See a 20-year historical record of predicted bean leaf beetle mortality in central Iowa. However, in general, Iowa experienced a warm 2011-12 winter, with predicted bean leaf beetle mortality ranging from only 30% to 53%.

A combination of cold winter temperatures and the high adoption of insecticide seed treatments now being used by farmers on the soybeans they plant have drastically curbed bean leaf beetle populations throughout much of the state in recent years.

How will the warm winter and lack of snow cover affect insect survival?

Bean leaf beetle adults are susceptible to cold weather and will die when the temperature is below the minus 10 degrees centigrade mark, but these beetles have adapted to winter by protecting themselves in leaf litter and insulating snow cover. An overwintering survival model was developed by research entomologists Lam and Pedigo at Iowa State University in 2000, and is helpful for predicting winter mortality based on accumulated subfreezing temperatures. Figure 1 is a map of predicted mortality in Iowa for the 2011-2012 winter.

Figure 1. Predicted overwintering mortality of bean leaf beetle based on accumulated subfreezing temperatures during the 2011-2012 winter (1 October 2011 – 8 March 2012).

In general, Iowa experienced a warm winter, with predicted mortality ranging from 30% to 53%. The predicted mortality estimates are the lowest since the overwintering model was developed. Many areas in the state had less than normal accumulated snow cover, which could increase actual adult mortality. A recent ICM News article discusses the implications for warm winters, lack of snow cover and insect survival.

Early-planted soybean fields should be monitored closely this year

Overwintering adults of the bean leaf beetle are strongly attracted to soybean plants and will move into fields with newly emerging plants, as shown in the photo (see figure 2). Early-planted soybean fields should be monitored closely this year, given the predicted likelihood of adult overwintering survival. Other fields of concern include food-grade soybean and seed fields where reductions in both soybean yield and seed quality can be significant. Information about managing bean leaf beetle and bean pod mottle virus is available on the Iowa State entomology website.

Keep this point in mind when you are scouting fields for signs of bean leaf beetle. Bean leaf beetle is easily disturbed and will drop from plants and seek shelter in soil cracks or under crop residue debris. Sampling early in the season requires you to be sneaky to estimate actual densities of bean leaf beetles that are present. Although the overwintering population of beetles rarely causes economic damage, their presence may be an indicator of a building up of the first generation and second generations of the beetles later in the growing season.

TAGS: Extension
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