Bean leaf beetles will slowly move from current feeding spots to soybeans as soybeans emerge in fields this month. Some research plots of soybeans around southern and central Iowa already have decent numbers of bean leaf beetles feeding on unifoliate soybean seedlings.
"In March, I predicted low overwintering mortality of bean leaf beetles based on our mild winter," says Erin Hodgson, Iowa State University Extension entomologist. "You may have noticed that beetle adults became active in alfalfa starting in April. They are strongly attracted to soybeans and will slowly move as the soybean plants emerge from the ground and start growing this month. The adults fly short distances (less than 170 feet on average) and infestations can be highly aggregated."
Overwintering beetles rarely cause economic damage, but may indicate trouble ahead
Bean leaf beetles are easily disturbed, and they will drop from the plants to seek shelter in soil cracks or under debris such as crop residue, explains Hodgson. Sampling your bean fields early in the season requires you to be sneaky to estimate the actual densities. You have to sort of sneak up on the beetles to count the number of beetles present and make your estimate. In some cases, you may just see leaf defoliation of the soybean plants and not notice any beetles (see accompanying photo). Although overwintering beetles rarely cause economic damage, their presence may be an indicator of the building of the first and second generations later in the season.
Maybe more important than defoliation, bean leaf beetles can vector bean pod mottle virus in soybeans. That is, these insects can carry a soybean disease called bean pod mottle virus and spread it to the soybean plants. "There can be a reduction in yield with bean pod mottle virus-infected soybean plants. That yield reduction results from reduced seed size and pod set," explains Hodgson. "This effect on soybean yield is most severe when the soybeans are infected as seedlings."
Bean pod mottle virus can cause a bleeding hilum or seed coat discoloration. Thus, food grade soybeans are at a higher risk for grading penalties (see photo). In soybean fields that have a history of persistent infestations of bean leaf beetles and of bean pod mottle virus, you should consider using an insecticidal seed treatment, says Hodgson.
For the past few years, many farmers in Iowa have started using insecticide-treated seed for the soybeans they plant. Insecticide seed treatment helps suppress bean leaf beetle populations and a significant amount of Iowa's soybean acreage is now planted with insecticide-treated seed. Overwintering beetles should be suppressed by the high adoption rate of insecticidal seed treatments, says Hodgson. However, the first and second generations of bean leaf beetles could still become a problem later on in the 2012 soybean growing season.
What if first-generation or second-generation beetles become a problem?
If first and second generation beetles do become an issue later on in the 2012 growing season, Hodgson says she will provide updated economic thresholds for farmers and crop consultants to use. She will make the new economic thresholds available through ISU Extension. The new economic thresholds can help you figure out whether or not it will pay to spray an insecticide on the population of beetles that is present in a field.
Economic thresholds are a good tool to use to make such decisions, she says. But the thresholds that were developed in years past need to be updated for use today because of the higher market values for soybeans. "Soybean prices are higher for soybeans today than when the economic thresholds were developed years ago," she notes. "But the calculations can be updated and the new economic thresholds will be made available by ISU, if and when they are needed," says Hodgson. Meanwhile, she advises you to scout your soybean fields and keep an eye out for bean leaf beetles. For more information about bean leaf beetle, visit this ISU Soybean Insects Guide website.