Indiana farmers are seeing higher numbers of a beetle that can damage soybeans and reduce yields, a Purdue Extension entomologist says.
The bean leaf beetle is common in the spring and early summer, but the second-generation, which typically does not damage plants or pods in late season, arrives in the fall, Christian Krupke said. This year, however, as some crops are late in maturing, farmers began noticing beetles feeding on the foliage and green pods.
"You can find bean leaf beetle every year in every field," Krupke said. "But for some reason, there are more beetles than usual late in the season this year."
"There is no reason to think this is the beginning of a trend of second-generation damage. This damage is also not on the same level as many other late-season pests, such as western bean cutworm."
Because bean leaf beetles can reduce yield, farmers should be aware of the economics of the field and, if necessary, spray insecticides, Krupke said. In addition to direct damage, the beetles often leave scars on pods, opening them to bacteria, moisture and disease.
Historically, the guideline for treating against bean leaf beetles has been when 5 percent of the pods in a field have feeding damage. Those levels are based on lower commodity prices, however and growers might want treat at lower damage levels now given the high soybean prices.
"It all depends on the commodity price," Krupke said. "If the prices go down, it's not worth it."
Before treating, Krupke said it's critical to note whether the beetles are still in the field, as they will soon stop feeding altogether and seek overwintering areas.
Once the pods turn yellow, bean leaf beetles will typically stop feeding on the crop and look for forage elsewhere. As the days become shorter, the beetles will go into an overwintering stage and cease to be a concern for growers, he said.
Before making pesticide decisions, growers should evaluate how much longer their crops will be vulnerable and the pre-harvest interval for control measures, Krupke said. Some insecticide applications must be completed several weeks before harvest.
To assess whether active feeding on pods is still occurring, growers should look for the beetles during the warmest part of the day when they are most active, he said.
"You actually want to see the beetles and not just damage to the leaves," he said. "It's important not to concern yourself with the aesthetics of the field, because all the late-season foliage is looking pretty ratty this time of year after weather, grasshopper feeding and other damage," he said.
More information about the bean leaf beetle and effective insecticides is available in the Sept. 16 issue of the Pest & Crop Newsletter at http://extension.entm.purdue.edu/pestcrop/