Frequent, heavy rains and saturated soils during egg hatch the last three springs have dramatically reduced adult rootworm populations, he says.
The increasing adoption of Bt hybrids for rootworm control also is likely contributing to the decline. These hybrids typically kill more than 90% of the corn rootworm larvae that feed on them.
In previous years, farmers used granular insecticides, which effectively protected the roots and crop, but didn't lower populations to the same degree.
The population decrease is significant because the pest in large numbers can cause stalk lodging. Lodged corn can result in harvest losses, slower harvest equipment speeds, increased drying costs and more significant volunteer corn problems the next season.
Because corn rootworm has had a history of adapting resistance to control methods, it is extremely important for growers embracing Bt technology next season to plant the recommended refuge.
"This particular pest has become resistant to many pesticides and crop rotation, so there's every reason to expect they could become resistant to Bt hybrids if given the chance," Krupke says. "The purpose of the refuge is to provide an area where susceptible pests can build up and therefore dilute any of those resistance genes."
Refuge requirements differ depending on the type of Bt corn, so farmers should pay extra close attention to label instructions.
More information about corn rootworm populations is available in the most recent issue of the Purdue Pest and Crop Newsletter.