"Control by the Bt trait requires that larvae eat some of the root. If the number of beetles in the field was high last year, there could have been a massive amount of eggs deposited in the field, resulting in a lot of visible root feeding this year. Even with a lot of root feeding, very few larvae will develop into adult beetles if the trait is working. If scouting shows high numbers of adults in the field and you suspect resistance to the Bt RW trait is a possible cause, this can be verified through Iowa State University."
Should you spray insecticide on fields to control adult corn rootworm beetles?
There are two reasons you might want to consider controlling adult beetles in a cornfield. First is to protect this year's crop by preventing excessive feeding on silks. The second reason, for continuous cornfields only, is to prevent beetles from laying eggs that could harm next year's crop. The eggs overwinter, hatch next year and the rootworm larvae chew on corn roots.
The economic threshold for controlling adult beetles depends on the cost of applying insecticides, but information from the University of Nebraska, as presented in this Monsanto guide suggests it may take more than five beetles per plant for silk clipping to lead to significant yield loss.
If you're planning to plant corn next year on a field that's in corn now, Purdue University suggests that if the scouted count is more than one beetle per plant, spraying an insecticide this summer may be justified in order to reduce the potential for rootworm damage in next year's corn crop.
If you do your own scouting, using this new app can make recording numbers quick and simple
Other information on corn rootworm management can be found in the February 2013 issue of ISA's On-Farm Update and in this Iowa State University rootworm management recommendation by Aaron Gassmann, an ISU entomologist.
Mueller reminds everyone that if you're doing your own scouting, using the new ScoutPro app for smart phone or tablet app can make recording numbers and locations in the field quick and simple. "ISA still has a number of copies of ScoutPro available free to ISA members," Mueller says. Email him to get your free app. firstname.lastname@example.org