Mortality from soil insecticides among Bt-resistant populations of western corn rootworm (Coleoptera: Chrysomelidae)

Western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), is a major pest of maize (Zea maize L.) in North America.  Larvae cause injury by feeding on the roots of maize, which leads to lodging and reductions in yield.  Historically, insecticides have been used to manage D. v. virgifera populations.  Current management of western corn rootworm larvae uses Bt maize and conventional insecticides.  Use of insecticides selects for resistance, and past research has documented resistance in western corn rootworm adults to organochlorine and organophosphate insecticides.  Elevated LD₅₀ values of bifenthrin, a pyrethroid, also have been observed in laboratory studies of field-collected adults.  While transgenic maize is a commonly used management tool, making up 76% of all maize planted in the United States, growers still use insecticides in management of larval rootworm.  Determining whether cross-resistance may exist between Bt and soil insecticides can inform management decisions for western corn rootworm.  To achieve this, larvae from Bt-resistant populations were subjected to laboratory-based bioassays that measured rates of mortality resulting from exposure to two insecticide classes: organophosphate and pyrethroid.  Larvae from four different Bt-resistant populations of D. v. virgifera and two susceptible populations were subjected to two different assays, one that tested formulated insecticides and one that tests technical-grade insecticides.  Results on mortality of Bt-resistant and susceptible larvae will be presented.