Geographic Variation in Mortality from Insecticides among Iowa 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 Iowa.  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.  Use of insecticides selects for resistance, and past research has documented resistance in western corn rootworm adults to organochlorines and organophosphates.  Elevated LD₅₀ values of bifenthrin, a pyrethroid, have also been observed in laboratory studies using field-collected D. v. virgifera 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.  Understanding the geographic variation in mortality of larval D. v. virgifera from conventional insecticides is an important step in the development of a resistance monitoring program and in detecting potentially resistant populations already present in the field.  To achieve this, samples of adult D. v. virgifera will be taken from several locations in Iowa, and larvae from these populations will be subjected to laboratory-based bioassays that measure rates of mortality resulting from exposure to a number of insecticides.  Plans to acquire D. v. virgifera and bioassay methods to be used will be presented.