Abundance and Frequency of Rotation Resistance for Two Species of Corn Rootworm (Diabrotica spp.) in Eastern Iowa

The western corn rootworm Diabrotica virgifera virgifera LeConte and the northern corn rootworm Diabrotica barberi Smith & Lawrence (Coleoptera: Chrysomelidae) are two significant insect pests of corn in North America. Both species have adapted to crop rotation in the Midwest, D. barberi through extended diapause and D. v. virgifera through oviposition in soybean fields. In 55 populations sampled over two years, we collected data on the abundance of D. v. virgifera and D. barberi and the prevalence of rotation resistance. During 2008 we collected data on abundance of both species and mapped the occurrence of rotation-resistant D. v. virgifera. In 2009 we expanded our goals by also mapping the distribution of rotation-resistant D. barberi. Our experimental design consisted of sampling transects across eastern Iowa. Each transect was an average of 249 Km in length and an average of 11 sites were evenly distributed along these transects. Three transects were sampled in 2008 and two in 2009. Soybean fields were monitored with Pherocon AM sticky traps to detect rotation-resistant D. v. virgifera, and first-year corn was monitored with emergence cages to detect rotation-resistant genotypes of both species. We placed Pherocon AM traps placed in neighboring fields of continuous corn to determine the abundance of these species. We found that rotation-resistant D. v. virgifera was most common in the northeastern and east-central Iowa, although no populations were above the economic threshold. Rotation-resistant D. barberi was found throughout eastern Iowa.