Abundance and distribution of western and northern corn rootworm (Diabrotica spp.) and prevalence of rotation resistance in eastern Iowa.

The western corn rootworm Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae) and the northern corn rootworm Diabrotica barberi Smith & Lawrence (Coleoptera: Chrysomelidae) are major pests of corn (Zea mays L.). Historically, crop rotation has been an effective management strategy, but both species have adapted to crop rotation in the Midwest. For both species in eastern Iowa, we measured abundance and prevalence of rotation resistance using sticky traps and emergence cages in fields of corn and soybean (Glycine max L.). Based on currently available data, we calculated the economic thresholds for these pests at two Diabrotica spp. per trap per day in cornfields and 1.5 D. v. virgifera per trap per day in soybean fields. The economic injury level of rotation-resistant D. barberi was determined to be 3.5 adult insects per emergence cage per year. Peak abundance of rootworm adults in cornfields was below economic thresholds in the majority of fields sampled, suggesting that management of rootworm larvae in continuous cornfields may not always be necessary. Rotation-resistant D. barberi was found throughout eastern Iowa using emergence cages in first-year cornfields, however, the abundance was below levels expected to impose economic injury in 14 of 17 fields evaluated. The presence of rotation-resistant D. v. virgifera, as measured by the occurrence of this insect in soybean fields, occurred only in northeastern Iowa and was also below the economic threshold. These data suggests that crop rotation remains a viable pest management strategy in eastern Iowa.