Susceptibility of Western Corn Rootworm (Diabrotica virgifera virgifera, LeConte, Coleoptera: Chrysomelidae) Populations Collected in Nebraska to Bt Corn Events

To formally characterize the susceptibility of Nebraska WCR populations to rootworm-active Bt corn events, a study was conducted to evaluate the susceptibility of WCR field populations from Cry3Bb1 history fields to each commercial rootworm-active protein (i.e., Cry3Bb1, mCry3a, Cry34/35Ab1) and appropriate non-Bt isolines.  In 2012, nine WCR field populations were collected of which six were from fields with a Cry3Bb1 history and had unexpected root injury.  Six WCR colonies that had never been exposed to Cry toxins and three field populations from sites not experiencing unexpected injury to rootworm Bt hybrids were used as controls.  Single-plant assays were performed in the greenhouse and laboratory by infesting each potted plant with 12 F₁ neonates at V5 growth stage and then recovering larvae after 17 days.  For each population, 12 replicates of each Bt protein and associated non-Bt isoline were included in a randomized complete block design.  Larval survivorship was significantly affected by a significant interaction between field type and hybrid for populations reared on Cry3Bb1 and mCry3A but not Cry34/35.  Mean survivorship of larvae from problem Cry3Bb1 history fields when reared on Cry3Bb1 or mCry3A was not significantly different than mean survivorship on respective isolines without Bt.  This pattern was not evident when larvae were reared on Cry34/35 and associated non-Bt isoline.  Results suggest that WCR resistance to Cry3Bb1 has evolved in response to selection pressure and that a possible cross-resistance relationship exists between Cry3Bb1 and mCry3A. There was no apparent cross resistance relationship between Cry3Bb1 or mCry3A and Cry34/35Ab corn.