Field-evolved resistance to Bt toxin Cry3Bb1 and mCry3A in Nebraska western corn rootworm, Diabrotica virgifera virgifera (LeConte) (Coleoptera: Chrysomelidae) populations

A study was conducted during 2012 and 2013, 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 field populations were collected of which six were from fields with a Cry3Bb1 history and had unexpected root injury. Three field populations were from sites that had not experienced unexpected injury to rootworm Bt hybrids and served as field control sites. Additional controls included seven WCR colonies that had never been exposed to Cry toxins and one Iowa WCR population that was known to be resistant to Cry3Bb1. 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.  Larval survivorship was significantly affected by a significant interaction between population type and hybrid for populations reared on Cry3Bb1 and mCry3A but not Cry34/35Ab1. Mean survivorship of larvae from problem Cry3Bb1 history fields when reared on Cry3Bb1or mCry3A was not significantly different than mean survivorship on respective isolines without Bt. This pattern was not evident when larvae were reared on Cry34/35Ab1 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 Cry34/35Ab1 corn.