A Minnesota field population shows varying levels of resistance to both Cry3Bb1 and Cry34/35Ab1-expressing corn

The western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte, is widely managed by corn (Zea mays L.) expressing proteins from the bacteria Bacillus thuringiensis. However, under laboratory conditions, colonies of WCR have been selected for resistance to each commercially-available protein targeting this pest. In addition, feral populations of WCR have previously developed resistance in the field to two of the four proteins targeting this pest. In the current study, progeny of a WCR population collected from a Minnesota corn field that was planted to SmartStax were evaluated on corn hybrids expressing CryBb1 or Cry34/35Ab1 has revealed the development of a third resistance not previously documented from field selection. Data gathered from seedling assays, greenhouse pot assays, and diet-toxicity assays revealed this population is beginning to develop statistically significant resistance to Cry34/35Ab1 in addition to Cry3Bb1. In diet-toxicity assays, larvae of the MN population had resistance ratios of 4.71 and >13.22 for Cry34/35Ab1 and Cry3Bb1, respectively. In greenhouse pot assays and a seedling assay, the survival of the MN population was not significantly lower on corn expressing Cry34/35Ab1 when compared to survival on the near isoline but this difference was significant in control populations in the same assay. In all on-plant assays, larvae had smaller head capsule widths and lower weights when reared on corn expressing Cry34/35Ab1 when compared to larvae reared on the isoline. These results document an incomplete resistance to Cry34/35Ab1.