Comparing different assays for Bt resistance detection in western corn rootworm

The western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte, is widely managed by corn (Zea mays L.) expressing proteins from the bacteria Bacillus thuringiensis (Bt). In field conditions, populations of WCR have been selected for resistance to Cry3Bb1, mCry3A, and Cry34/35Ab1. Recently, the United States Environmental Protection Agency (EPA) has suggested that companies switch from a diet-toxicity assay to an on-plant assay for insect resistance monitoring (IRM) programs. The goal of this study was to evaluate the ability of three different assays to detect resistance to the four commercially available Bt proteins. The seedling mat, greenhouse pot, and diet-toxicity assays were evaluated through use of field and laboratory populations exposed to Bt protein in planta, as well as field and laboratory populations that had not been exposed to Bt proteins. Preliminary data indicates that the seedling mat assay was better able to detect resistance in most populations tested with less variable data than both single plant assays. Diet toxicity assays may be better at detecting earlier shifts in resistance based on resistance ratios.