The widespread planting of crops genetically engineered to produce insecticidal toxins derived from the bacterium Bacillus thuringiensis (Bt) places intense selective pressure on pest populations to evolve resistance. Western corn rootworm (WCR) is a key pest of corn and is managed through planting of Bt corn (Gray et al. 2009). Starting in 2009, western corn rootworm in Iowa has imposed severe injury to Bt corn in the field. Laboratory analysis of field populations collected in 2011 revealed resistance to corn producing Bt toxins Cry3Bb1 and mCry3A, and crossresistance between these toxins. These cases of resistance highlight the vulnerability of Bt corn to further evolution of resistance by western corn rootworm; and more broadly, point to need to use an integrated approach when managing this pest and to follow resistance management requirements.