Corn rootworm is a persistent and economically important pest in Iowa, with yield loss caused primarily by larval feeding on corn roots. Genetically modified corn with Bt traits kills western and northern rootworm larvae, and Bt-rootworm corn has been available since 2003. However, some Iowa farmers have observed severe root injury to Bt hybrids in continuous cornfields, and there has been documented resistance of western corn rootworm in Iowa to Cry3Bb1 corn and mCry3A corn since 2009 and 2011, respectively.