Farmers in dozens of countries have embraced crops genetically engineered to produce proteins from Bacillus thuringiensis (Bt) bacteria that kill some key pests yet are safe for people and wildlife. Although this biotech approach reduces reliance on insecticide sprays, thereby providing economic and environmental benefits, resistance to Bt crops has evolved in at least 11 species of pests. Thus, effective ways to combat such pest resistance are urgently needed.