Insecticide sales are surging after years of decline, as American farmers plant more corn and a genetic modification designed to protect the crop from pests has started to lose its effectiveness.

The sales are a boon for big pesticide makers, such as American Vanguard Corp. and Syngenta AG. But it has sparked fresh concerns among environmental groups and some scientists that one of the most widely touted benefits of genetically modified crops—that they reduce the need for chemical pest control—is unraveling. At the same time,...