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. 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, the resurgence of insecticides could expose both farmers and beneficial insects to potential harm. Until recently, corn farmers in the U.S. had largely abandoned soil insecticides, thanks mostly to a widely adopted genetic trait developed by Monsanto Co. that causes corn seeds to generate their own pest-killing toxins. Today, according to the U.S. Department of Agriculture, two-thirds of all corn grown in the U.S. includes a rootworm-targeting gene known as Bt. In 2011, however, entomologists at Iowa State University and the University of Illinois started to document rootworms that were immune to the Monsanto gene, and have found these resistant pests scattered across the Midwest. Now, many farmers have decided they need to spray their soil to kill any rootworms that have developed Bt resistance, as well as growing populations of other pests. Scott Greenlee, who farms 1,700 acres in Sac City, Iowa, said he planned to start using a soil insecticide this year after part of his crop succumbed to rootworms in 2012. The 53-year-old Mr. Greenlee, who had planted Monsanto's Bt corn, said the affected fields produced just 50 or 60 bushels per acre, about a third of his normal yield. It was a train wreck," he added.