A paper published last March, for instance, documented an unsettling trend: Corn rootworms are evolving resistance to the bacterial toxins in Bt corn. “I was surprised when I saw the data, because I knew what it meant—that this technology was starting to fail,” says Aaron Gassmann, an entomologist at Iowa State University and co-author of the report. One problem, he says, is that some farmers don’t follow the legal requirement to plant “refuge fields” with non-Bt corn, which slow the spread of resistant genes by supporting rootworms that remain vulnerable to the Bt toxins.