Hidden in the soil of Illinois and Iowa, a new generation of insect larvae appears to be munching happily on the roots of genetically engineered corn, according to scientists. It's bad news for corn farmers, who paid extra money for this line of corn, counting on the power of its inserted genes to kill those pests. It's also bad news for the biotech company Monsanto, which inserted the larvae-killing gene in the first place. In fact, the gene's apparent failure ... may be the most serious threat to a genetically modified crop in the U.S. since farmers first started growing them 15 years ago. The economic impact could be huge," says the University of Arizona's Bruce Tabashnik, one of the country's top experts on the adaptation of insects to genetically engineered crops. Billions of dollars are at stake. The scientists who called for caution now are saying "I told you so," because there are signs that a new strain of resistant rootworms is emerging. In eastern Iowa, northwestern Illinois, and parts of Minnesota and Nebraska, rows of Bt corn have toppled over, their roots eaten by rootworms. Entomologist Aaron Gassmann at Iowa State University, who authored the [new] paper, collected insects from some of these fields and found many with a greater-than-expected ability to tolerate Bt. The EPA is now recommending that ... farmers in areas where such damage has been observed to stop planting this kind of Bt corn altogether. Instead, those farmers will have to use other methods, such as spraying chemical insecticides, to control the rootworm.