U.S. regulators for the first time are proposing limits on the planting of some genetically engineered corn to combat ... a bug that ranks among the most expensive crop threats to U.S. corn farmers. The plan is aimed at widely grown corn varieties sold by Monsanto, the first to sell rootworm-resistant corn, and rival seed makers including DuPont and Dow Chemical. Such corn seeds have been genetically modified to secrete proteins that are toxic to destructive insects. The [Environmental Protection Agency's] proposal would require seed companies to limit some Midwestern farmers' practice of sowing fields with corn year after year in areas harboring resistant rootworms. The agency is taking a tougher stance because the industry's efforts haven't done enough. Genetically modified corn ... was planted on an estimated 80% of U.S. cornfields last year, up from 19% in 2000. Midwestern farmers' embrace of pest-resistant corn since the first varieties' launch in 1996 has diminished its power. Repeated exposure to the corn's bug-killing proteins means that the small number of rootworms that are able to consume the BT toxin and live can reproduce by the thousands and spread across fields that are used to grow corn year after year. Over large areas, the [modified] corn plants will lose effectiveness, and growers will be forced to rely much more on insecticides," said [University of Arizona entomology professor] Bruce Tabashnik. "That's bad for their bottom line, and it's bad for the environment."