LINCOLN, Neb. (AP) — Researchers say bugs are developing resistance to the widely popular genetically engineered corn plants that make their own insecticide, so farmers may have to make changes. The Lincoln Journal Star reports (http://bit.ly/1eC05SM ) that cases of rootworms eating roots of so-called Bt corn have been confirmed in Nebraska, Iowa, Illinois, South Dakota and Minnesota.

Iowa State University researchers say rootworms have developed resistance to two of the four genetic traits in corn plants that are engineered to kill rootworms. Iowa State professor Aaron Gassmann says the problem isn’t widespread yet, but farmers and seed companies should consider changing their approaches to pest control.

In areas where Bt corn has failed to control rootworms, farmers turned to insecticides. The USDA says 76 percent of all corn planted last year was Bt corn.