University of Arizona entomologists looked at data from six experiments to monitor pests in fields sown with transgenic cotton and corn in Australia, China, Spain and the United States.
They found evidence of genetic mutation among bollworms (Helicoverpa zea) in a dozen cotton fields sown in Mississippi and Arkansas between 2003 and 2006...The mutation entails a slight change in the bollworm's DNA to help it resist a toxin that the cotton plant exudes thanks to a gene inserted by biotechnologists.
These GM toxins are produced in nature by a widespread bacterium called Bacillus thuringiensis, which goes by the abbreviation Bt. The type of Bt toxin to which these bollworms have become resistant is called Cry1Ac.
Via::TerraDaily, "First evidence emerges of pest resistance to GM crops: scientists" Image credit::Wikipedia entry for Helicoverpa zea