The genetically modified cotton that was designed to make the Bt (that’s short for Bacillus thuringiensis, a bacterium) toxins that kill boll weevils has been thwarted by the bugs, according to new research to be published in Nature Biotechnology. The bollworm is the first pest to evolve resistance in the field to so-called Bt crops, according to University of Arizona entomologists.

Bt-resistant populations of bollworm (Helicoverpa zea) were found in more than a dozen crop fields in Mississippi and Arkansas between 2003 and 2006. The first resistant worms showed up just seven years after the genetically modified crops were introduced, in 1996, according to the analysis of monitoring studies.

“What we’re seeing is evolution in action,” said lead researcher Bruce Tabashnik.