Western corn rootworm in some Iowa fields have developed resistance to two of the three types of Bacillus thurinigiensis (Bt) toxin produced by genetically modified corn; therefore, entomologists are recommending that farmers still need to make use of age-old practices such as crop rotation to fight insect pests.

A report in the Proceedings of the National Academy of Sciences provides information learned by a team at Iowa State University led by Aaron Gassman, a university entomologist, which says rootworm beetles have developed resistance to two out of three Bt toxins that can be in GM corn.

“That’s two of the three toxins on the market now. It’s a substantial part of the available technology,” Gassmann reportedly told Brian Owens for an article appearing on nature.com.

The history of GM corn with Bt toxin began in 2003. Cry3Bb1, which provided protection against pests such as rootworm, was first approved for use in the U.S. in 2003. By 2009, farmers had started to see rootworm damage in their GM crops. In 2011, that damage had spread to containing a second toxin, mCry3A.

“In lab tests, Gassmann showed that this was a case of cross-resistance — worms that had become resistant to Cry3Bb1 were also resistant to mCry3A, possibly because the toxins share structural similarities and some binding sites in the insect’s gut,” wrote Owens.

Gassmann contends farmers should not rely exclusively on technology to fight pests and should periodically change the crop grown on a field to help disrupt the pest’s life cycle. “The rootworm can’t survive if the corn’s not there,” he is quoted as saying.

To read the article click here.