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Posted: April 2014

Western Corn Rootworm Not Killed by Bt Corn in Iowa

Bt corn has been genetically modified to create a toxin (which stems originally from the bacterium Bacillus thuringiensis) that is lethal to certain insects. Some varieties of Bt corn (Cry3Bb1, mCry3A, and Cry34/35Ab1) target the western corn rootworm, one of the most serious insect pests of corn (maize) in North America. The western corn rootworm is a beetle and both the larval and adult stages can damage corn. Cry3Bb1 Bt corn, sold by Monsanto Company, has been on the market since 2003. It initially proved effective in protecting the plants against rootworms. As a result, many farmers stopped rotating corn crops with soybeans, which had aided in reducing corn rootworm damage. Instead, they planted Bt corn every year on the same fields.

In 2009 some farmers in Iowa noticed that the western corn rootworm was damaging their Bt crops. Researchers from Iowa State University in a team led by Aaron Gassman began investigating the problem. (Gassman acknowledges that his research has received financial support from Monsanto Company; see Gassman et al. 2014). More fields with damaged Bt corn in different counties in Iowa were discovered in 2010 and 2011. The researchers demonstrated that the western corn rootworms from such fields survived just as well feeding on Bt corn as on corn that had not been genetically modified. The insects had clearly, between 2003 and 2009, developed resistance to the toxin. They also found that more larvae survived in the fields in 2011 than had survived in 2009, suggesting that “resistance in the field has persisted, if not increased, over time”(Gassman et al. 2014).

In 2011 Gassman and his colleagues also discovered that western corn rootworms had developed resistance to a different variety of Bt corn (mCry3A; sold by Syngenta) and that some larvae were resistant to both varieties.

In light of these findings, the researchers conclude that “current approaches for managing Bt resistance should be reexamined.”


Gassman, A. J., Petzold-Maxwell, J. L., Clifton, E. H. et al. (2014). “Field-Evolved Resistance by Western Corn Rootworm to Multiple Bacillus thuringiensis Toxins in Transgenic Maize,” PNAS. doi:10.1073/pnas.1317179111

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