You may have seen reports earlier this year that resistance to Bt corn by field populations of the western corn rootworm (WCRW) has been confirmed in Iowa.  

(For the earlier reports of resistance in the Midwest, see and

WCRW is sometimes called the billion dollar pest because of the extent of damage it causes to corn and cost of its control. 

In 2003 Monsanto introduced the first Bt toxin, Cry3Bb1, with activity against mid-season rootworms. Two other Bt toxins targeting mid-season rootworms, mCry3A from Syngenta and Cry34/35Ab1 from Dow Agroscience, have also been introduced since then. 

All these products have been effective at preventing rootworm damage across the Corn Belt. It should be noted that these Bt rootworm traits are not effective against the southern corn rootworm, which is a common pest of several crops in the Southeast.

Field observations of greater than expected root injury by WCRW to Bt corn with the Cry3Bb1 toxin have been reported in the last several years. A team led by Aaron J. Gassmann of Iowa State University has demonstrated the development of resistance to the Cry3Bb1 toxin by a field population of western corn rootworms in northeastern Iowa.

The journal article, “Field-Evolved Resistance to Bt Maize by Western Corn Rootworm,” was published July 29 in PLoS ONE (; Vol. 6, Issue 7, pp 1-7).