The wily western corn rootworm thrives in fields where farmers choose a risky combo of continuous corn and continuous use of the Cry3Bb1 trait found in Monsanto's YieldGard RW and VT Triple products.

And with these chewed roots comes a now quantifiable yield loss. In 2012, Nick Tinsley, a Ph.D. student of University of Illinois entomologist Mike Gray, analyzed root injury ratings and yield data collected over many years and locations, and determined that for each node of roots destroyed, a 15% yield loss could be expected. In a drought year, farmers can see up to 40% yield loss or more.

That was the sobering news delivered by Gray, following the 2012 season as he investigated Bt corn performance inquiries from the Midwest.

"We had reports that there may or may not be resistance yet from a crescent-shaped area that included eastern Nebraska and South Dakota, Minnesota, northeast Iowa and northwest Illinois," he says. "We will use plant bioassays to confirm if we have resistance to the CryBb1 protein."

After an EPA review of both university and Monsanto data on Cry3Bb1 western corn rootworm resistance cases of field failures resistance in Iowa and Illinois, the agency's IRM Team has concluded that western corn rootworm may not be completely controlled by Cry3Bb1 in certain parts of the Corn Belt. While “confirmed resistance” as defined in registration documents has not been met at this time, given the nature of the data, the manufacturer, Monsanto, has agreed to several actions and changes related to the registration of Cry3Bb1 products to address these matters:

  • Work with EPA to revise regulatory definition of "confirmed resistance." There are differences between Monsanto and EPA/the academic community over testing procedures to determine actual resistance. EPA has encouraged Monsanto to adopt Iowa State University Entomologist Aaron Gassmann's more sensitive on-plant assay instead of a diet bioassay.
  • Aggressively increase the availability of pyramided products containing multiple corn rootworm toxins to replace seed containing single-trait Cry3Bb1.
  • Convert the majority of its single-trait Cry3Bb1 portfolio to the recently available RIB product to focus on greater refuge compliance.
  • Continue to conduct and support grower education that demonstrate the economic and technology-preserving value of crop rotation as a best management practice (BMP), and submit proof to EPA.
  • Continue to successfully help farmers with performance issues to adopt a BMP. Monsanto reports that acreage of affected areas decreased from 75,000 acres to 45,000 acres from 2011 to 2012 (some may be due to drought). And Monsanto worked closely with all growers who had rootworm performance complaints in 2011 to help them adopt a BMP of crop rotation, plant a pyramided product, used an insecticide or opted to plant non-Cry3Bb1 corn. Read their backgrounder and grower guide:

The EPA plans to maintain effective oversight of these products to preserve their substantial benefits to agriculture and the environment. And it is confident that the measures taken will prevent a further reduction in susceptibility of corn rootworm population to the Cry3Bb1 toxin.

Bottom line, given the complexity of this pest and the control challenges, farmers need to step up both their management practices and their vigilance in order to succeed in every corn field. There is no easy way out.