A team of researchers at Iowa State University (ISU) recently confirmed western corn rootworms in northwestern Iowa are developing resistance to the natural pesticide in Bt corn, raising questions about the effectiveness of the genetically engineered corn.
Specifically, the team of ISU entomologists led by Aaron Gassman found evidence of root worm resistance to the Cry3Bb1 strain of protein toxin from Bacillus thuringiensis found in Monsanto Bt corn varieties.
"We report that fields identified by farmers as having severe rootworm feeding injury to Bt maize contained populations of western corn rootworm that displayed significantly higher survival on Cry3Bb1 maize in laboratory bioassays than did western corn rootworm from fields not associated with such feeding injury," the researchers said in an article published on July 29 in the scientific journal PLoS ONE.
However, there was no significant correlation among populations for survival on Cry34/35Ab1 maize and Cry3Bb1 maize, suggesting a lack of cross resistance between these Bt toxins.
A significant correlation was also found between the number of years Cry3Bb1 corn had been grown in a field and survival of western corn rootworm on Cry3Bb1 corn. Interviews with farmers indicated Cry3Bb1 corn had been grown for at least three consecutive years in all of the fields containing resistant corn borers.
The researchers concluded insufficient planting of refuges and non-recessive inheritance of resistance may have contributed to resistance. They noted a 2009 study done by the Center for Science in the Public Interest indicated only 50% of Bt maize planted in Midwest complied with Environmental Protection Agency requirements for refuge size and proximity to Bt fields.
"These results suggest that improvements in resistance management and a more integrated approach to the use of Bt crops may be necessary," the researchers said.
"These are isolated cases, and it isn't clear how widespread the problem will become," Gassmann told the Wall Street Journal in an interview. "But it is an early warning that management practices need to change."
There are already reports of apparent rootworm damage to Bt corn in other Corn Belt states as well. University of Illinois Entomologist Mike Gray last week reported he had verified severe root pruning on some Bt hybrids that express the Cry3Bb1 protein in northwestern Illinois fields that had been in continuous corn production for "many years".
"In light of the developments in Iowa and northwestern Illinois, I urge great caution in choosing hybrids offering corn rootworm protection as you make seed selections for 2012, " Gray says.
He suggested several 2012 alternatives for producers who have encountered unsatisfactory root protection this season with their Bt hybrids, including:
- Rotating to soybeans or another non-host crop.
- Applying a corn rootworm soil insecticide at planting.
- Using a Bt hybrid expressing a different corn rootworm Cry protein.
- Using a pyramided Bt hybrid that expresses multiple Cry proteins targeted against rootworms.
"Most important, though, for effective corn rootworm management is to consider a long-term, integrated approach that includes multiple tactics, such as adult suppression programs, use of soil insecticides at planting, rotation of Bt hybrids that express different Cry proteins, and rotation to non-host crops," Gray says, noting many producers had relied on a single tactic for too many years.
Editor’s note: Richard Brock, Corn & Soybean Digest's marketing editor, is president of Brock Associates, a farm market advisory firm, and publisher of The Brock Report.