Rootworm Reprisal

January 4, 2014 01:57 AM
Rootworm Reprisal

Bt resistance issue now includes mCry3A toxin

First, university entomologists confirmed that Western corn rootworm (WCR) was able to break through the efficacy of the Bt toxin Cry3Bb1. Now, as a number of entomologists feared, a second toxin, mCry3A, has succumbed to repeated pressure from WCR in some continuous corn fields.

Aaron Gassmann, Iowa State University researcher and professor, confirmed the latest rootworm resistance problem with mCry3A in Iowa this past summer. Likely in all cases, the practice of continuous corn and the repeated use of the single toxin in those fields have contributed to the control breakdown.

Control breach. "Bioassay data from 2011 showed that whether the rootworm was from a field with Cry3Bb1 or mCry3A, they were able to survive exposure to either of the two toxins," Gassmann says. 

Similar genetic makeup of the two Cry3 toxins currently available in corn hybrids contributes to the cross resistance. "Western corn rootworms respond to the toxins in similar ways, so if they’re resistant to one, they’re probably going to be resistant to the other," Gassmann explains.

That fact does not bode well for a third toxin, Cry34/35Ab1, which is exhibiting WCR-control weakness in the field, Gassmann says, although no resistance has been confirmed.

Action plan.
For the coming year, farmers need to think about diversifying their management practices for WCR and move away from relying on traits alone to manage the pest. Here are five recommendations from Janet Knodel, North Dakota State University Extension entomologist, for farmers to consider:

  • Practice crop rotation and avoid planting continuous corn.
  • Control any volunteer corn or grassy weed hosts.
  • Use a different Bt corn hybrid, preferably with multiple Cry proteins.
  • Use a soil insecticide at planting to ensure control of larvae and prevent corn lodging.
  • Monitor the efficacy of control. Dig up corn roots this summer to evaluate whether your WCR control measures are holding or slipping; plan your hybrid selections and management practices for the next year accordingly. 

    You can e-mail Rhonda Brooks at

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