Suspect corn rootworm in continuous corn? Rotate!
Fields that have been continuous corn with repeated use of one Bt trait are at risk for developing corn rootworm resistance.
Resistance isn’t just a problem in the western Corn Belt; it’s also a problem in Michigan. The first field with unexpected corn rootworm damage in corn expressing the Bt trait for corn rootworm control was found in 2012 in Michigan. In 2013, more fields were identified. Lodged corn, high populations of corn rootworm beetles, signs of leaf feeding and silk clipping were present in suspect fields. When field histories were examined, a common theme emerged: continuous corn fields that had been planted to either Cry3Bb1 or mCry3A hybrids for multiple seasons.
To preserve the effectiveness of the Bt traits in controlling corn rootworm, we must take action. Before going to the fields this spring, Michigan State University Extension encourages farmers to double-check their cropping plan. Fields where there is suspected corn rootworm damage should be rotated to something other than corn and all volunteer corn should be controlled. Just one year in a non-host crop such as soybeans will wipe out the population. Adjacent fields should also be rotated if possible. If not, monitor for corn rootworm.
In the event the field cannot be rotated out of corn, use a hybrid that either has a totally different Bt trait or use a conventional hybrid with an insecticide to control corn rootworm. MSU field crop entomologist Christina DiFonzo published the Handy Bt Trait Table that helps sort out the different traits available. This and other information on corn rootworm can be found at the MSU Field Crops Entomology website.
Producers of continuous corn are asked to monitor their fields in July for the presence of corn rootworm. Signs include root feeding, corn rootworm beetles, silk clipping, possible leaf feeding, and lodged corn. Report suspected corn rootworm fields to your seed dealer, Christina Difonzo or me, Marilyn Thelen.
To learn more about corn rootworm in the eastern Corn Belt, see “Consensus recommendation: Managing Western Corn Rootworm Resistance to Bt on the Fringe,” a white paper by university field crop entomologists from Michigan, Ontario, Ohio, Indiana, New York and Pennsylvania.