It came as no surprise when reports of severe root damage to Bt corn caused by the western corn rootworm trickled into Mike Gray’s office last summer. “The western corn rootworm is a story of resilience, flexibility and adaptation to many different management strategies,” said Mike Gray, University of Illinois Extension entomologist and professor of crop sciences. “Its ability to develop resistance to transgenic corn was only a matter of time.”
Western corn rootworms have made remarkable adaptations, he said. Originally discovered feeding on native prairie grasses in the Great Plains, it wasn’t until producers began growing corn-on-corn and using nitrogen fertilizers that it moved into Nebraska and Kansas cornfields.
“Western corn rootworms flourished on those nitrogen-rich plants and, within a few years, we had a serious insect to deal with on a regular basis,” he said.
Growers tried controlling western corn rootworms with soil insecticides, but resistance developed to the chlorinated hydrocarbons. New classes of insecticides were developed and worked for a while. But again, resistance developed. Crop rotation worked well for quite some time, but eventually it lost its effectiveness, too.
“The road was bumpy in our attempt to effectively control western corn rootworm,” Gray said. “By 1995, it caused widespread damageto rotated corn across east central Illinois and northwestern Indiana. One can understand the excitement when Bt hybrids for corn rootworms entered the market.”
Bt hybrids have become an effective management strategy for most growers, he said.
Last summer, Iowa State University’s Aaron Gassman reported the first field-evolved resistance to Bt by western corn rootworms. Shortly thereafter, Gray was called to northwestern Illinois where he confirmed severe corn rootworm pruning on Bt hybrids expressing the same Cry3Bb1 protein described in Gassman’s article.
Researchers are continuing to conduct statewide surveys to monitor this pest.
“The western corn rootworm story will continue to reveal new twists and turns,” Gray said.
For more information on managing western corn rootworm, visit the Pest Management and Crop Development Bulletin at bulletin.ipm.illinois.edu.