Severe pruning by western corn rootworm larvae to Bt (Cry3Bb1) corn root systems (northwestern Illinois, August 16).
Severe pruning by western corn rootworm larvae to Bt (Cry3Bb1) corn root systems (northwestern Illinois, August 16).

On August 16, I traveled to Henry and Whiteside counties in northwestern Illinois, responding to a request to verify severe corn rootworm pruning on some Bt hybrids that express the Cry3Bb1 protein. The fields have been in continuous corn production for many years, and the producers have relied on Bt hybrids that express the Cry3Bb1 protein as their primary protection against western corn rootworm injury. Lodged plants were common in many areas of the fields, and western corn rootworm adults were numerous and easy to collect. It was also easy to find plants with two to three nodes of roots completely destroyed. A shovel was not required for removing the plants from the soil.

Unfortunately, yield losses will be significant in these fields. In early July, severe storms swept through northern Illinois and caused significant lodging of many cornfields. Producers should take a close look at such fields, examining the roots for excessive pruning. If you find pruning greater than expected, notify your seed company representative. I also would be interested in hearing from you.

Severe root damage to NW Illinois Bt cornIn issue 18 of the Bulletin (August 5), I reported that field-evolved resistance by western corn rootworm to the Cry3Bb1 protein has been confirmed by Dr. Aaron Gassmann of Iowa State University. He collected resistant western corn rootworm adults from continuous cornfields in northeastern Iowa where significant root damage had occurred in producers' fields. The Iowa fields had been planted with Bt hybrids expressing the Cry3Bb1 protein. The situations in Iowa and Illinois share some common features. Adults were collected from the Illinois fields in question and will be further evaluated for potential resistance.

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. If you encountered less-than-satisfactory root protection this season with your Bt hybrid, consider the following alternatives for 2012.

  • Rotation to soybeans or another nonhost crop.
  • A corn rootworm soil insecticide at planting.
  • A Bt hybrid expressing a different corn rootworm Cry protein than one that may have performed poorly in your fields in 2011.
  • A pyramided Bt hybrid that expresses multiple Cry proteins targeted against corn 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 nonhost crops. Many producers have relied on a single tactic for too many years, and unfortunate consequences are beginning to emerge.

As harvest gets under way this fall, I suspect that more producers in northwestern Illinois will encounter lodged corn that has been severely damaged by western corn rootworms. As I learn more about this evolving situation, I will keep you informed.