By DEBORAH BEHRENDS
Illinois Correspondent

URBANA, Ill. — Severe root damage observed in Bt corn in northwestern Illinois in recent weeks should alert growers to carefully consider 2012 seed selection choices, said Mike Gray, University of Illinois extension entomologist.

On Aug. 16, Gray verified severe corn rootworm pruning on some Bt hybrids that express the Cry3Bb1 protein in Henry and Whiteside counties, located in northwestern Illinois.

The fields were in continuous corn production systems for many years, and the producers had relied upon Bt hybrids that expressed the protein as their primary protection against Western corn rootworm (WCR) injury.

Lodged plants were common in many areas of the fields, and WCR adults were numerous and easy to collect. He also found plants with two to three nodes of roots completely destroyed. A shovel was not required for removing the plants from the soil, Gray said.

 “Unfortunately, yield losses will be significant in these fields,” he added. “In early July, severe storms swept through northern Illinois and caused significant lodging of many cornfields.”

Russ Higgins, U of I extension educator in commercial agriculture, said yield losses, if there are any, can’t be blamed on one thing. Higgins, based at the Northern Illinois Agronomy Research Center in Shabbona, spoke off the cuff last week by phone from the Farm Progress Show in Decatur, Ill.

“I haven’t seen any numbers, but I can say, when the roots are gone, if you have saturated soils, you would expect to see more root lodging,” he said. “We’ve been pretty fortunate in northern Illinois; we’ve had some August rains and, by comparison, we’ve done pretty well.”

While he expects yield to be down from the past two years, Higgins doesn’t think it can be blamed on one problem. “There are all sorts of things happening this year: early rain, later planting, strong winds, torrential downpours, lack of rain in July.

“This year, more than any, we’ll see what happens when we pull into the field and actually start harvesting.”

Earlier this month, Aaron Gassman of Iowa State University confirmed field-evolved resistance by WCR to the Cry3Bb1 protein in an Iowa study. Resistant WCR adults were collected by Gassman from continuous cornfields in northeastern Iowa where significant root damage had occurred.

Those fields had been planted with Bt hybrids expressing the Cry3Bb1 protein, Gray said. The situations in Iowa and Illinois share some common features, he added. Adults worms were collected from the Illinois fields in question and will be further evaluated for potential resistance.

 “I urge you to be very cautious in your choice of hybrids offering corn rootworm protection, in light of these developments in Iowa and northwestern Illinois,” Gray said. “Many producers have utilized a single-tactic approach for too many years, and now unfortunate consequences are beginning to emerge.”
Gray urged producers to consider the following alternatives if they encountered less than satisfactory root protection with their Bt hybrid this year:

•Rotation to soybeans or another non-host crop

•The use of a corn rootworm soil insecticide at planting

•The use of a Bt hybrid that expresses a different corn rootworm Cry protein than one which may have performed poorly in your fields during 2011

•The use of a pyramided Bt hybrid that expresses multiple Cry proteins targeted against corn rootworms

•A long-term, integrated approach to corn rootworm management 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

More information can be found online at http://bulletin.ipm.illinois.edu