Rootworm resistance to Bt confirmed in Illinois

Western corn rootworm’s resistance to Bt corn was confirmed at the University of Illinois Agronomy Day.

U of I entomologist Mike Gray announced the western corn rootworm collected last year in fields in northwestern Illinois are resistant to the Cry3Bb1 protein.

Similar reports surfaced in Iowa in 2009 and later were confirmed by Iowa State University entomologist Aaron Gassmann.

Samples were taken last year in Henry and Whiteside counties in Illinois, where severe root injury was reported in some producers’ fields on Bt hybrids expressing the Cry3Bb1 protein. The Whiteside County sample tested positive for resistance.

"The unfortunate news is we see results that parallel those out of Iowa State that were published last year confirming the evolution of field resistance in those populations,” Gray said.

Suspected samples also have been collected in LaSalle and Cass counties in Illinois.

Gray provided the following recommendations for growers as they plan for the 2013 growing season:

Consider rotation to soybeans or another non-host crop;

Consider the use of a corn rootworm soil insecticide at planting, along with a non-Bt hybrid;

Consider the use of a Bt hybrid that expresses a different corn rootworm Cry protein than one which may have performed poorly in fields in 2012 or has been in use for several consecutive years; and

Consider the use of a pyramided Bt hybrid that expresses multiple Cry proteins targeted against corn rootworm.

"Most importantly consider a long-term integrated approach. Mix it up. Manage the population. Don’t rely on the same technology over and over,” Gray said.

"In every case where we’ve had Bt Cry3Bb1 fail, the growers had been in continuous corn for a long time, six, seven, eight years in a row. They had utilized that same trait every year. That’s a recipe for a problem.”

A multi-year Iowa State study noted “insufficient planting of refuges and non-recessive in inheritance of resistance may have contributed to resistance.

"These results suggest that improvements in resistance management and a more integrated approach to the use of Bt crops may be necessary. Continuous corn creates the ideal environment for western corn rootworm and the possibility of resistance.”

Western corn rootworm eggs over-winter in the soil. The eggs hatch in early June and the larvae feed on corn roots. Adults feed on corn silk and deposit eggs back into the cornfields for the next year.

The damage to the roots has a number of consequences to the plant, including disrupting its ability to take nutrients and water. It also makes the plant unstable and prone to lodging.