They're here: Bt resistant rootworms confirmed in IL

Western corn rootworm resistance to Bt corn has been confirmed in Illinois fields.

Kay Shipman

Published: Aug 17, 2012

University of Illinois agriculture entomologist Mike Gray’s news was as foreboding as the dark storm clouds gathering overhead. Western corn rootworm resistance to Bt corn has been confirmed in Illinois fields, Gray reported Thursday during Agronomy Day in Urbana.

That resistance in a Whiteside County field was specific to corn with Cry3Bb1 proteins, which leaves Illinois farmers a few Bt corn options. Gray stressed the need for farmers to protect those remaining options.

“There is a pretty limited lineup” of Bt hybrids, Gray said.

Ominous information about rootworm behavior also was reported by Joe Spencer, a research professor with the Illinois Natural History Survey. Illinois scientists learned rootworms don’t behave in Bt fields and refuges as entomologists had believed.

Spencer quipped that real rootworm behavior is “out of compliance” with the assumptions that have shaped non-Bt corn refuge requirements.

Fewer Bt-susceptible males move rapidly from refuges into Bt fields where they could mate with potentially resistant females, Spencer noted. This increases the likelihood that resistant females will mate with potentially resistant males and produce resistant offspring.

Blended Bt and non-Bt seed -- refuge-in-a-bag mixes -- result in more even distribution of potentially resistant and susceptible rootworm beetles in fields compared to block refuges of 5 and 20 percent, according to Spencer. This increases the likelihood of their mating and decreases the potential for resistant offspring.

Farmers plant non-Bt hybrids in refuges to increase survival of Bt-susceptible rootworms and reduce the potential development of Bt-resistant insects.

Gray’s recommendations for the 2013 growing season in fields with potential Bt-resistant rootworm problems included planting hybrids with Bt traits or pyramid traits, planting non-Bt hybrids and using a soil insecticides, or rotating to another crop.

“Mix it up for rootworms. Don’t use the same approaches,” Gray said.

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