Iowa reports first in-field resistance to Bt corn

The first documented case of in-field resistance to Bt corn targeting rootworms has been confirmed recently in Iowa. Aaron Gassman, Iowa State University entomologist, and other researchers had received multiple reports of high damage to Bt corn in northeastern Iowa.

The group collected adults and eggs from the area. Rearing the larvae in the laboratory on Bt hybrids revealed that the larvae were able to survive on Bt corn hybrids expressing the Cry3Bb1 toxin at levels similar to survival on non-Bt corn.

It has been speculated that insects would eventually build up resistance to the Bt protein over time since corn rootworm Bt toxins are not high dose toxins, which means many larvae survive exposure and reach adulthood.

Christian Krupke, Purdue University, entomologist reports, “Hybrids expressing this toxin include those formerly labeled as Yieldgard RW and VT3 hybrids. This toxin is also one of the proteins found in SmartStax hybrids. The good news is that the study tested the other major toxins deployed in North America against this pest, Cry34/35 (found in Herculex hybrids targeting rootworms and also in SmartStax hybrids), and no enhanced survival was found. Although Cry3bb1 and Cry34/35 toxins are different, they are similar enough that cross-resistance (where surviving exposure to one toxin confers some level of survival to another), was a possibility worth investigating. No evidence of cross-resistance was found in these rootworm populations.”

The question for entomologists now becomes determining how these insects are able to survive toxin exposure by discovering what combination of physiological and behavioral traits are occurring.

The majority of corn planted in the U.S. is Bt corn, and the Cry3bb1 toxin is the major one deployed against rootworms. There is no "putting the genie back in the bottle," and resistance in these areas is a problem that won't go away.

This situation will require producers, agronomists and crop advisors to be more diligent in scouting crops to be on the front line of defense against this new development.

“The fields to watch are those where the selection pressure upon the pest is highest: namely continuous Bt corn in areas of high rootworm pressure,” Krupke said. “Some northwestern and north-central Indiana fields may fit this description. In general, however, we have a more diverse cropping system than Iowa with a large proportion of fields rotating corn with soybeans. That helps delay resistance. Planting the recommended refuge certainly helps, and although compliance with the refuge requirements has been falling in recent years, this serves as a stark reminder of how important it is.”

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This is unfortunate but hardly surprising. The trait has been successfully used for 8 years since its commercial release and is favourably comparable to chemical usage i.e. soil application of CRW control chemicals. Insect-plant interactions have evolved over centuries where under natural conditions the plant would have been less palatable and the insect would have looked for alternatives for its food source. Farming & cropping technology in that sense will always need to evolve pest-management strategies to keep up the battle always slightly in favour of the farmer. So resistance management is a given in this industry.