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First In-Field Resistance to Bt Corn Targeting Rootworms Documented in Iowa

Gil Gullickson 08/10/2011 @ 1:27pm Crops Technology Editor for Successful Farming magazine/Agriculture.com

This report, written by Christian Krupke, Purdue University Extension entomologist, appeared this month in Purdue’s Pest & Crop Newsletter.

• Following reports of high damage in Iowa, lab studies revealed resistance to Bt hybrids expressing Cry3Bb1 toxin (found in Monsanto hybrids targeting rootworms).
• Field locations where resistance was documented were characterized by high rootworm pressure, with a history of continuous Bt corn planting.
• This highlights the importance of refuge planting, and Indiana producers should remain vigilant.

Regular readers of Pest&Crop may recall that we have mentioned a few times that corn rootworm Bt toxins are not high dose toxins – meaning that many larvae survive exposure and reach adulthood on each acre of these hybrids. This is one of the reasons that the refuge is so critical in stewardship of this valuable IPM tool. Those points were underscored by the publication last week of the research findings of Iowa State University entomologist Aaron Gassman and co-authors. After receiving persistent 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. 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 next questions to tackle involve untangling the mechanisms behind how these insects are able to survive toxin exposure – what combination of physiological and behavioral traits are at work here? Understanding these mechanisms will undoubtedly help find solutions and plan future control technologies.

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