WCR resistance confirmed in three more Illinois counties
With the addition of three more counties, a total of five Illinois counties have now reported confirmed cases of field-evolved resistance to Bt corn (Cry3Bb1 protein) by western corn rootworm, and a University of Illinois entomologist said now is the time to move forward aggressively with integrated pest management (IPM) strategies.
In August 2012, in cooperation with an Iowa State University lab, Mike Gray said he confirmed resistance in Henry and Whiteside counties in northwestern Illinois. Working with Joe Spencer of the Illinois Natural History Survey and utilizing single-plant bioassays with larvae collected last summer, Gray said resistance to the Cry3Bb1 protein has now also been confirmed in McDonough, Mercer, and Sangamon counties.
The suspected Bt-resistant larvae collected last summer were exposed to a hybrid expressing the Cry3Bb1 protein as well as its corresponding isoline (not expressing the Cry3Bb1 protein), Gray said. Larvae obtained from three control colonies of western corn rootworms also were used in the bioassays. The control larvae had never been exposed to corn rootworm Bt protein and were provided by the USDA North Central Agricultural Research Laboratory in Brookings, S.D.
Gray said there continues to be some controversy regarding the most appropriate procedures (plant-based vs. diet-based bioassays) that should be used to confirm whether or not resistance to a Bt protein has developed by an insect population. Citing a University of Arizona study that defines key terms regarding resistance to Bt crops and pesticides, Gray offered the following definitions that are useful in communicating with producers who have experienced greater than expected levels of damage to rootworm Bt hybrids in their fields. They include:
- “Resistance: genetically based decrease in susceptibility to a pesticide”
- “Field-evolved resistance (= field-selected resistance): genetically based decrease in susceptibility to a pesticide in a population caused by exposure to the pesticide in the field”
- “Practical resistance (= field resistance): field-evolved resistance that reduces pesticide efficacy and has practical consequences for pest control”
“It has become increasingly evident that some producers have experienced a loss of efficacy with some Bt hybrids in their fields in recent years,” Gray said. “To date, most of those fields have been in continuous corn production, and producers have not rotated traits. It’s also clear that ‘practical consequences’ have resulted due to the loss of efficacy associated with some Bt hybrids in problem fields.
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