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How Resistance Could Develop in Corn Rootworm (CRW)


The level of understanding of resistance development in CRW is low compared with that of ECB, however many studies are underway. Several factors still need to be investigated. There are a few key differences between the two insects in their exposure to resistance selection. CRW have two life stages exposed to Bt; larva and adult. Also, the dose that CRW is exposed to is low to moderate for the CRW Bt events, as opposed to high dose in the ECB Bt events. Though CRW Bt corn hybrids are effective at protecting the roots from larval injury, some adult emergence occurs from these Bt fields. Another biological difference is that CRW mate in the same field as they emerge, reducing the chance of genetic mixing from nearby non Bt fields.

A study done by Gassman et al. 2011 announced the first report of field evolved resistance of Western corn rootworm to Cry3Bb1 hybrids in parts of the US. Since this report was published, the number of fields with suspected resistance has increased and the geographical range of the resistance spreads across much of the MidWest US. Though no resistance has been confirmed here in Canada, these incidences point out the need for crop rotation and improved resistance management.

For these reasons, entomologists recommend a proactive approach to resistance management. Growers are encouraged to consider crop rotation, as it is the most effective method for controlling corn rootworm. Growers should avoid using Bt hybrids that contain only one trait to control corn rootworm and move towards stacked and pyramid hybrids that provide multiple traits against the pest.  The current requirements for refuge size and configuration for CRW Bt hybrids are similar to the ECB refuge requirement. However, the CRW refuge must be placed within or adjacent to the CRW Bt corn hybrid to accommodate the limited movement of CRW adults at mating.