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Issue No. 11, Article 3/June 15, 2012

Western Corn Rootworm and Bt Resistance: Resistance Experts Weigh In

In the latest issue of the Journal of Economic Entomology (Vol. 105, No. 3, pp. 767-776), titled "Delaying Corn Rootworm Resistance to Bt Corn," two internationally respected entomologists have provided strong recommendations to the U.S. Environmental Protection Agency about refuge requirements for Bt hybrids that offer corn rootworm protection. Drs. Bruce Tabashnik and Fred Gould, professors of entomology at the University of Arizona and North Carolina State University, respectively, make these specific recommendations (p. 767):

"We conclude that the current refuge requirements are not adequate, because Bt corn hybrids active against corn rootworms do not meet the high-dose standard, and western corn rootworm has rapidly evolved resistance to Cry3Bb1 corn in the laboratory, greenhouse, and field. Accordingly, we recommend increasing the minimum refuge for Bt corn targeting corn rootworms to 50% for plants producing one toxin active against these pests and to 20% for plants producing two toxins active against these pests. Increasing the minimum refuge percentage can help to delay pest resistance, encourage integrated pest management, and promote more sustainable crop production."

I have extracted some key points from Tabashnik and Gould's thought-provoking journal article:

  • "A primary reason for increasing the refuge percentage for Bt corn producing Cry3Bb1 is that it does not meet the high-dose standard against its key target pest, western corn rootworm." (p. 768)
  • "[S]election experiments have demonstrated rapid evolution of western corn rootworm resistance to Cry3Bb1 corn when adequate refuges are not provided." (p. 769)
  • "Because resistance is expected to evolve faster as the initial resistance allele frequency increases, the higher than expected initial frequency of resistance to Cry3Bb1 also supports increasing refuge percentage to delay resistance." (p. 769)
  • "In local areas where resistance to Cry3Bb1 is detected, more stringent measures will be needed, such as not planting Cry3Bb1 corn until restoration of susceptibility to this toxin is demonstrated." (p. 770)
  • "Also similar to results with Cry3Bb1, western corn rootworm quickly evolved resistance to corn producing either Cry34/35Ab1 or mCry3A in selection experiments." (p. 770)
  • "[T]he similar values of realized heritability for corn rootworm resistance to Cry3Bb1 and Cry34/35Ab1 suggest that it also could readily evolve resistance to these Bt toxins in the field." (p. 771)
  • "Thus, we recommend a 50% refuge for Bt corn plants producing a single toxin that kills corn rootworms, whether the toxin is Cry3Bb1, Cry34/35Ab1, or mCry3A." (p. 771)
  • "[F]or populations of western corn rootworm resistant to Cry3Bb1, plants producing Cry3Bb1 and Cry34/35Ab1 are not effective pyramids because of the reduced efficacy of Cry3Bb1." (p. 772)
  • "Key IPM recommendations for corn rootworms include crop rotation, rotation of Bt corn hybrids producing different toxins (e.g., do not plant Cry3Bb1 corn year after year in the same field), and judicious use of insecticides." (p. 772)
  • "Initial increases in refuge percentage may be limited by the availability of seed for corn that does not produce Bt toxins active against corn rootworms. If so, we propose phased refuge increases beginning in areas where resistance has been detected, resistance is suspected, or resistance is most likely based on historical planting patterns." (p. 772)
  • "We emphasize that these are hypothetical scenarios and, in principle, growers can make such conditions less likely by using IPM to reduce the reproductive output and population density of the pest." (p. 772)
  • "The lag time for increasing refuge seed production has been overlooked in the remedial action plans to address resistance to Bt corn." (p. 773)
  • "The situation with western corn rootworm illustrates that changes in the regulatory framework are needed to achieve faster responses to evidence of field-evolved resistance." (p. 773)

At this point in the season, it remains unclear whether more reports of greater-than-expected root injury to Bt corn expressing the Cry3Bb1 protein will occur across the Corn Belt. I will share any reports provided to me with readers. It also is very murky what response, if any, the U.S. Environmental Protection Agency will make if additional cases of Bt failures begin to occur. Tabashnik and Gould remind readers of their article that a majority of a 2002 Scientific Advisory Panel recommended a larger refuge (50% minimum) for the initial Bt corn rootworm hybrids expressing the Cry3Bb1 protein that entered the marketplace in 2003. This recommendation was not implemented; instead, a 20% structured refuge was put in place matching that for Bt hybrids in the Corn Belt targeted at lepidopteran pests such as the European corn borer.

But there is a critical difference: hybrids used for lepidopterans are high dose. Some will say, All this is water under the bridge--now what? To prolong the effectiveness of this tremendous technology as we move forward, growers must take more seriously than ever before the importance of integrating management tactics for corn rootworms. Thinking longer term is imperative. Technological transgenic advances beyond Bt proteins for corn rootworm control will not be available commercially until, most likely, the end of this current decade.--Mike Gray

Mike Gray

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