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:

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

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