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Title:   Author  item
                                            Hibbard, Bruceitem
                                            Sappington, Thomasitem
                                            GASSMANN, AARON - IOWA STATE UNIVERSITYitem
                                            Miller, Nicholas
  Submitted to: Illinois Corn Breeders School Proceedings
                                    Publication Type: Proceedings
                                    Publication Acceptance Date: 2/2/2009
                                    Publication  Date: 3/2/2009
                                    Citation: Hibbard, B.E., Sappington, T.W., Gassmann, A.J., Miller, N.J. 2009. Rootworm Management with Genetically Modified Corn:  Current Status, Potential for Resistance, and a Look Toward the Future. Illinois Corn Breeders School Proceedings. p. 157-161.Interpretive Summary: Transgenic corn (Bt corn) that controls the western corn rootworm (WCR) is a viable alternative to insecticides for managing the most economically important pests of corn.  The agricultural industry has adopted a high dose/refuge strategy as a means of delaying the onset of insect resistance to transgenic crops, but the Bt corn products developed for WCR control have been introduced with less than high-dose.  A greenhouse method of rearing WCR on transgenic corn was used in which ~25% of previously unexposed larvae survived (compared to 1 to 4% in the field).  After three generations of full larval rearing on Bt corn, WCR larval survival was equivalent on Bt corn and untransformed corn in greenhouse trials, and in diet toxicity bioassays, the lethal concentration that kills 50% of the population (LC50)was 22-fold greater for the selected colony than the control colony (reared on untransformed corn).  After six generations of selection, 11.7-fold larvae from the selected colony survived Bt corn in the field than for the control colony.  The results suggest that rapid response to selection is possible, emphasizing the importance of effective refuges for resistance management.
                                Technical Abstract: To delay evolution of insect resistance to transgenic crops producing Bacillus thuringiensis (Bt) toxins, nearby "refuges" of host plants not producing Bt toxins are required in many regions. Such refuges are expected to be most effective in slowing resistance when the toxin concentration in Bt crops is high enough to kill all or nearly all insects heterozygous for resistance. However, Bt corn, Zea mays, introduced recently does not meet this "high-dose" criterion for control of western corn rootworm (WCR), Diabrotica virgifera virgifera.  A greenhouse method of rearing WCR on transgenic corn expressing the Cry3Bb1 protein was used in which ~25% of previously unexposed larvae survived relative to isoline survival (compared to 1 to 4% in the field).  After three generations of full larval rearing on Bt corn (Constant-exposure colony), WCR larval survival was equivalent on Bt corn and isoline corn in greenhouse trials, and in diet bioassays with Cry3Bb1 protein on artificial diet, the LC50 was 22-fold greater for the Constant-exposure colony than the Control colony.  After six generations of greenhouse selection, the ratio of larval recovery on Bt corn to isoline corn in the field was 11.7-fold greater for the Constant-exposure colony than the Control colony.  Removal from selection for six generations did not decrease survival on Bt corn in the greenhouse.  The results suggest that rapid response to selection is possible in the absence of mating with unexposed beetles, emphasizing the importance of effective refuges for resistance management.  Similar experiments were conducted with mCry3A and will also be discussed for the first time in context of published data with Cry34/35Ab1-selected and the above Cry3Bb1 WCR colonies.


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