ARS | Publication request: Potential shortfall of pyramided Bt cotton for resistance management

Submitted to: Proceedings of the National Academy of Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 15, 2013
Publication Date: N/A

Interpretive Summary: There are an increasing number of reports on crop pests overcoming insecticidal proteins produced by the genetically modified crop plants. The stacking strategy has been utilized to develop crop cultivars that produce two or more bacterial toxins that kill the same pest. In the United States, cotton cultivars with two bacterial toxin genes have replaced the cultivars with only one-toxin gene. Although cotton plants producing two toxins are likely to be more durable than the plants producing only one toxin, the extent of this advantage may not always be true. One key assumption favoring the cultivars producing two toxins is that these cultivars will kill insects selected for resistance to one toxin. In the present research, we tested this assumption for a major pest-cotton bollworm on transgenic cotton plants producing two toxins. Selection with one-toxin cotton plants actually increased the bollworm survival on the cotton plants with two toxins, which contradicts the assumption. The concentration of the two toxins also declined during the growing season, which would also exacerbate this problem. Furthermore, meta-analysis of the results from 21 selection experiments with eight species of lepidopteran pests shows that some cross-resistance typically occurs between the two toxins tested. Incorporation of empirical data into simulation models shows that the observed departure from ideal conditions could greatly reduce the benefits of the stacking strategy for pests like cotton bollworm that have inherently low susceptibility to bacterial toxins and have been exposed extensively to one of the toxins in the cotton cultivars before two-toxin cultivars are adopted. To substantially delay pest resistance to bacterial toxins in such cases, complementary approaches may be needed to augment the stacking strategy, including relatively large refuges of non-transgenic host plants and multiple control tactics as part of integrated pest management.

Technical Abstract: To delay evolution of pest resistance to transgenic crops producing insecticidal proteins from Bacillus thuringiensis (Bt), the "pyramid" strategy uses plants that produce two or more toxins that kill the same pest. In the United States, two-toxin Bt cotton has replaced one-toxin Bt cotton. Although two-toxin plants are likely to be more durable than one-toxin plants, the extent of this advantage depends on several conditions. One key assumption favoring success of two-toxin plants is that they will kill insects selected for resistance to one toxin. Here we tested this assumption for a major pest, Helicoverpa zea, on transgenic cotton producing Bt toxins Cry1Ac and Cry2Ab. Selection with Cry1Ac increased survival on two-toxin cotton, which contradicts the assumption. The concentration of Cry1Ac and Cry2Ab declined during the growing season, which would tend to exacerbate this problem. Furthermore, meta-analysis of results from 21 selection experiments with eight species of lepidopteran pests shows that some cross-resistance typically occurs between Cry1A and Cry2A toxins. Incorporation of empirical data into simulation models shows that the observed deviations from ideal conditions could greatly reduce the benefits of the pyramid strategy for pests like H. zea that have inherently low susceptibility to Bt toxins and have been exposed extensively to one of the toxins in the pyramid before two-toxin plants are adopted. To substantially delay resistance in such cases, complementary approaches may be needed to augment the pyramid strategy, including relatively large refuges of non-Bt host plants and multiple control tactics as part of integrated pest management.