Location: Pest Management and Biocontrol ResearchTitle: Efficacy of genetically modified Bt toxins alone and in combinations against pink bollworm resistant to Cry1Ac and Cry2Ab) Author
Submitted to: PLoS One
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/3/2013
Publication Date: 11/7/2013
Publication URL: http://handle.nal.usda.gov/10113/57982
Citation: Tabashnik, B.E., Fabrick, J.A., Unnithan, G.C., Yelich, A.J., Masson, L., Zhang, J., Bravo, A., Soberon, M. 2013. Efficacy of genetically modified Bt toxins alone and in combinations against pink bollworm resistant to Cry1Ac and Cry2Ab. PLoS One. 8(11):e80496. Interpretive Summary: The pink bollworm, Pectinophora gossypiella, is an important insect pest of cotton whose populations in the U.S. and adjacent areas of northern Mexico are currently suppressed by an area-wide eradication program. A key component of the program involves the use of genetically modified or transgenic cotton producing specific toxins (insecticidal proteins) of Bacillus thuringiensis (Bt). These Bt toxins kill a number of major insect pests, including the pink bollworm, while having no impact on beneficial insects or other plants or animals. However, pink bollworm and some other insect pests have become resistant to these Bt toxins, which threatens the long-term success of genetically-modified Bt crops. Although resistance can be delayed through use of plants producing more than one Bt toxin, laboratory studies have shown that pests can develop resistance to combinations of these natural Bt proteins. We showed that novel modified Bt toxins, altered in the laboratory from natural Bt proteins, are effective against both resistant and susceptible pink bollworm. When fed to pink bollworm in the laboratory, some combinations of natural and modified Bt toxins were more effective than expected, compared with their combined effects when they were fed separately. These results suggest the modified Bt toxins may be effective for controlling Bt-resistant populations of these pests and could prolong the success of this critically important pest management tool.
Technical Abstract: Evolution of resistance in pests threatens the long-term success of transgenic crops that produce insecticidal proteins from Bacillus thuringiensis (Bt). Previous work showed that genetically modified Bt toxins Cry1AbMod and Cry1AcMod effectively countered resistance to native Bt toxins Cry1Ab and Cry1Ac in some pests, including pink bollworm (Pectinophora gossypiella). Here we report that Cry1AbMod and Cry1AcMod were also effective against a laboratory-selected strain of pink bollworm that was highly resistant to Cry2Ab as well as Cry1Ab and Cry1Ac. Mean resistance ratios based on the concentration of toxin killing 50% of larvae for the resistant strain relative to a susceptible strain were 210 for Cry2Ab, 270 for Cry1Ab, and 310 for Cry1Ac, but only 1.6 for Cry1AbMod and 2.1 for Cry1AcMod. To evaluate the interactions among toxins, we tested combinations of Cry1AbMod, Cry1Ac, and Cry2Ab. For both the resistant and susceptible strains, the net results across all concentrations tested showed slight but significant synergism between Cry1AbMod and Cry2Ab, whereas the other combinations of toxins did not show consistent synergism or antagonism. The results suggest that the modified toxins might be useful for controlling populations of pink bollworm resistant to Cry1Ac and Cry2Ab.