The relatively low susceptibility of Helicoverpa armigera to Cry1Ac, its history of resistance to chemical insecticides and the seasonal decline in expression of Cry1Ac in transgenic cotton necessitated the development of cotton expressing two insecticidal proteins to provide sustainable control of this multinational pest. To manage the resistance issue, it was essential that the second insecticidal protein have a significantly different mode of action to Cry1Ac. A common feature of resistance to Cry1A proteins in several species as well as H. armigera has been a change in the binding site. A study of binding sites for some Cry proteins in the brush border membrane vesicles (BBMV) of H. armigera and Helicoverpa punctigera was undertaken. The binding affinity for Cry1Ac was higher than for Cry1Ab, matching their relative toxicities, and Cry1Ac and Cry1Ab were found to share at least one binding site in both H. armigera and H. punctigera. However Cry2Aa did not compete with Cry1Ac for binding and so could be used in transgenic cotton in combination with Cry1Ac to control H. armigera and manage resistance. Variation in the susceptibilities of three different H. armigera strains to Cry1Ac correlated with the parameter B max /K com .
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