The bollworm can be controlled effectively with Bacillus thuringiensis transgenic cotton (Bt cotton) which is being applied worldwide. However, the insecticidal efficacy is not stable. Here we give a summary of research progress for the mechanism of the altered insecticidal efficacy, factors affecting the expression of insect resistance, agronomic practices on regulation of insecticidal efficacy in Bt cotton. To realize the transgenic potential of Bt cotton cultivars, future research may be

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... ucted by increasing synthesis and reducing degradation of Bt protein to maintain high insecticidal ability in the transgenic cotton by agronomic management.