Toxicity of Bacillus thuringiensis insecticidal proteins for Helicoverpa armigera and Helicoverpa punctigera (Lepidoptera: Noctuidae), major pests of cotton.

The susceptibilities of the major pests of cotton in Australia, Helicoverpa armigera and Helicoverpa punctigera, to some insecticidal proteins from Bacillus thuringiensis were tested by bioassay. A commercial formulation, DiPel, and individual purified insecticidal proteins were tested. H. armigera was consistently more tolerant to B. thuringiensis insecticidal proteins than was H. punctigera, although both were susceptible to only a limited range of these proteins. Only Cry1Ab, Cry1Ac, Cry2Aa, Cry2Ab, and Vip3A killed H. armigera at dosages that could be considered acceptable. There was no significant difference in the toxicities of Cry1Fa and Cry1Ac for H. punctigera but Cry1Fa had little toxicity for H. armigera. The five instars of H. armigera did not differ significantly in their susceptibility to DiPel on the basis of LC(50). However, there were significant differences in the susceptibility to Cry1Ac and Cry2Aa of three strains of H. armigera. Bioassays conducted with Cry1Ac and Cry2Aa showed that there was a small but significant negative interaction between these delta-endotoxins.