Evaluation of the synergistic activities of Bacillus thuringiensis Cry proteins against Helicoverpa armigera (Lepidoptera: Noctuidae)

• Susceptibility of Helicoverpa armigera to Bacillus thuringiensis Cry mixtures.
• Evaluation of all 15 binary combinations of six different Cry proteins.
• Cry1Ac–Cry2Aa and Cry1Ac–Cry1Ca mixtures were the most toxic to neonate larvae.
• Antagonistic interactions between Cry1Aa and Cry1Ab and between Cry1Aa and Cry1Ac.
• Synergism was observed between Cry1Ab and Cry1Ca and between Cry9Aa and Cry1Ca.

With the aim of identifying Cry proteins that would be useful in the management of the economically important lepidopteran pest Helicoverpa armigera, the larvicidal activities of binary combinations (1:1 ratios) of six Cry proteins (Cry1Aa, Cry1Ab, Cry1Ac, Cry1Ca, Cry2Aa and Cry9Aa) were evaluated against H. armigera neonate larvae using droplet feeding bioassays. Determination of the LD50 values of individual Cry proteins and mixtures of Cry proteins enabled assessment of the nature of the interactions between Cry proteins in H. armigera. There was a more than 6000-fold difference between the LD50 values of the Cry protein mixture with the lowest larvicidal activity and the mixture with the highest larvicidal activity. Cry1Ac and Cry2Aa mixtures and Cry1Ac and Cry1Ca mixtures had the highest larvicidal activity against H. armigera, with Cry1Ac and Cry1Ca interacting synergistically. Differences in the magnitudes of the antagonistic interactions observed for different binary mixtures of Cry1A-class proteins are consistent with a model of more than one binding site for some Cry1A-class proteins in H. armigera. Binary combinations of Cry1A-class and Cry9Aa proteins showed additive interactions in neonate larvae of H. armigera, whereas combinations of Cry1Ca and Cry9Aa were statistically synergistic. The results suggest that products containing mixtures of Cry1Ac and Cry2Aa or Cry1Ac and Cry1Ca may be useful components of H. armigera pest management programs.

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