Helicoverpa armigera is one of the most important pests worldwide. Transgenic crops with toxin genes from Bacillus thuringiensis (Bt) have been deployed on a large scale to control this pest. The insecticidal activity of Bt is probably influenced by the insect midgut microbes, which vary across crop hosts and locations. Therefore, we examined the role of gut microbes in pathogenicity of Bt toxins in the H. armigera. Antibiotic cocktail was used for the complete elimination of the H. armigera gut microbes. Activated Cry1Ac, Bt formulation, and transgenic cotton resulted in larval weight loss and increase in mortality, but pretreatment of larvae with antibiotic cocktail significantly decreased larval mortality and increased the larval weight gain. Activated Cry1Ac and Bt formulation inhibited the activity of proteases in midgut of H. armigera larvae but showed no such effect in the larvae pretreated with antibiotic cocktail. Five protease bands in activated Cry1Ac and two in Bt formulation-treated larvae were inhibited but no such effect in the larvae pretreated with antibiotic cocktail. Cry1Ac protein was detected in Bt/Cry1Ac protoxin-fed larval gut extract in the absence of antibiotic cocktail, but fewer in larvae pretreated with antibiotic cocktail. The activity of antioxidant enzymes and aminopeptidases increased in larvae fed on Bt toxin, but there was no significant increase in antioxidant enzymes in larvae reared on toxin protein in combination with antibiotic cocktail. The results suggest that gut microbes exercise a significant influence on the toxicity of Cry1Ac and Bt formulation in H. armigera larvae. The implications of these results have been discussed in relation to development of insect resistance to Bt transgenic crops deployed for pest management.

Helicoverpa armigera (Hubner) (Lepidoptera: Noctuidae) is one of the most important pest worldwide. Bacillus thuringiensis (Bt) toxins have used as a biopesticide or deployed in transgenic plants for controlling this pest. We examined the biological activity of Cry1Ac, Cry1Ab and Bt formulation in different larval instars of H. armigera with respect to larval development and proteolytic activity to pinpoint the most susceptible instar, and the insect response to ingestion of Bt toxins. In the presence of Bt toxins, the larval mortality and weight loss increased in a dose-dependent manner, and the maximum effect was observed in neonates. Active Cry1Ac toxin resulted in greater mortality and weight loss in all the larval instars. Total protease, trypsin and chymotrypsin activities declined in the presence of Bt toxins as compared to the untreated control in all the larval instars. Ten protease isozymes were observed in the untreated control larvae in second, third and fourth instars. Maximum protease isozymes inhibition was observed in Bt toxin fed neonates. Inhibition of protease activity increased with the concentration of Bt toxins. In presence of Bt toxins, aminopeptidase activity increased from II to IV instar and alkaline phosphatase activity decreased from II to III and then increased in IV instar. The pathogenicity of Bt was greater in early larval instars of H. armigera than the later instars, suggesting that application of Bt formulation or deployment of Bt toxins in transgenic plants should be directed against the early larval instars.