Comparative study of Bacillus thuringiensis Cry1Aa and Cry1Ac delta-endotoxin activation, inactivation and in situ histopathological effect in Ephestia kuehniella (Lepidoptera: Pyralidae).

A comparative study of different steps in the mode of action of the individual Bacillus thuringiensis kurstaki BNS3 Cry1Aa and Cry1Ac delta-endotoxins on E. kuehniella larvae was performed in order to investigate the origin of the difference in the response of this larvae to each of the latter. Proteolytic activation was shown to be one of the main steps impaired in E. kuehniella tolerance to Cry1Aa. The absence of two proteinase activities as well as an altered activity level observed in the case of Cry1Aa would be the consequence of proteinase-mediated tolerance of E. kuehniella to this toxin. In situ binding and histopathological effect analyses allowed concluding that the binding of the toxin to BBMV receptors is the key step in E. kuehniella tolerance to Cry1Aa toxin. The latter was slightly bound to apical membranes of epithelial cells that remained intact, whereas Cry1Ac was tightly bound to completely damaged cells basal membranes.