Degradation of Bacillus thuringiensisδ-endotoxin in host insect gut juice

When Bacillus thuringiensis subsp. kurstaki HD-1 and subsp. sotto δ-endotoxins were activated from their protoxins in 50% Choristoneura fumiferana gut juice, the amount of 60–65-kDa toxin protein produced was much less than that produced in 1% gut juice. In bioassays against Bombyx mori larvae, the toxicity of activation mixtures with 50% C. fumiferana gut juice was less than activation mixtures with 1% gut juice. Differences between 50% and 1% B. mori gut juice in the amount of activated toxin produced from protoxin were much less pronounced. The present study may explain, in part, why B. thuringiensis subsp. kurstaki HD-1 is more toxic to B. mori than to C. fumiferana larvae, and suggests that the enhancement of B. thuringiensis δ-endotoxin insecticidal activity by protease inhibitors may be the result of reduced toxin degradation in the insect midgut.