Abstract: The effects of larval midgut fluid on the toxicity of Bacillus thuringiensis parasporal crystal were reviewed in this paper. The characteristics of larval midgut juice, including pH value, the composition and activity of proteases, affect not only the crystal solubility and efficiency of protoxin activation，but also its insecticidal specificity. On the other hand, excessive degradation of protoxins in midgut juices was responsible for the low susceptibility of some insects. On the contrary, the retardation of protoxin activation in midgut juice contributes to the resistance of some insects against Bt. Moreover, the precipitation effect of gut juice on protoxins was another factor to lower Bt toxicity.

Key words: Bacillus thuringiensis, midgut fluid, protease, parasporal crystal, insecticidal activity