What triggers activation of protoxin to active Bt toxin of Bacillus thuringiensis in a boll worm?

Alkaline pH of gut triggers activation of protoxin to activate Bt toxin of Bacillus thuringiensis in bollworm.

The toxin is produced in B. thuringiensis in an inactive form known as protoxin. The produced protoxins which after ingested by the insect, the cotton bollworm gets converted to the active form, toxin. This is due to the alkaline pH of the insect's alimentary canal. This simultaneously induces swelling in the gut and the worm dies.