Bt toxin kills insect by

Soil bacterium Bacillus thruingiensis (Bt) produces proteins that kill certain insects like lepidopterans (tobacco budworm, armyworm), coleopterans (beetles) and dipterans (flies, mosquitoes). Bacillus thuringinesis forms some intracellular protein crystals. These crystals contain a toxic insecticidal protein. The Bt toxin protein exists as an inactive toxin and is converted into an active form due to the alkaline pH of the alimentary canal that solubilities the crystals. The activated toxin binds to the surface of midgut epithelial cells and creates pores which cause cell swelling and lysis and finally cause the death of the insect.