Some strains of Bacillus thuringiensis produce proteins that kill certain insects such as lepidopterans (tobacco budworm, armyworm), coleopterans (beetles) and dipterans (flies, mosquitoes).

B. thuringiensis forms protein crystals during a particular phase of their growth. These crystals contain a toxic insecticidal protein.

The Bt toxin protein exist as inactive protoxins but once an insect ingest the inactive toxin. It is converted into an active form of toxin due to the alkaline $\mathrm{pH}$ of the gut which solubilise the crystals. The activated toxin binds to the surface of midgut epithelial cells and create pores that cause cell swelling and lysis and eventually cause death of the insect.

Specific $Bt$ toxin genes were isolated from Bacillus thuringiensis and incorporated into the several crop plants such as cotton.

The choice of genes depends upon the crop and the targeted pest as most. $Bt$ toxins are insect group specific.

The toxin is coded by a gene cry $IAc$ named cry. There are a number of them.

For example, the proteins encoded by the genes cry $IAc$ and cry $IIAb$ control the cotton bollworms that of cry $IAc$ controls corn borer.