Bt cotton

Bacillus thuringiensis forms Cry protein crystals in an inactive protoxin form.
The alkaline pH of the insect gut solubilises the crystals and converts the protoxin into the active toxin.

Bt cotton carries cry genes from $Bacillus\ thuringiensis$; the proteins encoded by the genes cryIAc and cryIIAb control the cotton bollworms.
Because the crop is protected from bollworms, the loss is reduced and the yield is high, with less use of insecticide.
The toxin is specific: Bt toxins are insect-group specific, and those in Bt cotton act on lepidopteran bollworms, not on aphids, beetles or dipterans (flies and mosquitoes).
(The protein encoded by cryIAb controls the corn borer.)
Hence Bt cotton shows high yield and resistance to bollworms.