Even if you keep Bascillus turingiensis localized in the initial spraying, it will spread far and wide and kill an almost infinite number of moth and butterfly larvae for years to come.

What happens is the infected caterpillar becomes a shell filled with bacterial spores. The case ruptures and billions of spores escape and are carried away by the wind. They will re-infect other caterpillars forever.

The BT developed in labs is so much more virulent that the original strains that were found in nature that the moths and butterflies don't have a chance.

The US moth and butterfly population has been almost destroyed by BT, not by habitat destruction or chemical pesticides as some people say. The number of butterflies is down to about 1% of what we had in the middle of last century, mostly due to the use of Bascillus turingiensis.