Bt is a natural pesticide we've been using on
our squash and cucumbers for a couple of years now. It was discovered
in 1901 in Japan by Ishiwata, but didn't make it to the commercial
market till the 1950's.
The strain we use only targets moth larvae, certain leaf eating worms
and Gypsy moths, and is safe for animals, humans, and other insects.
We use the liquid type that you mix with water. The shelf life is only
a few years in the bottle if you keep it out of the sun, but I've read
the powder version will last longer.
Bt is a living organism and must be mixed with water that is not too
hard (alkaline). If your water is hard you'll need to add some lemon
juice or other citric acid to bring the pH where you want it.
It takes a couple of days for the bug to die once it's ingested, but
this isn't a problem because they stop eating right away due to the
damage done to their stomach lining.