My tomatoes are frequently attacked by tomato hornworms. I read about using Bt, but I wasn't sure it would be effective. Doesn't it take days to work? I wondered if the worms would continue their destruction while I waited for the Bt to affect them. Also, does Bt prevent the hornwoms from attacking? I wondered if I should use it frequently to prevent the attacks -- my plants are often stripped before I realize the worms have arrived.