BT. Bacillus thuringiensus. If you broadcast it you'll kill the butterflies in your yard, but if you mix water with a few teaspoons in a 1 to 2 gallon backpack sprayer, along with some organic fertilizer or compost tea and maybe add a little molasses and spray it on the plants you'll make the plants happy and give the hornworms a meal that they can't digest and they stop eating and die. If you do this now before they're big then you probably won't spot the little ones when they are there and then die. If you do it when there are big ones working then you'll sometimes see them in place, then slowly loose their grip and kind of dangle from the limb before they shrivel and drop off. If you do it every week or 10 days you should keep most of them away. I'm patrolling for them now, because this is when they start turning up.

Thuricide is a brand, and Green Light has it also. Keep it in the fridge once you open it, but don't let it freeze. You can get granular mixes, I think, but for this purpose, you want it on the leaves that they're going to munch, so spray it.

SRS