P.AllenSmiths Blog. He recommends a "bio" pesticide. Which is all good and well, but
not selective. It can also inhibit the growth of good bugs too.
I recommend interplanting tomatoes with BASIL. I've interplanted basil and tomatoes for over five years and have not had one tomato horn worm. IF you didn't plant basil with your toms this year, you can always cut your basil and lay the stems, leaves and all, on the ground at the feet of the tomato plant and water in. Leave the basil there to compost into the soil. A rhubarb tea or tansy tea should also help.