Just keep a sharp eye out for denuded branches on your tomato plants, then look carefully for the caterpillar. They're not hard to find since they're so huge and bright green. If you don't get them, they can completely de-foliate a large plant in a few days. Mind you, tomatoes are so resilient, at the school garden we left one plant for the hornworms, as a learning experience for the kids. Then when they went underground to pupate that completely naked plant sprouted new leaves and went right on to produce tomatoes. (we did help it along with some extra fert and TLC) So moral of the story is, they don't always kill the plants. But they are voracious.
