Tomato hornworm are heavy feeders and 1-2 worms can defoliate a five-foot tomato plant in less than three days.
Predatory bugs and parasitic wasps do attack the larvae (Look for white cocoons on the body of the hornworm - do NOT kill the parasitized hornworm as that also reduces the number of future predators on the hornworm.), and helps keep hornworm populations under control. For none parasitized larva, physically remove the larva from the leaves and crush them by stepping on them. Farmers should monitor the moth using blacklight trap and spray with insecticides recommended in your state.