Tomato fruitworms may take an occasional bite out of tomato leaves, but they are primarily interested in flowers and fruit. When a tomato plant is completely defoliated by a caterpillar, the culprit is the tomato hornworm.
Tomato hornworms are the larvae of the hawk moth. These mottled grey moths have a large, furry body, orange spots on their sides and a wingspan of up to 5 inches. Hawk moths don't harm tomato plants, but their presence tells you to be on the lookout for hornworms.
Hornworms are pale green with white V-shaped markings down their sides and "horns" at the tail end. They grow up to 5 inches long before entering the pupal stage. Hornworms are destructive and can completely defoliate a tomato plant.
Hornworms have several natural predators. Green lacewings and ladybeetles attack small, young hornworms. Braconid wasps lay eggs on young hornworms and the wasps feed on the hornworms when they hatch. You can recognise a parasitised hornworm by the white cocoons that project from his back. Leave parasitised hornworms in your garden to preserve the beneficial wasps.
Handpicking is an effective control measure. Hornworms are harmless to humans but you may prefer to wear gloves when handpicking. Bt (Bacillus thuringiensis) is effective if applied when the hornworms are young. Apply Bt according to the package's directions.