Tomato Hornworm (Manduca quinquemaculata) is one of the largest caterpillars found in the vegetable garden, reaching up to about four inches long. Despite their size, their green colouring makes them remarkably difficult to spot among tomato foliage. 

They feed voraciously on leaves, usually beginning near the top of the plant, and large hornworms can strip entire stems bare. They may also chew holes in developing tomatoes. Dark green or black droppings on the leaves are often the first clue that one is hiding nearby.

The caterpillars eventually leave the plant and burrow into the soil, where they pupate. They emerge as magnificent five-spotted hawkmoths—large, powerful-flying sphinx moths with narrow wings and five yellow-orange spots along each side of the abdomen. The moths mate and lay pale green eggs on tomato and other nightshade-family plants, beginning the cycle again.

How to Fix It
Look for droppings: Dark pellets on leaves or the ground can help you locate a well-camouflaged hornworm above them.
Hand-pick them: For a home garden, physically removing hornworms is usually all that's necessary.
Check plants regularly: Look especially at the upper foliage and along stripped stems.
Leave parasitized hornworms alone: If a hornworm is covered with small white parasitoid-wasp cocoons, leave it in place. The wasps are beneficial and will help control future hornworms.
Remove nightshade weeds: Horsenettle, jimsonweed and other nightshade-family weeds can serve as alternate hosts.
Consider the moth: If damage is tolerable, remember what that enormous green bastard becomes — a spectacular native hawkmoth.