The tomato hornworm is a large pale-green caterpillar with white and black markings. The caterpillar can reach 3 1/2 to 4 inches in length when fully mature. A spike on one of the "tail" gives the caterpillar the name hornworm. The hornworm feeds on the leaves and new stems of the tomato plant, causing extensive damage. During July and August they also occasionally feed on the fruit.
The hornhorm can cause extensive damage in a short period of time; it is a ferocious eater. This will sometimes catch gardeners off guard, because the tomato plant is fine one day and almost devoured a couple days later.
Hornworms are the larvae for the sphinx or hawk moth. The adult moth lays eggs on the undersides of tomato leaves in late spring.
The eggs hatch in six to eight days and the larvae pass through five or six stages, maturing fully in three to four weeks. The fully grown larvae then burrow into the soil to pupate.
The pupa may remain in the soil all winter and emerge as a moth the following spring, or, if the weather conditions are suitable, the moth may emerge from the pupa in two to four weeks. The moths emerge from the soil, mate, and then begin to deposit the eggs of the next generation on tomato plants.
There are a number of natural factors which help to control tomato hornworm populations. One of the most common parasites in home gardens is a small, parasitic braconid wasp. Many wasp larvae feed inside the hornworm, eventually killing the caterpillar. The cocoons containing pupae of these wasps are visible as small white projections on the hornworm's body. Larvae with cocoons sometimes move sluggishly, but seldom cause additional feeding damage. They should be left on the plant so emerging adult parasites can attack other hornworms.
Hand-picking the hornworms from infested plants in the garden provides safe and effective control in small gardens. It is often surprisingly difficult to find these large larvae on the plants. Their large brown droppings are generally readily apparent, however. Once you find one larva, others are much more easily found.