The Tobacco Hornworm would be better called the Tomato Hornworm, since this is most commonly observed munching on garden tomato plants. The caterpillars are incredible eaters and can quickly strip a tomato plant of all its leaves — and it eats young shoots and green tomatoes, too! The adults are called the Carolina Sphinx Moth — they are large and somewhat attractive flower feeders, less commonly seen than the caterpillars.

There is another moth caterpillar that goes by the name of Tomato Hornworm (Manduca quinquemaculata), but this is less commonly seen on tomatoes than Manduca sexta. They are similar, but you can tell them apart fairly easily: Manduca sexta has 7 white slashes on its side, while Manduca quinquemaculata has 8 white V-shaped marks (a slash with an extra horizontal line).

On tomato in my garden in Durham, NC 9/3/2013.

Unfortunately for the caterpillars, but fortunately for the tomato plants, many or even most of them get parasitized by a braconid wasp (Cotesia congregata) that lays numerous eggs inside the caterpillars; the wasp larvae burrow out and form white cocoons on the outside of the body. This eventually kills the caterpillar. If you want to better protect your tomatoes from the hornworms, only pluck off the healthy caterpillars.

Durham, NC 9/3/2013.

Caterpillars will also feed on other members of the Solanaceae (nightshade family), such as tobacco, horsenettle, and ground-cherry (tomatillo).

Durham, NC 9/3/2013.

Durham, NC 9/3/2013.