Hornworms are the large, leaf-eating larval stage of hawk moths or sphinx moths. These plump, green caterpillars are well-known to gardeners and agricultural producers for their voracious appetites. The adult moths are strong, nocturnal flyers that resemble small hummingbirds as they hover over flowers to feed on nectar. Their life cycle and widespread presence are closely tied to the availability of certain plants across the Western Hemisphere.
Identifying the Two Main Pests
Gardeners most commonly encounter two species: the Tomato Hornworm (Manduca quinquemaculata) and the Tobacco Hornworm (Manduca sexta). Both species feed on similar plants and can grow up to four inches long, but they are distinguished by the color of the horn-like projection at the rear of their body.
The Tomato Hornworm has a tail spine that is black or dark blue. Along its sides, the larva possesses eight white markings, each shaped like a “V” or chevron. The adult moth, known as the five-spotted hawkmoth, has five pairs of prominent yellow-orange spots along the sides of its abdomen.
The Tobacco Hornworm is recognized by its red or orange-red tail spine. This species displays seven white diagonal stripes along each side of its body, often featuring a slender black border. The adult Carolina sphinx moth has six pairs of yellow-orange spots on its abdomen.
Geographic Origins and Current Range
The genus Manduca, which includes both hornworm species, is native to the Americas, with the ancestral origin traced to Central America. From this region, the species have radiated out, establishing vast and overlapping distributions across the continent. Their current range extends from southern Canada through the United States, Mexico, Central America, the Caribbean, and into parts of South America.
The Tobacco Hornworm (M. sexta) is found throughout the Americas and is more prevalent in the southern United States and Gulf Coast states. It thrives in warmer climates, extending across the Caribbean down to Argentina and Uruguay. Its presence in northern areas is often due to annual migration, as the moths are strong fliers.
The Tomato Hornworm (M. quinquemaculata) is more common in the northern regions of its range, frequently encountered in the northern United States and southern Canada. Its distribution extends south into Mexico and Central America but is less common along the deep southern US coastlines. This difference in distribution is a function of its tolerance for cooler temperatures during the overwintering pupal stage.
The Hornworm Life Cycle and Host Plants
The geographic presence of hornworms is intrinsically linked to their four-stage life cycle and specialized diet. The cycle begins when the adult moth lays eggs, typically singly on the underside of host plant leaves. The larval stage emerges and feeds voraciously for three to four weeks, passing through five to six growth stages, or instars.
Once fully grown, the larva drops from the plant and burrows into the soil to pupate, where it forms a hard, reddish-brown case. This pupal stage is how the insect overwinters in colder climates, remaining dormant until the spring. In warmer regions, hornworms can complete two to three generations per year, with adult moths emerging to mate and restart the cycle.
Both hornworm species are specialist feeders, consuming only plants from the nightshade family (Solanaceae). This dietary requirement dictates where they are found, as they must locate these plants to survive. Preferred host plants include crops like tomato, tobacco, potato, eggplant, and pepper. The adult moths are capable of migrating hundreds of miles, allowing them to rapidly colonize new areas.