The hornworm, a substantial, bright green caterpillar often encountered in gardens, is the destructive larval stage of a much larger insect. This garden pest belongs to the moth family Sphingidae, commonly known as sphinx moths or hawkmoths. These insects undergo complete metamorphosis, transforming from a voracious leaf-eater into a robust, night-flying pollinator. Determining which specific moth emerges requires distinguishing between the two most common hornworm species.
The Identity of the Hornworm Moth
The adult moth’s identity depends on the specific hornworm species found in the garden. The Tomato Hornworm (\(Manduca\ quinquemaculata\)) develops into the Five-spotted Hawkmoth, while the Tobacco Hornworm (\(Manduca\ sexta\)) turns into the Carolina Sphinx Moth. Both adult moths are large, heavy-bodied fliers, often reaching a wingspan between three and five inches.
These moths possess mottled wings of gray, brown, and white, providing effective camouflage while they rest during the day. A difference between the two species is visible on the abdomen, which features a distinct pattern of yellow-orange spots along the sides. The Five-spotted Hawkmoth exhibits five pairs of these spots, while the Carolina Sphinx Moth typically displays six pairs.
Both hawkmoths are nocturnal and are known for their hummingbird-like flight behavior as they hover near flowers. They feed on nectar using an exceptionally long, coiled proboscis, acting as pollinators for deep-throated, night-blooming flowers. The adult stage focuses on reproduction and nectar feeding, a shift from the leaf-devouring habits of the caterpillar.
The Complete Transformation Cycle
The transformation from a hornworm to a flying moth involves the complex process of complete metamorphosis. Once the larva has finished its three to four weeks of intensive feeding, it reaches its largest size, triggering the next stage. The mature hornworm then descends from its host plant and burrows several inches deep into the soil.
Underground, the larva creates a chamber where it sheds its skin one last time to become a pupa, a hard, dark-brown, elongated case. This pupal stage is recognized by a distinctive loop or “handle” structure extending from the head end, which encases the developing mouthparts. The pupa is a period of internal reorganization as larval tissues break down and reform into the adult insect structure.
The duration of the pupal stage varies based on the season and climate. Hornworms that pupate during the summer months generally emerge as adult moths in about two to four weeks, starting a second generation. Those that pupate later remain dormant, overwintering in the soil until warmer temperatures in the spring trigger their emergence.
Distinguishing Between Hornworm Larvae
Since the two moths originate from visually similar caterpillars, identifying the larva is the only way to know which adult moth to expect. Both the Tomato Hornworm and the Tobacco Hornworm grow to an impressive length of four inches and share the same prominent, spine-like protrusion on their posterior end. This projection, the “horn,” is the most obvious feature, but its color is the differentiator.
The Tomato Hornworm larva has a black or bluish-black horn, and its body markings consist of eight white, V-shaped chevrons along its sides. The Tobacco Hornworm larva displays a red or orange-red horn. This species has seven diagonal white stripes on its body, which are typically bordered by a thin black line.
Both species feed primarily on plants in the Solanaceae family, including tomato, pepper, eggplant, and potato. Therefore, the host plant does not reliably distinguish the species. Careful observation of the horn color and the pattern of the white side markings is necessary for accurate identification.