The large, bright green caterpillar often discovered stripping the foliage from nightshade plants, such as tomatoes and peppers, is a common garden pest known for its voracious appetite. Though frequently called a “tomato worm,” this larva is one stage in the complete life cycle of a fascinating insect. This destructive phase is temporary, leading to a much different creature. Understanding the creature’s full development is the first step toward effective garden management.
Which Worm Are You Seeing
The garden pest referred to as the tomato worm is typically one of two closely related species: the Tomato Hornworm (Manduca quinquemaculata) or the Tobacco Hornworm (Manduca sexta). Both grow to an impressive size, reaching up to four inches long, and are colored a vibrant green, which serves as camouflage. They are distinguishable by the markings on their bodies and the color of the prominent, harmless spine, or “horn,” at their rear end.
The Tomato Hornworm features eight white, V-shaped markings along its sides and has a black horn. Conversely, the Tobacco Hornworm has seven diagonal white stripes and a reddish-orange horn. While the names suggest a preferred host, both species readily feed on tomato, pepper, eggplant, and potato plants. The damage caused by just a few larvae can quickly reduce a healthy plant to bare stems, making identification and control important.
The Pupal Stage
Once the hornworm reaches maturity, typically after three to four weeks of feeding, it drops from the plant. The larva then burrows four to six inches deep into the soil to begin the pupal stage. This transitional phase occurs within a hard, reddish-brown casing where the caterpillar reorganizes itself completely.
The pupa is identifiable by a unique structure called the proboscis sheath. This sheath contains the developing mouthparts of the adult moth and appears as a prominent, curved loop, resembling a pitcher handle. In warmer climates, this stage may last only a few weeks, leading to a second generation. In cooler regions, the pupa remains dormant in the soil over the winter, emerging as an adult moth the following spring.
The Adult Moth Form
The tomato worm ultimately transforms into a large, fast-flying insect known as a Sphinx Moth or Hawk Moth (family Sphingidae). The Tomato Hornworm emerges as the Five-spotted Hawk Moth (Manduca quinquemaculata), and the Tobacco Hornworm becomes the Carolina Sphinx Moth (Manduca sexta). These moths are robust, often having a wingspan between four and five inches, with mottled gray-brown coloration that provides excellent camouflage.
The two adult species are distinguished by the number of yellow or orange spots on their abdomens. The Five-spotted Hawk Moth has a row of five yellow spots down each side. The Carolina Sphinx Moth is characterized by six pairs of yellow-orange spots. These moths are primarily active at dusk, and their rapid, hovering flight while feeding on nectar often causes them to be mistaken for hummingbirds.
Interrupting the Life Cycle
Gardeners can manage this pest by targeting the different life stages with non-chemical methods. The large, mature larvae are susceptible to handpicking, which is an effective control measure in small gardens. Since the caterpillars are most active during the evening, a simple flashlight inspection helps locate and remove them.
During the larval stage, inspect for signs of natural parasitism, specifically Braconid wasps. If a hornworm is covered in small, white, rice-like cocoons, it should be left alone. These are the pupae of the beneficial wasp that will soon emerge to parasitize other hornworms.
The underground pupal stage can be targeted by shallow tilling the garden soil in late fall or early spring. This exposes the overwintering pupae to cold temperatures and predators. This practice significantly reduces the population of moths that emerge to lay eggs the following season.