The tomato worm is a common, oversized garden pest representing the larval stage of a large, night-flying hawk moth. This caterpillar is a serious threat to nightshade plants, including tomatoes, peppers, and eggplants, because of its voracious appetite. These pests can defoliate entire plants almost overnight, making them one of the most destructive insects a gardener might encounter.
Identifying the Tomato Hornworm
This caterpillar can reach up to four inches at maturity, making it one of the largest garden caterpillars in North America. Its bright green body features distinctive markings and a prominent, harmless “horn” on its posterior end. The caterpillar’s color blends seamlessly with the foliage of its host plants.
The term “tomato hornworm” refers to two closely related species: the Tomato Hornworm (Manduca quinquemaculata) and the Tobacco Hornworm (Manduca sexta). While both are large and green, their markings offer a clear distinction. The Tomato Hornworm has eight white, V-shaped markings along its sides, and its horn is typically black or dark blue. In contrast, the Tobacco Hornworm features seven diagonal white stripes, often with black margins, and its horn is reddish or orange-red.
The Life Stages of the Pest
The life cycle begins when the adult moth, known as a sphinx or hawk moth, lays tiny, pale green eggs, usually on the underside of a tomato leaf. These eggs hatch quickly, often within a week, and the larvae begin feeding immediately. The destructive larval stage lasts about three to four weeks.
During this time, the caterpillar progresses through five to six instars, with the majority of feeding occurring in the final stages. A single mature hornworm can consume significant foliage, often stripping leaves down to the stem and occasionally chewing holes in the fruit. The presence of large, dark, pellet-like fecal droppings, known as frass, is often the first visible sign of an infestation.
Once fully grown, the larva drops from the plant and burrows into the soil to pupate. The pest overwinters in this pupal stage, emerging as a moth the following spring or summer to begin the cycle anew.
Stopping Damage in the Garden
The most straightforward and effective method for small gardens is manual control, which involves picking the caterpillars off the plants. Because of their excellent camouflage, a helpful technique is to use a UV or black light flashlight at night, as the hornworms glow, making them much easier to spot. Once removed, destroy the pests by dropping them into soapy water.
Gardeners can employ cultural control by tilling the soil in late fall or early spring to disrupt the overwintering pupae. This exposes the pupae to the elements or predators, reducing the number of moths that emerge later.
Biological controls offer a sustainable way to manage populations, primarily through the parasitic Braconid wasp (Cotesia congregata). Female wasps lay their eggs inside the hornworm, and the larvae feed internally before emerging to spin tiny, rice-like white cocoons on the caterpillar’s back. Leave any hornworm with these cocoons in place, as the parasitized caterpillar is doomed, and the emerging wasps will hunt other hornworms. For targeted treatment, a biological insecticide containing Bacillus thuringiensis (Bt) can also be applied, which specifically affects small, feeding caterpillars.