“Tomato worms” are large, highly destructive caterpillars that target plants in the nightshade family, such as tomatoes, peppers, potatoes, and eggplant. These pests are the larval stage of sphinx moths (or hawk moths) and can reach up to four inches in length. They are notorious for their rapid and voracious feeding habits, capable of stripping a plant of its foliage quickly. Understanding these pests is the first step in protecting your garden from severe defoliation.
Identifying the Two Main Species
The two species responsible for garden damage are the Tomato Hornworm (Manduca quinquemaculata) and the Tobacco Hornworm (Manduca sexta). Both are similar in size and bright green, but they can be distinguished by their markings. The Tomato Hornworm has eight distinct white chevron-shaped markings on its sides that point toward the head. It also features a dark posterior spine, or “horn,” which is typically black or blue-black. In contrast, the Tobacco Hornworm has seven diagonal white stripes, often edged with black, that do not form a chevron shape, and its horn is usually orange-red or red.
Understanding Their Life Cycle and Destructive Habits
The life cycle typically involves one to two generations per growing season. Adult moths lay small, spherical eggs singly on the undersides of host plant leaves. After hatching, the larvae progress through five to six growth stages (instars) over three to four weeks.
The most significant consumption occurs during the final instar, where the caterpillar reaches its full size. This stage is responsible for the majority of defoliation, as larvae consume leaves, stems, and sometimes chew into developing fruit, causing substantial yield loss. When fully grown, the caterpillar drops from the plant and burrows four to six inches deep into the soil to pupate, overwintering before emerging as a moth the following spring.
Effective Management and Removal Strategies
Controlling an infestation begins with diligent monitoring, as the caterpillars’ excellent camouflage means that the first sign of their presence is often the damage itself. Gardeners should look for areas of heavy defoliation, especially near the top of the plant, and for dark, pellet-like droppings, called frass, on the leaves or soil below. Handpicking is an effective, non-chemical method for small-scale infestations, requiring the physical removal of the large caterpillars from the plant.
For more widespread issues, biological controls offer targeted solutions that preserve beneficial insects. The bacterium Bacillus thuringiensis subspecies kurstaki (Bt-k) is a naturally occurring control method approved for organic gardening. This product is sprayed onto the foliage and must be ingested by the caterpillar; once eaten, it causes the larvae to stop feeding almost immediately, leading to death within a few days.
Another powerful biological agent is the tiny parasitic braconid wasp (Cotesia congregata), a natural enemy of the hornworm. The female wasp deposits eggs just under the caterpillar’s skin, and the developing larvae feed internally until they emerge to spin small, white, rice-shaped cocoons on the host’s exterior. If a parasitized hornworm is found, it should be left on the plant, as it will die shortly, and the emerging adult wasps will help control future generations.
Additionally, a cultural control practice involves tilling the garden soil in late fall or early spring. This action physically disrupts and destroys the overwintering pupae, thereby reducing the number of adult moths that emerge to lay eggs.