Gardeners prize the taste of a homegrown tomato, but few pests destroy that promise as rapidly as caterpillars. These insects possess voracious appetites that lead to severe defoliation and fruit damage, sometimes overnight. A single generation can ruin a season’s harvest, leaving plants stripped of leaves and fruit scarred or hollowed out. Understanding the specific culprits and their feeding habits is the first step toward protecting a tomato patch and applying effective solutions.
The Primary Caterpillars Targeting Tomato Plants
The most notorious tomato pest is the Tomato Hornworm, a large caterpillar that can grow up to four inches long. This larva of the five-spotted hawk moth is pale green with distinct V-shaped white markings along its sides and a noticeable dark projection or “horn” at its rear end. While they will occasionally nibble on fruit, Hornworms primarily consume the foliage, often starting with the leaves at the top of the plant and moving downward. A close relative, the Tobacco Hornworm, is nearly identical but features diagonal white stripes and usually has a red horn, though both species inflict similar damage on tomato plants.
The Tomato Fruitworm, also known as the Corn Earworm, is another primary pest. These caterpillars are smaller than Hornworms, reaching about one to one-and-a-half inches in length, with highly variable coloration ranging from green and pink to brown, often featuring alternating light and dark stripes. Unlike the Hornworm, the Fruitworm’s preferred target is the developing fruit; it bores directly into the tomato, especially near the stem, causing significant internal damage and rot. A single larva is capable of moving between and damaging multiple tomatoes during its feeding period.
Recognizing the Signs of Infestation
Gardeners often spot the evidence of an infestation before locating the well-camouflaged caterpillars. The most noticeable sign of a large caterpillar like the Hornworm is heavy defoliation, where entire leaves or stems have been stripped bare. Since Hornworms begin feeding on the upper parts of the plant, the damage often appears as a sudden removal of canopy foliage.
A more definitive clue is the presence of “frass,” the caterpillars’ droppings. Hornworms produce large, dark green or black pellets that resemble small grenades, often found scattered on the leaves immediately below the feeding zone. For the Tomato Fruitworm, the sign of damage is a small entry hole on the fruit, usually near the stem, with a trail of dark frass left around the opening. This indicates the caterpillar has tunneled inside, causing a watery internal cavity that leads to premature ripening and rot.
Physical Removal and Biological Control Methods
Once an infestation is confirmed, immediate action involves physically removing the pests. Handpicking is highly effective, especially since Hornworms are large and do not sting or bite. Since these pests are nocturnal feeders and blend seamlessly with the foliage during the day, inspecting plants in the evening or early morning is productive. For Hornworm infestations, using a UV or black light at night can be helpful, as the caterpillars fluoresce and become much easier to spot.
For biological control, the naturally occurring soil bacterium Bacillus thuringiensis (Bt) is a targeted and effective treatment. The kurstaki strain (Btk) specifically targets and harms caterpillars without affecting beneficial insects, pets, or humans. When a caterpillar ingests the Btk, the bacterial proteins paralyze the gut wall, causing the insect to stop feeding within hours and die from starvation within a few days.
Another biological control method involves preserving the activity of native parasitic insects, such as the Braconid wasp. The female wasp lays her eggs beneath the Hornworm’s skin, and the resulting larvae feed internally. The most recognizable sign of this beneficial activity is a Hornworm covered with dozens of small, white, rice-like cocoons attached to its back. An infected Hornworm should be left alone, as it will soon die, and the emerging adult wasps will help control future generations of pests.
Proactive Measures to Deter Pests
Proactive measures focus on preventing pests from establishing themselves, targeting adult moths which lay the eggs that hatch into caterpillars. These moths often seek out debris and soil to overwinter as pupae. Deep tilling the garden soil in the fall or early spring can disrupt this pupal stage, significantly reducing the number of adult moths that emerge in the growing season. Practicing crop rotation is also important, as it prevents the pests from emerging in the same location where they overwintered.
Physical barriers, such as lightweight row covers, can be installed early in the season to prevent the adult hawk moths from laying eggs on the tomato plants. The covers must be removed when the plants begin to flower to allow for necessary pollination. Companion planting can be used as a deterrent strategy by masking the tomato plant’s scent or attracting beneficial insects. Interplanting herbs like basil or borage near tomatoes may help repel the egg-laying moths, while plants like dill or yarrow attract parasitic wasps, which act as a natural defense against potential infestations.