The tomato hornworm represents one of the most destructive pests a tomato gardener can encounter. These voracious caterpillars target nightshade family plants, including tomatoes, peppers, potatoes, and eggplant. A single hornworm can consume an enormous amount of foliage in a short time, often leading to rapid defoliation that can severely impact a plant’s ability to produce fruit. Because of this high level of damage, immediate intervention is necessary to protect your harvest.
Identifying the Threat and Its Damage
Confirming you have a tomato hornworm involves recognizing both the pest and the distinct damage it causes to the plant. The larvae are large, reaching up to four inches long at maturity, and are a bright green color that provides camouflage among the foliage. The primary distinguishing feature is a prominent “horn” or spine on their rear end, which is typically black or dark blue in the tomato hornworm and red in the tobacco hornworm.
Finding the caterpillar itself can be challenging due to its excellent camouflage, but the damage is unmistakable. Hornworms consume entire leaves and new stems, often leaving behind a defoliated stalk that looks stripped bare. They tend to start feeding on the upper portions of the plant, which may be the first place you notice missing foliage.
The most reliable way to locate the camouflaged pest is by searching for its droppings, known as frass. Hornworm frass is characterized by large, dark green or black pellet-like droppings found on the leaves below where the caterpillar is feeding, or on the soil at the base of the plant. Following this trail of frass upward will often lead directly to the hidden hornworm.
The Critical Decision: When to Leave the Hornworm Alone
While the impulse is to immediately remove every hornworm, a notable exception exists involving the parasitic braconid wasp, Cotesia congregata, a natural enemy of the hornworm. If you find a hornworm covered in small, white, rice-like capsules, you should leave it undisturbed on the plant.
These small white structures are the cocoons of the parasitic wasp larvae, which have finished feeding inside the hornworm and emerged to pupate. By leaving the parasitized hornworm in place, you allow the beneficial wasps to complete their life cycle and hatch. This new generation of wasps will then seek out and parasitize other hornworms in your garden, providing a long-term, natural pest control solution for the rest of the season.
Immediate and Organic Removal Strategies
When a hornworm is not parasitized, immediate removal is necessary to prevent further damage to your plants. The most direct method for the home gardener is manual removal, or hand-picking. This involves carefully inspecting the plant, especially the upper foliage and stems, and physically taking the caterpillar off the plant.
A useful tactic is to search in the early morning or evening when hornworms are more active. Alternatively, using a UV blacklight at night can help, as the caterpillars fluoresce brightly under ultraviolet light. Once removed, the hornworm should be disposed of humanely, such as by dropping it into a container of soapy water, which ensures it cannot return to the garden. Never crush the pest on the plant, as this can spread plant pathogens.
For larger infestations where hand-picking is impractical, organic biological controls offer a targeted solution. The bacterium Bacillus thuringiensis subspecies kurstaki (Bt) is highly effective against hornworms and other caterpillars. Bt is a stomach poison that must be ingested by the caterpillar to be effective, paralyzing its digestive system.
The spray should be applied to the foliage late in the evening since hornworms feed actively at night and Bt quickly degrades in sunlight. A significant advantage of using Bt is that it is a highly selective pesticide, meaning it targets only caterpillars and is safe for beneficial insects like bees, wasps, and ladybugs.
Long-Term Prevention and Management
Shifting focus to long-term management can greatly reduce the likelihood of future infestations. The hornworm overwinters as a pupa buried several inches deep in the soil beneath the host plant. Tilling the top few inches of the soil in late fall or early spring is a highly effective cultural practice that exposes these pupae to the harsh elements and predators, significantly reducing the number of adult moths that emerge.
Another strategy is to prevent the adult moths, known as Sphinx or Hawk moths, from laying eggs. Using physical barriers, such as lightweight row covers or fine netting, early in the season can successfully block the moths from reaching the plants.
Companion planting also works to deter the adult moths by confusing them or masking the tomato plant’s scent. Planting aromatic herbs like dill, basil, and borage, or flowers such as marigolds, near your tomato plants can help repel the moths. These combined preventative measures help break the hornworm life cycle, offering sustained protection for your garden.