Growing tomatoes can be a rewarding garden activity until you discover the tell-tale signs of an infestation: stripped leaves, munched fruit, and dark droppings. These destructive “worms” are actually the larval stage of moths, and their presence can quickly undermine a season’s worth of effort. They are voracious eaters that cause significant damage in a short period. Understanding how to identify these pests, implement preventative strategies, and use targeted removal techniques is the most effective approach to protecting your harvest.
Identifying the Common Tomato Pests
The two primary caterpillar pests home gardeners encounter are the Tomato Hornworm and the Tomato Fruitworm, each causing distinct types of damage. The Tomato Hornworm is a large, smooth caterpillar, growing up to four inches long (about the size of a finger). This pest is bright green, allowing it to camouflage against the foliage, and features eight white, V-shaped markings along its side and a prominent dark horn on its posterior end. Hornworms are known for defoliating plants quickly, eating entire leaves and stems, with damage often starting at the top of the plant.
The Tomato Fruitworm is generally smaller, reaching about one to one-and-a-half inches at maturity, and features a spiky texture. Its color varies widely (green, yellow, brown, or black), with alternating light and dark stripes running the length of its body. Unlike the Hornworm, the Fruitworm bores into the fruit itself, typically entering near the stem end. Evidence includes small entry holes, often with dark, moist excrement (frass) visible around the opening, causing internal rot and making the fruit unusable.
Proactive Measures for Prevention
Implementing preventative cultural practices early in the season can significantly reduce the likelihood of a major infestation. One method involves using fine netting or row covers to create a physical barrier over the young plants. This material must be in place before the adult moths begin laying eggs, but remove it when the plants start flowering to allow for pollination.
Both the Hornworm and the Fruitworm pupate in the top few inches of soil over winter. Tilling the garden plot in the late fall or early spring can expose these overwintering pupae to predators or harsh weather. This action reduces the population that emerges as adult moths the following season.
Crop rotation is important, as planting tomatoes in the same spot year after year allows pests to establish and build up populations in the soil. Switching to a non-host plant for one or two seasons helps break this cycle and limits the pests’ food source.
Strategic companion planting can confuse or deter adult moths from laying eggs on the tomato plants. The strong scent of plants like basil, marigolds, and dill can mask the smell of the tomato plant, which the adult sphinx moth uses to locate a host. Marigolds, specifically the French variety, also attract beneficial parasitic wasps that prey on the Hornworm.
Regular monitoring and pruning serve as preventative care by limiting hiding spots and catching problems early. Removing the lower leaves of the tomato plant, especially those touching the soil, prevents pests from climbing easily and improves air circulation. Inspecting the underside of leaves for eggs or small larvae every few days can prevent a serious outbreak.
Targeted Intervention and Removal Techniques
Once an infestation is present, the most immediate intervention is manual removal, or handpicking. Because large Hornworms are well camouflaged, the best way to find them is to follow the trail of large, black droppings, or frass, left on the leaves or the ground below. Handpicking should be done in the early morning or late afternoon when the caterpillars are most active, and the pests can be disposed of in a container of soapy water.
For widespread infestations, a targeted biological control agent is Bacillus thuringiensis (Bt) var. kurstaki (Btk). Bt is a naturally occurring soil bacterium that is harmless to humans, pets, and beneficial insects. It must be ingested by the caterpillar, where a crystal protein paralyzes the insect’s digestive system, causing it to stop feeding and eventually die.
Btk is most effective when applied to young larvae, as they consume the treated foliage more readily than mature ones. Since sunlight can quickly degrade the spray, the best time for application is late in the day or evening, ensuring the product remains active when the caterpillars feed at night. Repeat applications are often necessary every seven to ten days, especially after rain or overhead watering, until the pest pressure decreases.
Encouraging the presence of natural enemies provides a powerful, long-term biological defense. Tiny Braconid wasps are a gardener’s ally; they parasitize the Hornworm by laying eggs inside its body. If you discover a Hornworm covered in small, white, rice-like cocoons, leave it alone. These cocoons are the wasp pupae emerging from the caterpillar, and allowing them to hatch releases more beneficial wasps into the garden to control future pests.
Chemical insecticides are generally considered a last option for home gardeners, as they can harm beneficial insects and pollinators. If intervention is necessary and other methods have failed, choose a product with a short residual effect and a high degree of specificity to caterpillars. Always read the product label carefully to ensure it is safe for use on edible plants and follow all instructions regarding application rates and harvest intervals.