The sudden appearance of large, chewed sections on tomato plants often indicates an infestation of caterpillars, particularly the Tomato Hornworm (Manduca quinquemaculata). These pests are capable of rapidly defoliating entire plants and can also chew into the ripening fruit. Controlling these larvae requires swift intervention to protect the current harvest. This guide provides effective methods to eliminate these garden invaders and implement long-term protective measures.
Identifying the Pest and Manual Removal Techniques
The first step in controlling these pests is accurately identifying the culprits and their signs. The most damaging caterpillar is the Tomato Hornworm, a large, bright green larva that can grow up to four inches long and possesses a distinctive horn-like projection on its rear end. This color provides excellent camouflage against the tomato foliage, making direct visual detection difficult.
The clearest sign of an infestation is the presence of large, dark green or black droppings, known as frass, found on the leaves or the soil beneath the plant. Since these caterpillars feed most heavily on the upper parts of the plant, look for bare stems where leaves once were, a sign of rapid defoliation.
Manual removal, or handpicking, is the most immediate and safest control method for small gardens. Caterpillars are often easier to spot during the cooler parts of the day, such as early morning or late evening. For a more effective search after dark, an ultraviolet or black light can be used, as the caterpillars’ skin fluoresces brightly under the UV spectrum. Once removed, the pests should be dropped into a container of soapy water for disposal.
Applying Biological and Organic Treatments
For larger infestations or when manual removal is insufficient, biological treatments offer a targeted and environmentally sound solution. The most effective organic treatment is a spray containing Bacillus thuringiensis subspecies kurstaki (Bt-k), a naturally occurring soil bacterium. Bt-k is a highly selective microbial insecticide that targets only caterpillars, making it safe for humans, pets, and beneficial insects.
The insecticidal action of Bt-k is dependent on ingestion by the caterpillar. Once consumed, the bacterium releases a protein crystal that paralyzes the digestive system. The caterpillar immediately stops feeding and dies within a few days.
Another set of organic options includes Neem oil and insecticidal soap, which are best used against younger, smaller caterpillars. Insecticidal soap is a contact killer, disrupting the insect’s cell membranes. Neem oil, derived from the seeds of the neem tree, acts as a repellent and disrupts the caterpillar’s feeding and growth hormones. Proper application for both requires thoroughly coating all plant surfaces, including the undersides of leaves, and should be done in the evening to prevent potential leaf burn from the sun.
Strategies for Future Prevention
Long-term control focuses on disrupting the pest’s life cycle before they can emerge to cause damage. Tomato Hornworms overwinter as pupae buried four to six inches deep in the soil. Tilling the garden soil thoroughly in the fall, after harvest, or in early spring can physically expose or damage up to 90% of these pupae.
A foundational preventative measure is practicing crop rotation, which involves avoiding planting tomatoes or other nightshade family members (Solanaceae), such as peppers and eggplants, in the same location year after year. This disrupts the pest’s ability to emerge from the soil and immediately find food, starving the newly emerged larvae.
Physical barriers offer an additional layer of protection early in the season. Lightweight floating row covers placed over young plants can prevent the adult moth, known as the five-spotted hawk moth, from landing on the foliage and laying eggs. These covers must be removed once the plants begin to flower to allow for necessary pollination.