The tomato hornworm, Manduca quinquemaculata, is a large, destructive caterpillar that poses a significant threat to plants in the nightshade family, including tomatoes, peppers, eggplant, and potatoes. As the larval stage of the Five-spotted Hawk Moth, this pest is known for its voracious appetite, capable of causing rapid and extensive defoliation of garden plants. A single mature hornworm can consume several leaves and even scar developing fruit. Successfully controlling this common garden pest involves a combination of immediate physical removal and long-term biological and cultural strategies.
Immediate Action: Scouting and Physical Removal
The bright green coloration of the tomato hornworm allows it to blend seamlessly with plant foliage, making visual detection difficult despite its large size. The most reliable initial sign of their presence is the appearance of large, dark green or black fecal pellets, known as frass, often found on the leaves below the feeding area or on the ground. Gardeners should also look for branches that have been stripped bare of leaves, indicating active feeding.
Hornworms typically hide along the main stems or on the underside of leaves, especially during the heat of the day. When scouting, physically inspect the plant, moving from the top where the most recent damage occurs downward. A highly effective technique for locating these camouflaged pests is to use an ultraviolet (UV) flashlight after sunset.
The hornworm’s body contains phosphors that cause them to glow brightly under UV light, making them easy to spot. Once located, they can be removed by hand, ideally wearing gloves, and dropped into a container of soapy water to ensure they are killed. This physical removal is the most direct and non-chemical way to protect plants from immediate damage.
Targeted Treatments Using Biological Controls
When hornworm populations are too high for effective hand-picking, targeted treatments utilizing biological agents offer an organic solution. The soil bacterium Bacillus thuringiensis (Bt) is a microbial insecticide highly effective against young caterpillars. When a hornworm ingests the bacteria on treated foliage, the Bt produces a crystal protein that paralyzes the insect’s digestive system.
The hornworm stops feeding almost immediately, although it may take several days for the caterpillar to die from starvation. Because Bt is only toxic to caterpillars that eat it, the product is harmless to beneficial insects and safe for human application. For best results, the spray should be applied thoroughly to the plant foliage, preferably in the late evening, as sunlight can rapidly degrade the active bacterial spores.
Natural enemies also play a significant role in population control, particularly the parasitic Braconid wasp, Cotesia congregata. The adult female wasp injects eggs into the hornworm’s body, and the resulting larvae feed internally. When the larvae are mature, they chew their way out and spin dozens of small, white cocoons on the caterpillar’s back.
These white structures are often mistaken for eggs, but they are the pupal stage of the developing wasps. A hornworm found with these cocoons should be left on the plant, as it has been neutralized and will soon die. Allowing the wasps to emerge ensures they will seek out and parasitize more hornworms, establishing a self-sustaining biological control within the garden.
Long-Term Strategy: Cultural and Preventative Gardening Practices
Implementing preventative cultural practices can significantly reduce the potential for future hornworm outbreaks. Tomato hornworms typically spend the winter months in the soil as reddish-brown pupae, burrowing about three to four inches deep. A highly effective control method is to till the garden soil in the late fall after harvest or in the early spring before planting.
Tilling exposes the overwintering pupae to the harsh elements and to predators, which can result in up to 90% mortality of the next generation. This mechanical disturbance breaks the hornworm’s life cycle before the adult moths emerge to lay eggs. Another important practice is to rotate crops annually, avoiding the planting of tomatoes or other nightshade family members in the same location.
Hornworms will also feed on related weeds, such as jimsonweed and nightshade, before moving to cultivated crops. Maintaining weed control removes these alternate host plants and limits the initial food sources for newly hatched larvae. Companion planting also offers a layer of protection by deterring the adult Five-spotted Hawk Moth from laying eggs.
Plants with strong scents, such as basil and marigolds, are believed to repel the moths, and planting them interspersed among the tomato rows can reduce the attractiveness of the area. Dill and parsley, while not directly repelling the moth, can attract beneficial predatory wasps, which helps to increase the natural enemy population in the garden.