The tomato hornworm is one of the most destructive garden pests, capable of stripping a tomato plant of its foliage almost overnight. These large caterpillars feed voraciously on leaves, stems, and even the fruit of tomatoes and other nightshade relatives like peppers and eggplants. This guide details practical, science-backed methods to control and prevent hornworm infestations.
Identifying the Pest and the Damage
The tomato hornworm (Manduca quinquemaculata) is a large caterpillar, reaching up to four or five inches in length, and is colored a pale green that blends with tomato foliage. A distinguishing feature is the dark, pointed, horn-like spike protruding from its rear end, which is harmless to humans. The closely related tobacco hornworm (Manduca sexta) is similar but has a red horn and diagonal white stripes rather than the tomato hornworm’s V-shaped white markings.
The first sign of an infestation is often not the pest itself, but the damage it leaves behind. Hornworms consume entire leaves, starting at the top of the plant, often leaving only the bare central stem, or mid-rib. Another clear indicator is the presence of large, dark, blocky droppings, known as frass, which can be found on leaves below the feeding site or on the soil surface. The size of these pellets increases with the size of the caterpillar, helping to pinpoint the location of the hidden pest.
Immediate Physical Removal Methods
Hand-picking is the most direct and effective method for immediate hornworm removal, especially in small gardens, though their camouflage makes this a challenge. Inspect the plant thoroughly, looking for bare stems, fresh frass, and recently chewed leaves. Hornworms tend to be found on the underside of leaves or clinging tightly to stems, often favoring the upper portions of the plant.
A highly effective technique for locating these camouflaged pests is to hunt them at night using a blacklight, or ultraviolet (UV) light. Hornworms glow a bright, fluorescent green under UV light, making them easy to spot. This nighttime inspection allows gardeners to find smaller caterpillars that might be missed during the day. Once removed, the worms should be dropped into a container of soapy water, which ensures they are killed.
Targeted Biological and Chemical Treatments
When infestations are too widespread for hand-picking alone, targeted biological treatments offer an effective solution that minimizes harm to beneficial insects. The most common biological control is Bacillus thuringiensis (Bt), specifically the kurstaki strain (Btk), which targets only caterpillars. Btk is a naturally occurring soil bacterium that must be ingested by the hornworm to be effective.
Once consumed, the bacterial toxins become active in the alkaline environment of the caterpillar’s gut, paralyzing the digestive system. The caterpillar immediately stops feeding and starves, though death may take a few days. Btk should be applied thoroughly to the foliage, especially the undersides of leaves, and is most effective when the hornworms are small.
Natural predators also play a significant role in controlling hornworm populations. The tiny, non-stinging Braconid wasp (Cotesia congregatus) is a specialist parasite of the hornworm. The female wasp injects her eggs just under the caterpillar’s skin, and the larvae feed internally before chewing their way out to spin tiny white cocoons on the hornworm’s exterior.
A hornworm covered in these cocoons is near death. It should be left in the garden to allow the new generation of beneficial wasps to emerge and continue the natural control cycle.
Broad-spectrum chemical insecticides are discouraged for hornworm control due to their indiscriminate impact on beneficial insects and pollinators. If an extreme infestation necessitates a chemical approach, it should be considered a last resort and applied with caution. Choosing an insecticide with a shorter residual effect can help mitigate the impact on the garden ecosystem.
Strategies for Long-Term Prevention
Preventing future hornworm generations involves disrupting their life cycle. Tilling the garden soil in late fall after harvest or in early spring before planting can expose or destroy the hard, brown pupae. This physical disturbance can eliminate a large percentage of the overwintering population, reducing the number of adult moths that emerge to lay eggs in the spring.
Physical barriers prevent the adult sphinx moths from reaching the plants to lay their eggs. Lightweight floating row covers can be placed over young tomato plants and secured at the edges. Since tomatoes are self-pollinating, the covers can remain in place for much of the season, acting as a physical shield against the egg-laying moths.
Strategic planting can also help manage the pest population by deterring the adult moths or attracting beneficial predators. Companion plants like basil, which has a strong aromatic scent, can confuse the moths looking for tomato foliage. Plants with small, nectar-rich flowers, such as dill, yarrow, or sweet alyssum, attract the Braconid wasps, encouraging them to stay and patrol the area for pests.