The tomato hornworm (Manduca quinquemaculata or M. sexta) is a large caterpillar known for its voracious appetite for plants in the nightshade family, particularly tomatoes. A single hornworm can rapidly defoliate a plant, causing significant damage. The natural world provides effective checks on these pests through a variety of predators, parasites, and microbial agents. Understanding these natural enemies is a sustainable way to manage hornworm populations in the garden.
Parasitic Wasps: Nature’s Biocontrol Agents
The most distinctive natural enemy of the tomato hornworm is a tiny parasitoid known as the Braconid wasp, Cotesia congregata. This small, non-stinging wasp uses the caterpillar as a host for its offspring, a process that ultimately prevents the hornworm from reaching maturity. The adult female wasp injects numerous eggs directly into the hornworm’s body, typically targeting younger larvae.
The wasp larvae hatch inside the caterpillar and begin to feed on its internal tissues for about two weeks. The parasitized hornworm continues to feed, but its growth is arrested, and it consumes significantly less plant material.
When the developing wasp larvae are ready to pupate, they chew their way out of the hornworm’s skin. They then spin small, white, cottony cocoons that appear externally on the hornworm’s back, often mistaken for eggs. The hornworm, weakened, will not survive this process, but the cocoons protect the next generation of beneficial wasps. If a gardener finds a hornworm covered in these cocoons, the best action is to leave it in place, allowing the adult wasps to emerge and seek out more hornworms.
Active Hunters: Insect and Vertebrate Predators
Beyond parasitic wasps, a range of generalist predators actively hunt and consume hornworms at various life stages. Several beneficial insects target the smaller eggs and young larvae, helping to reduce the population before it causes widespread damage. These include the larvae of green lacewings and lady beetles, which prey on the eggs and smaller caterpillars.
Paper wasps (Polistes species) are significant insect predators, actively hunting hornworm larvae to feed their young. They tear off pieces of the caterpillar to carry back to their nests, making them an effective biological control agent. Other predatory insects, like ground beetles, often attack the hornworm pupae that overwinter in the soil.
Vertebrate predators also manage hornworm numbers. A variety of birds, including cardinals, bluebirds, and crows, forage for the large, protein-rich caterpillars. Chickens and turkeys, if allowed access to the garden, are particularly effective at consuming both the larvae on the plants and the pupae in the soil. Attracting these avian allies can be encouraged by placing bird feeders nearby or by planting habitat that offers shelter and nesting sites.
Microbial Solutions
A widely used tool for hornworm control is the microbial agent Bacillus thuringiensis (Bt). This naturally occurring soil bacterium is a biological insecticide that specifically targets caterpillars. The most common strain used for hornworms is Bacillus thuringiensis subspecies kurstaki (Btk).
When a hornworm ingests foliage treated with Btk, the bacterial spores release a crystalline protein known as a Cry toxin. The alkaline conditions within the caterpillar’s midgut activate this protoxin, which binds to specific receptors on the gut wall. This binding creates pores in the gut lining, causing paralysis of the digestive system.
The hornworm stops feeding almost immediately, leading to its death. Because this toxin requires the specific alkaline gut environment and receptors found only in certain insects, Btk is considered safe for humans, pets, and beneficial insects like bees and adult parasitic wasps. It is most effective when applied to foliage and ingested by young hornworms, requiring reapplication after rain or every five to seven days.