Stephen Albert
Tomato hornworms are green caterpillars up to 4½ inches long with a black horn protruding from the tail and eight V-shaped white marks along the side.
A related hornworm is the tobacco hornworm with a red horn on the tail and seven white diagonal lines with a black border.
Hornworms feed on leaves, stems and fruit of nightshade family plants—tomatoes, eggplants, potatoes, peppers and others. Feeding hornworms can kill plants.
Hornworms are the larvae of large, gray moths with wingspans of 4 to 5 inches.
Hornworms easily blend in with the foliage of the plants they attack but they leave behind black pellets of excrement which are easy to spot. They often feed upside down on the bottom of leaves.
Moths emerge from soilborne pupae in early summer. Adult moths lay eggs on the underside of leaves. The eggs hatch in a week then the larvae hornworms feed for a month.
Moths lay pale green eggs singly on the leaf undersides. After feeding the caterpillars inter the soil and form brown, shiny, 2-inch-long pupae. The pupae can overwinter. There are four generations a year.
Target Plants: Eggplant, pepper, potato, tomato.
Feeding Habits and Damage: Hornworms chew leaves leaving behind holes and sometimes only midribs. They also chew stems and gnaw green fruits.
Organic Controls: Handpicking is perhaps most effective; handpick and drown in soapy water. Apply Bacillus thuringiensis (Bt) while caterpillars are still small. Pyrethrum, rotenone, and spinosad sprays are most toxic and a last resort, but these sprays also kill beneficial insects.
Organic Control Calendar: Here’s what you can do seasonally to control tomato hornworms:
- Before planting: Turn soil 3 to 5 inches to expose overwintered pupae; they will die in the cold or be eaten by birds.
- At planting time: Give the soil a final shallow cultivation to expose pupae. Plant dill as a trap crop to draw moths and hornworms away from the main crop.
- While crops develop: Handpick caterpillars from foliage and destroy them. Repel hornworms with hot pepper spray; spray the solution directly on the pest. Apply Bt or spinosad when caterpillars are small. Attract braconid wasps to the garden; female braconid wasps lay eggs under the skin of hornworms; when the wasp eggs hatch the larvae feed on the hornworm literally eating hornworms alive. Leave parasitized hornworms in the garden to allow new generations of braconid wasps to appear.
- After harvest: Remove old plants and overgrown weeds. Turn soil 4 to 6 inches deep to expose pupae to cold and birds.
Natural Predators: Braconid and trachogramma wasps are small wasps that parasitize hornworms. Other insect predators are assassin bugs and praying mantids. Animal predators include orioles, barn swallows, blackbirds, bluebirds, downy woodpeckers, flycatchers, phoebes, and sparrows, also moles, skunks, and toads.
Scientific name: Manduca quinquemaculata (tomato hornworm), Manduca sexta (tobacco hornworm).
