Hornworms

Overview

The tomato hornworm, Manduca quinquemaculata, and the tobacco hornworm, M. sexta, are common pests of tomato, tobacco, eggplant, pepper, and potato throughout most of the United States. The hornworms are large (up to 4 inches long), bright green caterpillars with diagonal white stripes and a prominent horn at the rear. The two species have slightly different markings. The tomato hornworm has 8 diagonal white stripes on each side; the horn is straight and black. The tobacco hornworm has 7 diagonal white stripes; its horn is curved and red.

Symptoms and Diagnosis

Larvae of both species consume entire leaves and small stems and may chew large pieces of green fruit. Hornworm damage is obvious when the infestation is moderate to heavy because of the large amount of defoliation. Search for the large caterpillars. Large, black droppings on the leaves or ground beneath the plant will usually indicate the presence of hornworms.

Life Cycle

Hornworms overwinter in the soil as hard-shelled, brown pupae. Large adult moths, known as sphinx or hummingbird moths, emerge in May or June and deposit spherical green eggs on the undersides of leaves of host plants. The larvae hatch a week later and feed on foliage and fruit for three to four weeks until reaching full development. Pupation occurs in the soil and adults emerge 2 to 4 weeks later to lay second-generation eggs. In Missouri, there may be one or two generations depending on location.

Integrated Pest Management Strategies

1. Handpick caterpillars. Handpicking is usually all that is needed to control these pests in a home garden. Larvae are most easily located in the early morning, often on the exterior of the plant. Leave any caterpillars with small white cocoons on their backs; they are being parasitized by a braconid wasp, which will soon produce more wasps to control them.

2. Biological control. Bacterial insecticides containing Bacillus thuringiensis kurstaki (Btk) are effective when larvae are smaller. It may take several days for the caterpillars to die, but feeding generally stops shortly after treatment.

3. Cultivation. Disking or rototilling after harvest destroys pupae in soil, reducing overwintering numbers.

4. Chemical control. An application of carbaryl (Sevin) or permethrin will effectively control hornworms if handpicking is impractical. However, chemical control will also reduce the number of beneficial insects. Avoid getting these types of insecticides on the fruit if it is to be consumed.

Organic Strategies

Strategies 1 and 3 are strictly organic approaches. For an organic approach to Strategy 2, consult the Organic Materials Review Institute (OMRI™) for appropriate Bacillus thuringiensis (Bt) products.