Tobacco Hornworm

Tobacco Hornworm, Manduca sexta, referred to as the tobacco hornworm or as the hawkmoth or sphinx moth (adult), is a common insect in a wide variety of habitats, such as tobacco fields and vegetable gardens. Tobacco and tomato hornworms are the common large caterpillars that defoliate tomato plants. Their large size allows them to strip a plant of foliage in a short period of time, so they frequently catch gardeners by surprise. They are quite similar in appearance and biology.

The tobacco hornworm is more common in the southern United States, especially the Gulf Coast States. Its range extends northward to New York. The tomato hornworm, in contrast, is uncommon along the Gulf Coast, but relative to tobacco hornworm is more likely to be encountered in northern states. They both occur in many areas, and may appear together on the same plant.

Hornworm eggs are spherical to oval in shape, measure about 1.50 mm in diameter, and vary in color from light green to white. Eggs are deposited principally on the lower surface of foliage, but also on the upper surface. Duration of the egg stage is two to eight days, but averages five days.

Life Cycle: Hornworms overwinter in the soil as pupae. Moths of this overwintering generation begin to emerge in early June and may continue to emerge as late as August. Nocturnal in habit, hornworm moths frequently can be seen hovering over plants at dusk. At night, eggs are deposited on the underside of leaves. Each moth deposits one to five eggs per plant visit and may lay up to 2,000 eggs.

Larvae emerge about 4 days later, depending upon temperature. After feeding for 3 weeks, hornworms burrow into the soil and spend 3 weeks after which a new generation of moths emerges. Heavy egg deposition is common in August and early September due to a peak in overwintering moth emergence along with that of a second or possibly third brood.

Damage: These important tobacco pests consume large quantities of leaf tissue, particularly as fifth instars. Two or more healthy larvae can completely defoliate a tobacco plant, leaving only midribs and stem. Severe damage most commonly occurs during late July and August.

Tobacco plants and other Solenaceous crops are rich in alkaloids. The most studied alkaloid is nicotine in the tobacco plant. The nicotine in the tobacco plant leaf is toxic to most insects, however, the Tobacco hornworm is able to succefully feed on this plant. Hornworms have a special mechanism for selectively sequestring and secreting the nicotine, so ,basically, it is not toxic to them.

Control: “Tomato” hornworms are easily controlled by garden insecticides. A biological control organism that is also highly effective and sold commonly is Bacillus thuringiensis. Hornworm larvae can also be hand-picked although they are surprisingly difficult to detect because of their cryptic coloration. Larvae tend to feed on the exterior parts of plants during shadier periods, near dusk and dawn, when they may also be more readily observed and destroyed.

Natural enemies of these pests are abundant. Polistes spp. wasps prey on the larvae and several wasp parasitoids Trichogramma spp Cotesia congregata, Hyposoter exigua are sometimes effective. In Florida, Trichogramma pretiosum was released to control larvae at a rate of 378,000/acre at 3-day intervals and high levels of egg parasitism were attained.

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