Egg - Hornworm eggs are smooth, spherical, and about 1.3 mm in diameter. Light green at first, they turn white before hatching.
Larva - Mature tobacco hornworm larvae usually have green bodies with fine, white hairs and seven diagonal stripes on each side; the posterior horn is usually curved and red. Tomato hornworm larvae have eight V-shaped markings on each side; the horn is straight and black. Both species are about 75 to 85 mm long when fully grown.
Pupa - Pupae are brown, hard, spindle-shaped, and about 50 mm long. They have a curved, pitcher-handle-like tongue case. The tongue case of the tomato hornworm is longer and more curved than the tongue case of the tobacco hornworm.
Host Plants - Tobacco is the principle host of hornworms, though other plants of the family Solanaceae are consumed, such as tomato and horsenettle.
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.
Life History - 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.
In North Carolina, at least two and one-half generations occur each year. Early generations are potentially damaging to marketable tobacco. Later ones feed after harvest on noncommercial suckers. However, these last generations are important because they produce the overwintering pupae. Pupae are stimulated to enter diapause (the resting period) after the second weed of August by the shorter day lengths they encountered as larvae.
Use of cultural practices is very important. Early planted tobacco, proper (not excessive) nitrogen fertilization, sucker control, stalk destruction, and fall plowing all help to reduce overwintering populations.
Hornworms should be treated with insecticides when infestations exceed the economic threshold level of 5 or more large, unparasitized larvae, 2.5 cm (1 inch) or longer, found per 50 plants. Parasitized hornworms (with small white cocoons) eat less and are counted at one-fifth of a larva. If applications are necessary during harvest, make them immediately after rather than before priming. For specific chemical recommendations, consult the current North Carolina Agricultural Chemicals Manual.