Tobacco hornworm
Tobacco hornworm

Hornworms are common large caterpillars that defoliate tomatoes and other solanaceous plants.

Hornworms are quite similar in appearance and biology. The tobacco hornworm (Manduca sexta L.) is more common in the southern U.S., especially along the Gulf Coast. The tomato hornworm (M. quinquemaculata Haworth), is more likely to be encountered in northern states.

Hornworm eggs are spherical to oval in shape and vary in color from light green to white. Eggs are deposited principally on the lower surface of foliage. The larva is cylindrical in form and bears five pairs of prolegs in addition to three pairs of thoracic legs. The most striking feature of the larva is a thick pointed structure or “horn” located dorsally on the terminal abdominal segment. The tobacco hornworm develops seven straight oblique whitish lines laterally. The white lines are edged with black on the upper borders, and the “horn” is usually red in color.

The tomato hornworm is similar. But instead of seven lateral bands, it bears eight whitish or yellowish V-shaped marks laterally, and pointing anteriorly. The V-shaped marks are not edged in black. The “horn” of the tomato hornworm tends to be black in color. There normally are five instars.

Mature larvae drop to the soil at maturity and burrow into the soil, where they pupate. The adults of both species are large moths with stout, narrow wings. Both species are dull-gray or grayish-brown in color, though the sides of the abdomen usually are marked with orange-yellow spots. The hind wings of both species bear alternating light and dark bands.

Survival And Spread
The number of annual generations is three to four in Florida, but two generations per year is common over most of the range of these species. The life cycle can be completed in 30 to 50 days, but often is protracted due to cold weather or diapause. Several Solanum spp. weeds are reported to serve as hosts. Adults imbibe nectar from flowers of a number of plants.

Hornworm larvae are defoliators, usually attacking upper plant parts consuming foliage, blossoms, and green fruits. The larvae of hornworms attain a large size and are capable of high levels of defoliation. About 90% of the foliage consumption occurs during the final instar. Larvae blend in with the foliage and often are not observed until they cause considerable damage at the end of the larval period. Hornworms are not normally considered to be pests of commercial crops in Florida, but may occasionally damage crops.

Management Methods
Chemical insecticides or bacterial insecticide (Bt) are applied to the foliage for larval suppression. The mature caterpillars are difficult to kill; so young larvae should be targeted. As with other insects scouting is important to detect infestations before serious damage occurs.

For growers, there are a wide variety of excellent materials available for the control of hornworm and other lepidopteran species. Natural enemies of these pests are abundant. Polistes spp. wasps prey on the larvae and several wasp parasitoids (e.g., Trichogramma spp., Cotesia congregata, Hyposoter exigua) are sometimes effective.