The size of these garden pests allow them to quickly defoliate tomatoes, potatoes, eggplants, and peppers. Occasionally, they may also feed on green fruit. Gardeners are likely to spot the large areas of damage at the top of a plant before they see the culprit. Hornworms are often difficult to see because of their protective coloring. Not much for the heat of direct sunlight, they tend to feed on the interior of the plant during the day and are more easily spotted when they move to the outside of the plant at dawn and dusk
The tobacco hornworm larva (Manduca sexta) is generally green with seven diagonal white lines on the sides and a curved red horn (above). The tomato hornworms (Manduca quinquemaculata) have eight V-shaped marks on each side and their horn is straighter and blue-black in color (below). These "hornworms" are the larvae of hawk or sphinx moths, also known as hummingbird moths. The tobacco hornworm is the most commonly seen of the two, but both can be present on the same plant.
The presence of the hornworm may be noticed because of the large, black droppings (frass) that accumulate on the ground beneath the affected plants and the huge defoliation they do.
Hornworm damage usually begins to occur in midsummer and continues throughout the remainder of the growing season.
Handpicking. The large size of hornworms makes it easy to get hold of them. Once removed from the plant, they can be destroyed by snipping them in half with shears or dropping them into a bucket of water.
Rototilling. Turning up the soil after harvest will destroy any pupae that may be there.
Biological. Bacillus thuringensis, or BT (e.g., Dipel, Thuricide), is also considered very effective, especially on smaller larvae. Spray it as a precaution. Natural enemies, such as the parasitic wasp that lays its eggs on the hornworm's back, are common. If found, such worms should be left in the garden so the emerging wasps can parasitize other hornworms.
Insecticides. Hornworms can be controlled with carbaryl, permethrin, spinosad insecticides. Read the label carefully before using any insecticide.