Description: Common throughout North America, the tomato hornworm (Manduca quinquemaculata) is one of the most destructive pests of tomato, potato, pepper, eggplant and tobacco plants. They consume entire leaves, small stems, and sometimes chew pieces from fruit. Despite their large size, hornworms are often difficult to spot because of their protective coloring. Growers will often find large areas where feeding has occurred before they see this garden pest. Damage is most often noticed in midsummer and continues throughout the remainder of the growing season.
Likely to be the largest caterpillars you’ll see in the vegetable garden, tomato hornworms (3-4 inches long), are green with seven diagonal white strips and a black or red horn projecting from the rear. Adults are large (4-5 inch wingspan), heavy-bodied moths. They are gray or brown in color with white zigzags on the rear wings and orange or brownish spots on the body. Also called a sphinx or hawk moth, they fly quickly and are able to hover like a hummingbird.
Tip: To find the larvae hidden among plants, look for black droppings (frass) on the leaves and ground and spray the foliage with water. The caterpillars will thrash about and give away their hiding spots.
Life Cycle: Overwintering occurs in the soil as dark brown pupae. Adult moths emerge in late spring, mate and deposit spherical green eggs on the underside of leaves. In 5 days hatching begins and the larva passes through five or six stages before reaching full growth in 3-4 weeks. These larvae eventually burrow into the soil where they transform into the pupal stage. Adults develop in 2-4 weeks and work their way to the soil surface, where they mate and begin laying eggs for the next generation of hornworms. There are two generations per year.
Tomato Hornworm Control: Because they are so large hornworms are most often controlled in home gardens by handpicking. Once removed from the plant, they can be destroyed by dropping them into a bucket of soapy water. Beneficial insects including lacewings, braconoid and trichogramma wasps, and ladybugs attack the eggs. For best results, make releases when pest levels are low to medium. If populations are high, use a least-toxic, short-lived natural pesticide to establish control, then release predatory insects to maintain control. Both Dipel Dust (Bacillus thuringiensis, var. kurstaki) and Monterey Garden Insect Spray (Spinosad) are very effective, especially on young caterpillars (larvae). If pest levels become intolerable, spot treat with botanical insecticides. Roto-tilling after harvest destroys overwintering pupae in the soil. This is especially effective since pupae are large and not buried very deeply in the soil. Results have shown that greater than 90% mortality is caused by normal garden tilling.
Note: If you have caterpillars that have parasitic wasp cocoons attached to them, don’t destroy them! Collect them instead and allow them to eat unwanted or volunteer tomatoes until the wasps hatch inside. Now you’ve got an army of free, natural predators to work for you.