Hornworms on tomatoes

Ken Johnson

There are several different types of caterpillars that will feed on tomatoes. The most well known, and probably most dreaded, are the tomato (Manduca quinquemaculata) or tobacco (Manduca sexta) hornworms. These large (up to four inches long) green caterpillars have a prominent “horn” on their rear end (thus their name) and can do quite a bit of damage to tomato plants.

Hornworm damage usually begins to occur midsummer and continues throughout the remainder of the growing season. Because of their size, they can quickly defoliate tomatoes and less commonly potatoes, eggplants and peppers. They may also occasionally feed on green fruit.

You’ll likely spot the damage caused by these caterpillars long before you see the culprit. Hornworms are often difficult to see because of their protective coloring. They prefer to stay out of the heat and direct sunlight, so they tend to feed on the interior of the plant during the day. They are more easily spotted when they move to the outside of the plant at dawn and dusk. The presence of the hornworm may also be noticed because of the large, black droppings (frass) that accumulate on the ground beneath the affected plants.

If you find one of these caterpillars and are want to know what species you have here is how you can tell them apart. The tobacco hornworm larva is generally green with seven diagonal white lines on the sides and a curved red horn. The tomato hornworms have eight V-shaped marks on each side and their horn is straighter and blue-black in color. The tobacco hornworm is the most commonly seen of the two.

These hornworms are the larvae of hawk or sphinx moths, also known as hummingbird moths.  The adult moths emerge in mid- to late spring. The moths of both of these species are very large with wingspans of up to five inches and are capable of flying long distances. After mating, females lay small, pearl-like eggs individually on tomato foliage and leaves of other hosts (pepper, potato, eggplant and some weeds of the nightshade family). The caterpillars hatch and usually pass through five larval stages, or instars, in about one month. Fully grown larvae pupate in the soil. There are usually two generations produced each summer; larvae of the second generation overwinter as pupae.

There are several different ways you can control these pests in your garden. Hand picking is a good way to control these caterpillars, especially in smaller plantings. Rototilling the soil after you are done growing for the year is a very effective management strategy too. It has been shown to destroy up to 90 percent of overwintering pupa. Pesticides can also be used to manage these pests; make sure to read and follow all label directions.

Nature also does a good job at helping control populations of hornworms. Eggs and small larvae will be eaten by a variety of different insects. Hornworm caterpillars are also attacked by small parasitic wasps. The wasps will lay eggs in the hornworm larvae. When the wasp eggs hatch the larvae consume the internal organs and fluids of the caterpillar. When the wasp larvae complete their development, they form white, silken cocoons on the outside of the caterpillar’s body. When the adult wasps emerge from the cocoons, the caterpillar will die. If you these parasitized hornworms, leave them be in order to conserve the beneficial wasps. The wasps that emerge will find other hornworms to parasitize.

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