How to Control Hornworms

Be they tomato or tobacco, hornworms are a hungry bunch.

Jessica Walliser July 10, 2012

There’s a lot of information in the scientific names of insects. Take two common pests of tomatoes: the tomato hornworm, Manduca quinquemaculata, and tobacco hornworm, Manduca sexta. The Latin root of the genus name Manducameans “to chew,” something these pests are notorious for doing.

Each of the contiguous 48 states hosts one or both of these hornworm species. Pests of plants in the nightshade family, including tomatoes, tobacco, potatoes, and peppers, the larvae of hornworms can quickly defoliate a plant. These caterpillars earned their common name because of a soft spike or “horn” protruding from their hind end. Tomato hornworms have a dark spike and eight white Vs positioned sideways down the length of their body. Tobacco hornworms display a red horn and seven diagonal white lines along each side. Both species can reach 4 or more inches in length and spend several weeks feeding on plants before migrating down to the soil to burrow in and pupate. The brown pupae, which are occasionally unearthed while working in the garden, are 1 to 2 inches long and have a curved handle-like protrusion that encases the developing mouthpart or proboscis.

Adults of all hornworm species are commonly called sphinx, hawk, or hummingbird moths. While other nonpest species fly and feed during the day, tomato and tobacco hornworm adults fly only at night, when they can be spotted feeding on flower nectar. Tomato and tobacco hornworm moths are a mottled gray-brown with heavy bodies and a wingspan of 4 to 5 inches.

Tomato and tobacco hornworm caterpillars leave behind dark pellets of excrement, which are often spotted before the caterpillars. Damage is commonly noted first on the tops of plants. The larvae take shelter on or under interior leaves during the heat of the day, making them a challenge to find.

Hornworms frequently fall prey to nonstinging parasitic wasps that use them as hosts for their young. Tiny braconid wasps, less than 1⁄8 inch long, deposit eggs into the caterpillar’s body. The resulting larval wasps feed on the hornworm’s innards until they emerge to spin cocoons and pupate on the outside of the hornworm. If you spot a hornworm with white, ricelike sacs on its back, don’t disturb it. The caterpillar has already stopped feeding and will soon die. You can encourage these little beneficial wasps by interplanting tomatoes and other susceptible crops with flowering herbs in the carrot and mint families, such as dill, fennel, cilantro, basil, sage, oregano, and thyme. Adult parasitic wasps are attracted to their nectar.

Nonparasitized hornworm caterpillars can be controlled by hand-picking and with organic biological pesticides like spinosad and Bacillus thuringiensis (BT).

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