AUBURN UNIVERSITY, Ala. — Caterpillars come in many shapes, sizes and species. They may also appear harmless to the unfamiliar eye. However, one particular species is a wolf in sheep’s clothing for gardeners and farmers: the hornworm.

Hornworm Species

Tomato hornworm moth.

Although named as a worm, the hornworm is actually a huge caterpillar. Manduca quinquemaculata, commonly known as the tomato hornworm, is a familiar pest in the Southeast. Another species is the tobacco hornworm, Manduca sexta, which closely resembles the tomato hornworm. The tomato hornworm has green margins on its white stripes, and its horn is blue or green in color. The tobacco hornworm has black margins on its white stripes with a red horn.

Hornworms emerge from eggs on the underside of plant leaves. At this stage, they are yellow or white and contain no markings on their body. Alabama Cooperative Extension System Entomologist Xing Ping Hu said the larvae eat leaves, molt four times and get bigger after each molting.

“Hornworms can grow up to 4 inches in length,” Hu said.

As they mature, their bodies molt and simultaneously turn green, giving them their V-shaped markings and iconic horn-like tail. Hornworms will overwinter and emerge as adult moths in the spring. Adult moths are giant and robust bodied, with a wingspread of 4 to 5 inches. They are grayish brown with yellow spots on the sides of their body.

Damage

The hornworm typically feeds on tomatoes. However, this caterpillar is known for feeding on many other crops in the Solanaceae family aside from tomatoes.

“Their favorite plant is the tomato, but they have also been found on pepper, eggplant and potato plants,” Hu said. “Additionally, many weeds can serve as alternate hosts, including horsenettle, jimsonweed and nightshade.”

Their natural green color serves as the perfect camouflage in row crops and gardens. This worm is a voracious eater and can defoliate an entire plant in a matter of days.

“Young and mature caterpillars blend in with leaves very well and might not be noticed until most of the damage is done,” Hu said. “However, as they feed, they create dark green or black droppings that are clearly visible.”

Hornworm Control

There are multiple ways that gardeners and farmers can prevent damage from hornworms. It is important for gardeners to use the method that is most beneficial to their garden setting.

Environmental Adjustment

Removing weeds can reduce the opportunity for hornworm egg sites. Regular garden and crop surveys are encouraged to ensure the insect does not have the necessary hosts to reproduce. Tilling the soil at the beginning and end of each gardening season will also help destroy overwintering larvae and pupae.

Physical Removal

Picking hornworms by hand is usually the only necessary management in most cases. Caterpillars are harmless to people and pets. Hu said gardeners can drop them into soapy water or feed them to chickens if they have a backyard flock.

Natural Enemies

Parasitized hornworm by a braconid wasp.

Some natural predators of hornworms include lady beetles, green lacewings and paper wasps. The most prominent foe of the worm is the braconid wasp. This species of wasp will lay eggs on the hornworm and parasitize their body. The wasp eggs hatch into larvae that then feed on the caterpillar’s body, later creating protruding rice-like cocoons. It appears as if the caterpillar is carrying grains of rice on its back.

Hu said if you see this occurring, it is wise to leave the hornworms in the garden to allow the adult wasps to emerge. At this stage, the parasitized caterpillars lose the ability to eat and will die. Later, the wasp will look for other hornworms to parasitize.

Chemical Application

Pesticides are generally not necessary. However, if other options are not effective or practical, you may consider applying a product to kill smaller worms before severe defoliation. The insecticide of choice is Spinosad, a natural product that is approved by the Environmental Protection Agency, or an insecticidal soap, rather than using a residual insecticide in home gardens.

More Information

Education of pests is crucial to the success of any garden or crop. To ensure that you are ready to combat garden invaders, gather more information from the Alabama Extension website at www.aces.edu.