tobacco hornworm (69333 bytes)

The Tomato Hornworms

By Judy Sedbrook, Colorado Master GardenerSM, Colorado State University Cooperative Extension, Denver County

Tomato/tobacco hornworms are the largest caterpillars found in this area and can measure up to 4 inches in length.  The prominent "horn" on the rear of both gives them their name.

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 found in this region and may even be present on the same plant.

tomato hornworm larva  (12646 bytes)

Tomato Hornworm

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.

hornworm with frass (5190 bytes)

Hornworm with frass

Hornworm damage usually begins to occur in midsummer and continues throughout the remainder of the growing season.

CONTROL:

  • 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.

Parasitic wasp cocoons on hornworm by W.W. Morgan (53551 bytes)

Parasitic wasp cocoons on hornworm

  • Insecticides. Hornworms can be controlled with carbaryl, permethrin, spinosad insecticides. Read the label carefully before using any insecticide.

Photo Credits:

Tobacco hornworm:  Stephen Younge

Tomato hornworm:  Paul M. Choate, University of Florida

Hornworm with frass: Oklahoma State University, Dept. of Entomology

Parasitized hornworm: W.W. Morgan

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Date last revised: 01/05/2010