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.
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
Hornworm damage usually begins to occur in midsummer and continues throughout the remainder of the growing season.
Parasitic wasp cocoons on hornworm
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
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010