Devil In Disguise : Tomato Horn Worm

By Judy Sedbrook, Colorado Master Gardener, 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 often 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 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 dropping them into a bucket of soapy 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. *** "
The following photo is of the parasitic wasp larvae on the hornworm.

Life Cycle: Homworms overwinter as pupae in the soil. Adults begin emerging in mid-May in most years. Eggs are deposited on the undersides of leaves at night. Each moth deposits 1 to 5 eggs per plant visit and may lay up to 2,000 eggs. First generation larvae are present by late May or early June. After feeding for 3 weeks, the larvae burrow into the soil and pupate. First generation adults are emerging by mid-July and second generation larvae are present from early August to early October.

With our high temperatures right now, most tomato plants are unable to set fruit. Many of my friends have remarked that their plants are now being destroyed by the hornworm.
I hope the above info will be of some comfort and will help next season.