While watering the two tomato plants in my great expectations garden, I noticed a few missing leaves and some black, barrel-shaped frass (insect excrement) beneath the plants. Certain I'd quickly find the hornworm culprits, I looked, and looked, and … I'm always amazed at how well these large caterpillars can remain hidden from our probing eyes!
Two types of hornworms may be found feeding on tomato plants: Tomato Hornworm (Manduca quinquemaculata) and Tobacco Hornworm (M. sexta). The caterpillars are called "hornworms" because of the prominent horn-like projection rising from the upper surface at the end of their abdomen.
Both hornworms will feed on tomatoes as well as several other closely related plants in the Solanaceae family including eggplants, peppers, potatoes, tobacco, and certain weeds. Backyard vegetable gardeners need to be alert for the symptoms of feeding activity by these luminous green caterpillars which includes missing leaves and stems, hunks bitten out of developing fruit, and the aforementioned frass on leaves and the ground beneath infested plants.
Tomato and tobacco hornworm caterpillars are the larvae of hawk or sphinx moths (family Sphingidae). Indeed, tomato hornworms eventually pupate to become the 5-Spotted Hawkmoth; the "quinque" in the specific epithet refers to the five spots on the moth. Both types of hornworms can grow to a truly impressive size of 4" in length and 1/2 - 5/8" in diameter. However, despite their size, these cleverly camouflaged caterpillars may go undiscovered for weeks owing that to their coloration and white markings. Both hornworms have white diagonal lines along their sides. The tobacco hornworms have a series of white diagonal lines while the lines on tomato hornworms appear as a series of white sideway "V's".
The caterpillars can be controlled through hand-picking and doing the "caterpillar stomp dance." Hornworm caterpillars are also subject to the depredations of several predators and parasitoids. Paper wasps, yellow jackets, and other wasps will grab them, chew them up, and take the remains to their nests to feed their larvae.
The tiny parasitoid wasp, Cotesia congregata (Family Braconidae) inserts its eggs into the caterpillars and the resulting wasp larvae consume the hornworms from the inside out. Just before the hornworms die, the full grown wasp larvae erupt through the upper epidermis to form oval, white, silk pupal cocoons. Rows of these white cocoons sprouting from tobacco and tomato hornworms are a well-known and a welcomed sight to home gardeners. Of course, the parasitized caterpillars should be left alone. They will do little to no feeding, and the wasp cocoons represent the potential future demise of numerous other hornworms.