Hornworm is a name used for a group of caterpillars. These caterpillars have a hornlike appendage on the upper part of what looks like their last body segment. Hence, these immature insects are “worms with a horn.”

Hornworms metamorphose into an adult insect known as a moth. Such moths have several common names. One name is hawk moth because the moths are fast flyers. Another name, hummingbird moth, reflects the fact that these insects feed at flowers in hummingbird fashion. The name Sphinx moth is based on the family name of the insect, which is Sphingidae. That name was inspired by the behavior of the caterpillar that sometimes rears up on its prolegs, exposing its true legs and face and, in so doing, sort of resembles the Egyptian Sphinx.

Two species of Sphingidae are named Manduca sexta. and Manduca quinquemaculata. These insects are known as the tobacco hornworm and the tomato hornworm, respectively, and are closely related. The caterpillars both feed on plants from the Solanaceae family, which includes potatoes and
tobacco.

The caterpillars look similar, but tobacco hornworms have seven white diagonal lines on the side of the body and a red horn; tomato hornworms have eight such lines and a black horn.
The presence of hornworms on tomato plants in many gardens means that many people, including me, have some experience with these insects. I had the job of picking hornworms from tomato plants when I was a kid. I found the process kind of fun. Looking for the caterpillars that blended in nicely with the tomato foliage was a challenge. But the best part was getting to kill the worms by stomping them on the ground. If you stomped the worms just right, you could project the green innards toward a target, say a brother or sister. That was real incentive for a young boy to help with the garden pest control detail!

Some people even claim to have become entomologists because of encounters of a close kind with hornworms. Entomologist Howard Ensign Evans, the author of several popular books on insects, including “Life on a Little Known Planet” and “Wasp Farm,” makes such a claim.

According to Evans, “As a farm boy in the Connecticut Valley, I grew up with bean beetles and corn borers.

My first job was picking off tobacco hornworms for a penny each…”
Former head of the Department of Entomology at Purdue University, John V. Osmun purports to have been exposed at the tender age of nine months to a tobacco hornworm by his brother. According to family members, even at such a young age, future entomologist Osmun responded by gently stroking the hornworm on the back.

In his 1921 book, “Knowing Insects Through Stories,” naturalist and author Floyd Bralliar relates that, as a shoeless boy, he remembers discovering pupae of tomato hornworms when his father plowed the soil of the garden in the springtime. By keeping the pupae as pets, he learned about life cycles of insects when a moth emerged.
Over the years, the very hornworms that irritated gardeners and intrigued budding entomologists have also served as wonderful research animals. In fact, one wag once commented that hornworms are the entomologists’ white lab rats.

By studying hornworm moths, researchers have been able to show that cold-blooded insects can warm their bodies through heat generated by wing vibration. This activity allows a moth to fly at cool air temperatures. Much early research on how hormones regulate insect development was conducted on hornworm caterpillars. Also, this insect was used to understand the mechanisms of larval resistance to nicotine insecticide. Hornworm caterpillars that eat tobacco leaves filled with nicotine are not killed, so researchers used the insect to discover why this was so.

Hornworms have also been used to help entomologists understand how an insect is immune to certain kinds of diseases and parasites.
So, the next time you find a hornworm caterpillar on your tomato plant, think about why it is called a hornworm, or that it will become a hummingbird moth or that it is a widely used research animal, or that school children have learned something about nature by raising such a creature. I say, “Three cheers for the hornworm!” Like most creatures, even a pest insect, has a redeeming quality or two.

Readers with questions or comments for Tom Turpin may write to him in care of this publication.

This farm news was published in the April 23, 2008 issue of the Farm World, serving Indiana, Ohio, Illinois, Kentucky, Michigan and Tennessee.