Last summer I found three tomato hornworms feeding on my tomato vines. I was impressed with their bright colors and 4-inch length and wondered just what kind of a moth these turn into when they complete metamorphosis.
Hornworms, which are not really worms, have a horn on their tail. Tobacco hornworms look similar but have a red horn on their tail. As pupae, they overwinter in garden soil and hatch the following summer.
Most people become upset to see caterpillars damaging their tomatoes. Instead of killing them, just move them to a better spot.
As I often do when I find caterpillars, I brought the hornworms into the house and fed them tomato leaves. They were in the final process of being a caterpillar before entering the pupa stage. They were frantic to eat and then start walking. All the butterflies I have raised go through this process.
The tomato hornworm will dine on any member of the nightshade family, which includes potatoes, eggplants, peppers and, of course, tomatoes. One friend let them feast on her potato leaves. Since the edible part of the plant is underground, she saw no reason to disturb them.
Most butterfly and moth caterpillars have specific plants they feed on. The butterflies and moths lay their tiny eggs on a leaf of the host plant. The eggs hatch and the larvae grow to full size within weeks. Monarchs take two weeks to grow to 250 times the size they were when hatched. The tomato hornworm probably grows by 500 times, from egg to mature worm.
Shortly after I put the hornworms in a container with leaves and some damp moss, they buried themselves in the moss and turned into large brown pupae.
During the winter I checked on them every couple of weeks and moistened the moss if it seemed dry.
There is a difference between a pupa and a cocoon. The pupa emerges when the larva sheds its final skin or coat. In contrast, cocoons are formed when the moth weaves a silk coat over its entire body. The silk shrinks and the moth sheds its coat inside the cocoon. Silkworms exhibit this behavior. On the Internet, I learned that there are people all over the country who help the tomato hornworm to complete its life cycle. Many post their experiences and advice on the Garden Web forums. They suggest keeping the pupa in a sealed container in the refrigerator so it does not break the pupa stage before summer. When it does, it will emerge as the adult five-spotted hawkmoth (Manduca quinquemaculata), also known as the hummingbird moth. This moth is 5 to 6 inches across and is a night pollinator of deep-throated flowers. Unless you are prowling your garden in the dark, you probably will not see it.
These giant moths can cause problems for farmers growing vegetables in the nightshade family. Chemical controls exist for the worms, but the pests aren’t noticeable until the end of the larval stage, when they change colors. The chemical controls also kill other insects so I would never use them.
Better to rely on Integrated Pest Management techniques and let the insects control each other. I have seen a wasp remove a worm from a tomato plant section by section. The wasp was supplying its own nest with food for its newly laid eggs once they hatched.
You can also hand-pick the worms when you see them, but I must warn you that they bite. This happened to me not long ago. I was so surprised that I dropped the worm, and I have no idea where it landed.
A parasite called the braconid wasp lays eggs inside the caterpillar. As the hornworm eats and grows, the wasp eggs are dining on the caterpillar and eventually kill the host.
This past May, I removed the hornworm pupae from my refrigerator and put them in a butterfly cage, still buried in moss with the top of the container open. One pupa cracked open and lost all its fluid shortly afterward.
I read that the tomato hornworm pupa emerges from the soil after the soil reaches summer temperature and the host plants have grown. I wanted to see this moth emerge before I released it. So far, my pupa has not moved, but I have high hopes that it soon will and that I will witness one of nature’s wonders.
Winter Vegetable Workshop
Napa County Master Gardeners will be teaching a “Vegetables in Winter” workshop on Saturday, Aug. 18, from 9:30 to 11:30 a.m., at the University of California Cooperative Extension Office in Napa. Learn what winter vegetables to grow and how to grow them.
Free Composting Workshop
In conjunction with the City of Napa, Napa County Master Gardeners will hold a free workshop on “Yard Waste Prevention and Home Composting.” The workshop is Saturday, Aug. 18, from 9 to 10 a.m., at the White Barn in St. Helena. Discover how to convert yard waste into rich compost for your garden. Become familiar with tools and types of bins for home composting. Learn about using grass clippings to add valuable nutrients to your soil. Pre-registration is required. Register online. Phone registrations are not accepted.
Master Gardeners are volunteers who help the University of California reach the gardening public with home gardening information. Napa County Master Gardeners are available to answer gardening questions in person or by phone, Monday, Wednesday and Friday, 9 a.m. to noon, at the UC Cooperative Extension office, 1710 Soscol Ave., Suite 4, Napa, 253-4221, or from outside Napa toll-free at 877-279-3065. Or email your garden questions by following the guidelines on our web-site. Click on Napa, then on Have Garden Questions?