A Keen Eye Catches the Hornworm

By Barbara Damrosch

Sometimes a pest is so intriguing you almost wish it would appear in your garden. Almost. Take the tomato hornworm, which feasts on tomato plants and their relatives, such as peppers and eggplants. In its larval form, it is a huge caterpillar, up to 4 inches long. Sometimes the damage it does is slight, if there are few larvae and many plants. But because of its size, it can be voracious. If you notice foliage missing from the tops of the plants and even nibbles on the green fruits, hornworms might be the culprits. If so, you'll also find their dark-colored droppings on leaves below their feasting area, which will help you to locate them.

Finding them is harder than you'd think. A tiny aphid is more visible. Tomato worms have perfected the art of mimicry, and with their pale-green color they look exactly like part of the tomato plant itself. If you find one, you'll recognize it by the upright black horn that it carries on its hind segment and the white V-shaped markings on its sides. The tobacco hornworm, which has similar looks and appetites, has a red horn and the markings are diagonal stripes.

When hornworms have grown to maturity, they pupate in the soil, turning into sphinx moths and hawk moths, with five-inch wingspans. These are also called hummingbird moths and are often mistaken for hummingbirds as they hover before an open flower, buzzing their wings energetically during pollination. Don't harm them. You might have the moths without the larvae ever being a problem.

If they become a problem, take comfort in the fact that many allies, such as green lacewings and lady beetles, prey on hornworms by eating the larvae while they are small. Encourage these beneficial presences by keeping your yard poison-free. Mature hornworms are often parasitized by wasps. If you find a big green caterpillar covered with what looks like grains of rice, leave it be. A braconid wasp has laid eggs on its body, and these have hatched into larvae that consume the caterpillar from within. These larvae then pupate inside the white cocoons that you see and turn into adult wasps. The wasps will finish off their host and then go on to parasitize others.

For hornworms without the cocoons, handpicking is the best defense. Many people find this job extremely distasteful, even those people not normally repelled by creepy-crawlies. When poked, a tomato hornworm will start to emit a bright-green goo. I have also heard that a hornworm, when disturbed, will often make a noise (a sort of clicking sound), so shaking the plant a bit might help you find the worm's hiding place. I have not heard this with my own ears, though I would like to. In someone else's garden.

Article copyright of Barbara Damrosch. Originally published June 26, 2008 in The Washington Post and reprinted with permission. Creative Commons photo credit: NatureFreak07