Bugs in the Garden. Hornworm: Friend or Foe? Friend and Foe

Do you grow tomato plants? Who is your worst enemy in that case? Probably the tomato hornworm or the tobacco hornworm. Those fat green caterpillars with a funny looking horn at the rear end, red in the case of tobacco hornworms, blue in tomato hornworms. Besides that minor difference they look very similar; they both attack your precious plants with gusto and grow healthy and vigorous in such a diet. They also feed on tobacco, potato, peppers and other plants of the Nightshade family.


The moths that emerge from those caterpillars after they are done growing and after a period of rest called pupation are called hawk moths or sphinx moths. They are very beautiful, mostly brown and white with orange markings on the sides of their bodies, five in the tomato hornworm and six in the tobacco one; once again not a big difference.

Naturally you hate the worms, fear to find and infestation of them in your garden and would like to exterminate them forever. Fortunately, Nature has provided a few enemies of hornworms that help you in your battle for control but you are very likely not to know them and even accidentally eliminate your little helpers in an effort to combat the worms. So, it is important to know both.

Occasionally you find a hornworm all covered with white little oval things that you might mistake for eggs. Those are not eggs at all; they are your friends. It is fascinating to learn how they got there, although I must warn you: be prepared for some gore.


There is a tiny wasp, so small that if you drop a handful of them on a white piece of paper they look like the marks you could make with a regular pen, no bigger than some scribbles. This wasp lays its eggs inside the body of a caterpillar. The poor thing feels the prick and goes back to eating and growing for a while, without knowing that it is doomed. Eventually it becomes lethargic and later catatonic. It stops moving but continues to cling to the stem of one of your tomato plants.

In the interim the wasp’s eggs have turned into minute grubs that start doing a lot of eating and growing of their own at the expense of the caterpillar. When they are fully grown, something out of the movie “Alien” takes place. Remember a scene, early in the movie? An astronaut, that had suffered an encounter with the alien creature, seems fully recovered. Then, all of the sudden, he screams and writhes in pain. A blood covered hideous creature, a parasite, the alien in question, bursts out of his chest and scurries away. They must have gotten the idea from the parasitic wasp.

The full grown grubs emerge from the body of the caterpillar after making little holes. They cling to the body for safety, and spin cocoons around themselves. By then the caterpillar is too far gone to care. The wasp’s larvae need a period of repose inside their cocoons to turn into winged adults. They emerge in a few days ready to go and infest another caterpillar. I told you that it wasn’t pretty!


But it works to your benefit. Be thankful to the wasps that keep the population of hornworms down and prevent serious damage to your tomato plants. Hold the pesticides! You don’t want to kill these friends of yours along with your enemies.

There is still another twist to this story. As I said, the unwanted pest that devours your plants turns into a handsome moth. The transformation is so complete that people even end up calling it by a different name; in the case of the tobacco hornworm it becomes the Carolina sphinx moth. It flies at night visiting flowers such as morning glories, sweet potatoes, Nicotiana, etc. in search of nectar. It hovers on front of them, unfurls its long tongue and drinks deeply. In doing so it pollinates them. It has gone from foe to friend, from tomato destroyer to pollinator. We have to think twice about our desire to eliminate the hornworm entirely. Perhaps Nature works best by keeping a delicate balance by means of parasites.