Mainers have found a powerful ally in the annual battle against tomato hornworms.
Tomato hornworms destroy plants before the fruit even has a chance to ripen. They are a common, albeit unwelcome, pest that feed on plants including eggplants, pepper plants and tomatoes. In Maine, they seem particularly fond of the tomatoes and thanks to their voracious appetite, they can strip a plant clean almost overnight.
But the braconid wasp — a tiny, parasitic insect — can kill hornworms almost as fast as the worms themselves destroy a tomato plant. Otherwise, the only way to eradicate hornworms is by picking them off your tomato plants one-by-one. It’s good news for gardeners who are experiencing a particularly bad year for the hornworms.
The adult female wasp will sting the hornworm and lay her eggs inside its body, according to Jim Dill, pest management specials with University of Maine Cooperative Extension. The wasp larvae develop inside the caterpillar and eat their way out, killing the hornworm in the process.
“While they are developing inside the caterpillar, the wasp larvae are slowly making it incapable of doing anything,” Dill said. “They are basically turning it into a mummy.”
Once the wasp larvae break free of their hornworm host, they spin tiny cocoons on its body.
Dill said people often mistake those white cocoons on a hornworm for the larvae themselves.
“Eventually the new wasps emerge from those cocoons and go looking for a new hornworm host,” he said.
Hornworm caterpillars are bright green with white diagonal stripes on their bodies. They have a small, pointed horn-like structure at the base of their body which gives them their name. They can grow to nearly 4 inches in length.
“A lot of people know what they are, but there are also a lot of people with new backyard gardens seeing them,” Dill said.
Dill urges anyone who sees hornworms with the telltale white cocoons on their bodies to leave them alone so the wasps can emerge and live to fight the pests another day.
The wasps themselves are harmless to humans, he said.
“They are so tiny if one was to land on you or try to sting you, you would probably not even know it,” Dill said. “But they are definitely parasitic on the hornworm that ends up dying.”
