Tobacco hornworms: Big, green and in the garden

            The big, meaty green caterpillars that many of us have been fighting to eradicate from our tomato, eggplant and pepper plants this summer make plenty of people squirm. In part it’s because they are among the largest caterpillars in the region, sometimes reaching close to three inches in length, with reddish horns that look like stingers (but aren’t). We also hate them for their voracious appetites and preference for consuming our favorite crops.

            But before you pick and squish every tobacco hornworm caterpillar you see, think about the beautiful giant moth they become and the important role they play as native insects in the environment. If you listen to Sam Jaffe, proprietor of The Catepillar Lab in Keene, N.H., you may reconsider plucking them from your plants at all and instead try to protect them.

            Once they complete their metamorphosis, tobacco hornworms become Carolina sphinx moths, large-bodied moths with a five-inch, coffee-colored wingspan that enables them to hover over flowers like hummingbirds. Jaffe says they have the longest tongues of any insect in New England, allowing them to feed on and pollinate deeper flowers than any other moth.

“They’re very impressive and long-lived, and they almost give off the impression of being mammal-like – fluffy and hairy with large eyes that reflect red in the light,” said Jaffe. “They’re special in a lot of ways.”

            And not just as adult moths. The caterpillars exhibit some pretty special behaviors, too. After hatching from tiny eggs on the underside of leaves, the green caterpillars decorated with diagonal white lines go through five stages of growth, called instars, growing larger with each stage.

By the time they reach their fifth instar, they can become quite bold, occasionally rearing up on their hind legs when disturbed and thrashing their head back and forth. Jaffe calls that behavior regal, though he admits that he would have trouble convincing tomato farmers of that.

He would have even more trouble if he pointed out that tobacco hornworms may even try to nip your finger if you let them, though their mandibles aren’t strong enough to break your skin. Those mandibles also produce an audible clicking sound that is believed to serve as a warning to predators.

            When the caterpillars have matured and finished feeding, they drop to the ground and dig an underground chamber in which to pupate, remaining there through the winter months until the moth emerges in the spring.

            From an ecological perspective, tobacco hornworms play numerous roles. The early stages of the caterpillar, when they’re still tiny, serve as food for a wide variety of birds, and larger birds and scavenging mammals will eat the mature caterpillars, too.  Other insects eat them as well, like stinkbugs and assassin bugs, which insert their uniquely-adapted mouthparts into the caterpillar and suck it dry.           

            And then there are the parasitic wasps. One variety injects its eggs inside the egg of the hornworm, and when the eggs hatch, the wasp larvae kill the hornworm egg and emerge as adult wasps. Another version lays its eggs in the caterpillar, and when the wasp larvae emerge, they spin cocoons on top of the caterpillar. So if you see a hornworm covered in tiny white cocoons, that’s a hornworm you shouldn’t kill, because all those cocoons will become more wasps that will kill more hornworms. It’s a natural process that keeps the caterpillar population under control. 

            If you still can’t make yourself love tobacco hornworms, put one in a jar and watch it for a few weeks. Jaffe calls them “great guest caterpillars” that are fun to observe and easy to rear. He says farm camps and schools with gardens can easily incorporate them into educational programs.

            Just don’t’ confuse them with the very similar tomato hornworm, which has a straight blue horn rather than the curved red horn of the tobacco hornworm. Tomato hornworms have mostly disappeared from the Northeast, so almost any hornworm you see will likely be the tobacco variety.

            While you can’t be faulted for removing hornworms from your crops, Jaffe hopes you will try viewing them through his eyes. If you do, you’ll see an important part of our natural heritage, one that is helping him demonstrate that caterpillars are fascinating insects that can be used to inspire the next generation to appreciate all that is wild.

This article first appeared in the Bennington Banner on September 9, 2016.