Pam’s Perspective
Pam Otto is the Manager of Nature Programs and Interpretive Services for the St. Charles Park District
Fall is in full swing these days, with animals everywhere getting ready for the winter that lies ahead. Birds are migrating; mammals are storing fat and/or food—I personally can vouch for that one. And the hornworms have put themselves to bed until spring.
I know this because there’s one napping on our front desk at Hickory Knolls. Or at least there was, until we tucked it back in again, in the mulch beneath some shrubs outside the front door.
The hornworm pupa, destined to transform into a sphinx, or hawk, moth next spring, came special delivery from the park district’s Primrose Farm. The little critter, looking quite unlike its more famous—or infamous—self, was dug up along with a potato. Being one of those articles of nature that folks just aren’t sure about, it ended up at our place.
So, what the heck is a hornworm?
“Hornworm” is the common name given to many species of caterpillars in the Sphingidae, or sphinx moth family. The name comes from the prominent spike, or “horn,” that rises from the rear of many of these larvae. Their other common name, sphinx moth, also can be traced to the caterpillars and their tendency to rear their heads back, sphinx like, when threatened. Hundreds of different species of hornworms/sphinx moths live in the eastern United States. But today we’re going to focus on only a couple.
I hate to even mention them, given their reputation among gardeners, but here we go: tobacco and tomato hornworms. Yep, those big, green caterpillars that devour tomato, pepper, eggplant and potato plants.
Even though none of us at Hickory Knolls are caterpillar experts, we’re certain that the hornworm pupa we inherited must be one of these two species. And, if I had to venture a guess, I’d go with tobacco hornworm, Manduca sexta. Also known as the Carolina sphinx moth, this is the species more common in our area. The tomato hornworm, Manduca quinquemaculata, which matures into the five-spotted hawkmoth, just isn’t seen as often, even though we have way more tomato plants than tobacco plants growing here.
Regardless of which one we have, it’s a given that it spent the last few days of its caterpillar-hood munching and crunching away on garden greenery. As these larvae grow and molt, their appetites increase right along with them. In fact, entomologists estimate that 90% of the damage hornworms cause is completed in their final instar, or developmental stage—the days leading up to pupation.
It’s that seemingly destructive behavior that’s given these fascinating creatures a black eye (or black ocellus, if you’re into technical terms). But, I’ll go out on a limb here and say that, while it’s easy to accuse the caterpillars of bad behavior, we’re really the ones to blame.
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Pam’s Perspective – Page 2
Tobacco and tomato hornworms are just like the raccoons, skunks, opossums, chipmunks, geese (choose the “nuisance” species of your choice) that are all around. They thrive here in suburbia because of the ideal conditions we’ve created.
By nature, these species feed on plants in Solanaceae—the nightshade family. While not unheard of, the plants in this family that are truly native to Kane County are rare. So I’d wager that a couple hundred years ago, tobacco and tomato hornworms in this area were few and far between.
But as Kane County grew, so did the list of plants these juicy-looking juveniles find irresistible. Besides that impressive list of garden plants, the caterpillars also feed on petunias; jimsonweed, an introduced and highly toxic behemoth of a plant; and nightshade—another plant we’re also responsible for bringing to this country.
So here we are today, living amid a landscape loaded with food for tobacco and tomato hornworms. When you think about it that way, it’s no wonder these caterpillars and, now, pupae, seem to be showing up with increasing frequency in gardens, mulch and compost piles.
What will be interesting to see is how the animals that benefit from hornworm fare in the coming years.
It’s true, not a lot of things eat tobacco hornworms. That plant family they feed on, Solanaceae, is well known for its toxic compounds; some of those properties are no doubt passed on to the caterpillars. But there’s one group of insects who are more than happy to have hornworms around: wasps.
Polistes spp., or paper wasps, frequently feed on hornworms, carrying young caterpillars back home to feed their own developing larvae. Next summer, if you notice a paper wasp nest hanging from your eaves, and you’ve got a garden, you might want to consider leaving it in place, as a means of biological hornworm control.
Braconid wasps also help keep hornworm populations in check. These small, solitary wasps come in a variety of colors—there are more than 15,000 described species—and pose no threat to humans. But they’re death on hornworms. Females of this family lay their eggs on the caterpillars, which are then literally eaten alive by the developing wasp larvae. If you ever find a hornworm that looks as though it has dozens of grains of rice embedded its back, leave it be—it’s actually an incubator for another generation of helpful wasps.
It’s true, the hornworm pupa that’s lying outside our door right now may survive the winter and develop into a large (wingspan of about 4 in.) hawk moth. And next spring that moth may flutter over to the community garden plots and lay eggs on the tomato, potato, pepper and eggplants growing there. But I’m not too concerned about the consequences. Last I checked, we had a pretty healthy population of paper wasps on the side of the building that faces the gardens…next summer, when the new colonies move in, those wasps will be happy to help themselves to hornworms.
Pam Otto, who apologizes profusely for use of the words “pupa” and “larva,” terms she has been told are not appreciated by people who eat while reading, is the manager of nature programs and interpretive services for the St. Charles Park District. She can be reached at 630-513-4399 or potto@stcparks.org.