As this season wraps up, we’re taking some time to reflect on the garden—what worked, what didn’t, and the lessons we learned along the way.
And this year, one lesson came in the form of a very large, very hungry green caterpillar.
If you’ve just discovered your first fat hornworm contentedly munching away on a tomato plant, you probably have a lot of questions. First off, what is it? Should you kill it? What will it become? Is that thing worth protecting? And, perhaps most importantly, at what cost?
We spent an entire season getting to know tomato hornworms, and my feelings toward them changed more than once along the way. At times I was delighted by them, at others thoroughly frustrated—and by season’s end, mostly awestruck by the intricate workings of nature.
From the title, you already know the path we chose: we offered mercy to the fat, wriggly beasts.
In this week’s article, I’ll tell you what happened—and let you decide whether you’d make the same choice.
Hornworms actually come in two forms: the tobacco hornworm (Manduca sexta) pictured here, and the closely related tomato hornworm (Manduca quinquemaculata). But no matter their taxonomy, both of these hornworms can prove surprisingly destructive.
First, Meet the “Tomato” Hornworm
Despite the name, the fat green caterpillar chewing through your tomato plant may not be a tomato hornworm, per se.
Two closely related species commonly feed on tomatoes: the tomato hornworm (Manduca quinquemaculata) and the tobacco hornworm (Manduca sexta). Tomato hornworms have a series of white V-shaped markings along their sides, while tobacco hornworms have straight diagonal white stripes and typically a reddish horn on the rear.
Most of the caterpillars we encountered this summer were tobacco hornworms—the species pictured throughout this article.
Either way, the basic story is the same. The caterpillar spends several weeks eating voraciously, eventually drops to the soil to pupate, and emerges as a large sphinx moth. In warm years, a second generation can appear later in the summer.
And that transformation is a big part of what makes these insects so difficult for me to dislike.
A Carolina sphinx moth, the adult form of a tobacco hornworm. This moth has just emerged from its pupa, and its wings are still soft, curled, and not yet ready to support flight. By nightfall, the wings will be fully expanded, hardened, and ready for flight.
From Hornworm to Sphinx Moth
The progression from hornworm to sphinx moth could have come straight out of an Eric Carle book, except this hungry caterpillar has healthy eating habits. No ice cream, cherry pie, or lollipops for him. He goes straight for the nice green leaves—your green leaves.
And in no time, he’s a very fat caterpillar.
Late this summer, while our photographer was in the garden, we watched a full-grown hornworm suddenly release its grip and drop from a tomato plant to the ground. Having spent plenty of time trying to pull these caterpillars from stems, I can confirm that falling is no accident. When a hornworm wants to hang on, it hangs on.
A mature hornworm falls to the soil and burrows down to pupate.
But once a caterpillar has finished feeding, it leaves the plant and heads for the soil, where it burrows down and forms a pupa.
Eventually, it emerges as a large sphinx moth—also called a hawk moth or hummingbird moth.
Sphinx moths are powerful, fast-flying insects built for hovering at flowers. Many are active around dusk or after dark, using their long mouthparts to reach nectar deep inside tubular blooms. In the process, they can serve as important pollinators of night-blooming plants.
I got to see that relationship firsthand nearly a decade ago, when we grew a large bed of flowering tobacco. As evening settled in and the flowers released their fragrance, sphinx moths would begin appearing over the plants, hovering from bloom to bloom. They were absolutely breathtaking.
I suspect that experience is at least partly responsible for my reluctance to squish the fat green caterpillars I find on my tomatoes today.
Unfortunately, knowing what a hornworm will become doesn’t make it any less hungry.
Tomato plants heavily defoliated by hornworms. Damage is most common near the ends of branches where foliage is young and most tender.
How Much Damage Can Hornworms Really Do?
I should probably set the stage by confessing to a serial crime: I planted a few more tomato plants than I typically do...like twice as many. Actually, it was 400 plants in total.
And while I managed to conceal that little secret from my husband—the resident tomato twiner—for quite some time, the hornworms figured it out pretty quickly.
At first, a few missing leaves didn’t bother me much. Tomato plants are vigorous, and I was willing to sacrifice a little foliage in exchange for a few more sphinx moths in the garden.
Then the hornworm population exploded.
Within a surprisingly short period of time, entire sections of plants had been stripped bare. Some stems were reduced to little more than petioles, and eventually the caterpillars moved beyond the foliage and began taking bites out of the tomatoes themselves.
That kind of damage matters. A mature tomato plant can tolerate some defoliation, but hornworms are enormous caterpillars with equally enormous appetites. When enough of them are feeding at once, the loss of leaf area reduces the plant’s ability to photosynthesize just when it should be supporting a heavy crop of developing tomatoes.
It’s impossible to isolate the hornworms from all the other variables that change from one growing season to the next—weather, disease pressure, and even the particular varieties we grow. But if I had to make my best estimate, I’d say our tomato crop took at least a 30% hit in yield during the worst of the infestation.
In other words, this wasn’t just cosmetic damage. It was a real production problem.
The easiest way to locate a hornworm is to look for its poop, more politely called "frass". Hornworm frass is almost black with a shape that resembles, quite literally, a tiny grenade.
Follow the Poop: How to Find Tomato Hornworms
For creatures that can grow nearly as long as your finger, hornworms are surprisingly good at disappearing.
Their green bodies blend almost perfectly into tomato foliage, and I’ve stood inches away from a full-grown caterpillar without seeing it. Often, the first thing you notice isn’t the hornworm itself—it’s the damage it leaves behind.
Look for bare stems, missing leaves, and large dark droppings—called frass—on the foliage or ground below. Hornworm frass is surprisingly large, especially once the caterpillars reach full size. If you find a fresh pile, start looking directly above it. There’s a good chance the culprit is sitting somewhere nearby.
Once you know what to look for, finding them becomes a bit of a treasure hunt. Check along the undersides of stems and leaf petioles, particularly near fresh feeding damage. And don’t assume the caterpillar will be sitting on the most obviously damaged branch—they can travel farther through a plant than you might expect.
At the height of our infestation, the frass became one of the easiest ways to tell where the hornworms were feeding, even when the caterpillars themselves remained perfectly camouflaged.
It starts with a few innocent looking worms, but before long a minor infestation can become a major one. In our case, numbers quickly approached the hundreds and likely ran into the thousands by season's end.
A Ticking Time Bomb
For a while, it seemed like we might have weathered the worst of it.
The first wave of hornworms disappeared into the soil, the tomato plants began putting on new growth, and the sphinx moths started making their evening appearances around the garden.
Then the next generation arrived.
This time, my patience was wearing a little thinner. We had already watched the first round strip huge sections of foliage and take a noticeable bite out of our tomato crop, so when fresh caterpillars began appearing, I started plucking a few from the plants.
But there was a problem: there were so many of them, and this second generation was smaller and much harder to spot.
I could stand in front of a badly damaged plant, know perfectly well that a caterpillar had been feeding there, and still struggle to locate it among the stems and leaves.
Fortunately, something else in the garden had gotten very good at finding them.
And unlike me, it wasn’t interested in simply carrying them away.
An adult braconid wasp (Cotesia congregata) that has just emerged from its cocoon. Credit: Beatriz Moisset, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
The Braconid Wasp, A Gardener's Best Friend
Just as the second generation was becoming difficult to manage, I began finding caterpillars covered in clusters of tiny white cocoons. Then there were more. And more. Before long, the tomato plot looked like a miniature war zone—zombie caterpillars everywhere.
These were the work of tiny braconid wasps, and compared with me, they operated with almost unnerving precision.
While I could stare straight at a tomato plant and still be fooled by a hornworm’s sophisticated camouflage, the wasps had another trick up their tiny little sleeves.
They could smell them.
Alright, wasps don’t have sleeves—or noses, exactly. And technically, much of what they’re detecting comes from the tomato plant itself. When a caterpillar begins chewing, the damaged plant releases a distinctive blend of volatile compounds into the air. Parasitoid wasps can use those chemical signals to home in on plants where suitable caterpillars are feeding.
Lesson, kids: don’t chew with your mouth open.
Once a female wasp closes in, camouflage offers the hornworm very little protection. She locates the caterpillar and deposits her eggs inside its body, beginning a process that is equal parts fascinating and horrifying.
And suddenly I began looking at all those hornworms a little differently.
The wasps were finding caterpillars I never would have spotted—and they were finding them in astonishing numbers. Within a relatively short time, the hornworm population that had seemed ready to overwhelm the tomato plot began to collapse.
I was relieved.
And also, admittedly, a little heartbroken for the caterpillars.
But before we get to that part of the story, we need to clear up one of the most common misconceptions about those strange white structures covering their backs.
Although commonly misidentified as eggs, those tiny white structures on a hornworm are actually cocoons of the Braconid wasp Cotesia congregata. Here, the cocoons have been uncapped by the emerging adult wasps. Once airborne, adult females will mate and go out in search of more hornworms to parasitize.
What Are Those White Things on a Hornworm?
If you’ve ever found a hornworm covered in little white capsules, you may have heard them described as wasp eggs.
They aren’t.
By the time those white structures appear, the eggs have already hatched inside the caterpillar. The developing wasp larvae spend part of their lives feeding within the hornworm before eventually chewing their way out through its skin.
Then they get to work.
On one occasion this summer, I actually caught this process in action. A tiny, maggot-like wasp larva had just emerged and was busy spinning the silky cocoon that would eventually surround it. It was cool, gross, and surprisingly difficult to look away.
Those finished white structures are cocoons, not eggs. Inside each one, a wasp larva pupates before eventually emerging as a tiny adult parasitoid wasp.
And we were lucky enough to photograph that part too.
The absence of cocoons doesn't mean that a hornworm hasn't been parasitized. Impaired hornworms appear a brighter green color and are usually lethargic with heads pulled away from the stem and sometimes curled. Cocoons appeared on these two hornworms just days after this photo was taken.
Zombie Caterpillars—Is It Dead Yet?
The white cocoons make a parasitized hornworm easy to identify, but they appear very late in the process. For much of the wasps’ development, the larvae are still hidden inside the caterpillar.
At one point, I watched a cluster of five hornworms within just a few square feet. Only two carried visible cocoons, but all five seemed unusually sluggish and had taken on a brighter green appearance.
Within a few days, I had my answer: cocoons appeared on the remaining caterpillars.
After watching hundreds of hornworms this season, I began noticing a fairly consistent set of clues.
Signs a hornworm may already be parasitized:
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It becomes lethargic. Parasitized caterpillars move less and often seem sluggish when disturbed.
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It feeds less often. Healthy hornworms seem to eat almost constantly; suspicious ones may sit in the same place for long stretches.
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Its color changes. In our gardens, parasitized hornworms often became a noticeably brighter, almost lime green.
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It looks thinner. They gradually lose the plump, well-fed appearance of a healthy caterpillar.
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The feeding damage gets old. If the chewed edge in front of its mouth has browned rather than appearing freshly cut and green, it may not have eaten for quite some time.
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White cocoons appear. Once those show up, there’s no question—the caterpillar has been parasitized.
By contrast, a healthy hornworm is usually plump, minty green, and actively feeding. Large caterpillars may rest for longer stretches, but you can often see the dorsal vessel pulsing beneath the skin, a sign that they're still metabolically active.
At this point, I only remove hornworms that look healthy and are actively feeding. If one seems sluggish, thin, unusually bright green, or otherwise “off,” I leave it alone.
Every parasitized caterpillar left on the plant has the potential to produce dozens of tiny wasps—and those wasps are far better at finding hornworms than I am.
These days, the garden is almost hauntingly quiet, and the few hornworms that remain are clearly not doing well. Feeding has nearly stopped, while the plants have begun pushing out fresh foliage and setting new fruit.
Where Things Stand Now
As I write this, the tomato plot looks remarkably different than it did just a few weeks ago.
I went out this morning looking for hornworms and couldn’t find a single caterpillar that appeared healthy and actively feeding. The few I did find showed signs of parasitism, and after a summer when hornworm numbers likely reached into the thousands, that feels almost unbelievable.
The tomato plants are recovering too. Fresh, dark green foliage is filling in many of the bare stems, and the plants are still setting tomatoes—just in time, I hope, for one last good flush before frost.
And should another brave hornworm decide to raise its head, there is now an army of parasitoid wasps waiting for it.
That may be the part of this whole experience that impressed me most. I spent weeks trying to spot caterpillars hidden among the foliage. The wasps found them with an efficiency I couldn’t begin to match.
Nature eventually restored the balance.
It just didn’t do it on my timetable.
Pest or important pollinator? I’ll let you decide. Hornworms can certainly create problems—especially for the gardener—but after watching this season unfold, I’m convinced nature has some remarkably effective ways of keeping them in check.
Should You Kill Tomato Hornworms?
I’m still not sure there’s one right answer.
We paid a real price for leaving so many hornworms alone. Our plants were heavily defoliated, yields suffered, and for a while the population was clearly beyond what the tomato plot could comfortably support.
But I also suspect we’re ending this season in a better position than we started it. We now have a thriving population of parasitoid wasps that knows exactly where to find hornworms—and that may pay dividends in future seasons.
My biggest takeaway is this: if you dramatically change the environment, expect nature to need a little time to catch up.
By jumping from our usual tomato planting to roughly 400 plants, we essentially created an enormous new area of hornworm habitat in a single season. The hornworms responded quickly. Their natural enemies eventually responded too, but there was a lag—and during that lag, the tomatoes took a beating.
For gardeners who want to preserve the moths without sacrificing an entire crop, a sacrificial tomato plant or two might offer a useful middle ground. Caterpillars could be relocated there when feeding pressure becomes excessive, leaving enough hosts in the garden for parasitoid wasps while protecting the bulk of the crop.
As for next year, I’m still not entirely sure what I’ll do. Cutting our tomato planting dramatically could change the balance all over again. Will the wasps we raised this year be waiting for a much smaller hornworm population next summer? Or will fewer tomato plants simply concentrate the damage?
I don’t know yet.
And perhaps that’s the final lesson: a garden isn’t a system we control so much as one we continually negotiate with.
Not ready to throw in the towel? Good on you—gardening is a lesson in resilience. Check out our collection of heirloom tomato seeds and start planning next year’s garden.
