As if it wasn’t gross enough—those fat, green caterpillars that were ravaging your tomato plants have now sprouted some sort of alien-like white eggs.
What gives?
First, the good news: those aren’t eggs.
They’re the cocoons of tiny parasitoid wasps, and their appearance is a sign that nature has already begun dealing with your hornworm problem for you.
The bad news—at least for the caterpillar—is that by the time those white cocoons appear, the real action has already been going on for quite some time.
Earlier, a female braconid wasp located the hornworm and deposited her eggs inside its body. The larvae hatched there, developed inside the still-living caterpillar, and eventually emerged through its skin. Once outside, each larva spun one of those conspicuous white cocoons and began the final stage of its transformation into an adult wasp.
Yes, it’s every bit as gruesome as it sounds.
It’s also one of the most effective forms of natural pest control you can have in a tomato garden.
In this week’s article, we’re taking a close-up look at the strange relationship between hornworms and braconid wasps—what those white cocoons really are, what’s happening inside the caterpillar, and why you should think twice before removing a parasitized hornworm from your plants.
An adult braconid wasp (Cotesia congregata) emerging from its cocoon onboard a parasitized hornworm. If it is a female, it will mate and go out in search of more hornworms to parasitize. Photo Credit: Elijah Keith 2026
How a Braconid Wasp Parasitizes a Hornworm
The process begins long before those white cocoons appear.
A female braconid wasp first has to find a suitable hornworm. As we discussed in our previous article, she can use chemical signals released by a tomato plant as the caterpillar feeds to help zero in on her target.
Once she finds one, she inserts her ovipositor into the caterpillar and deposits a cluster of eggs inside its body.
And here’s where things get especially strange.
Along with her eggs, the wasp also injects particles from a symbiotic bracovirus. The virus helps suppress the hornworm’s immune defenses and alters its development, creating an environment where the wasp larvae can survive instead of being attacked by the caterpillar’s immune system.
The eggs hatch, and the tiny larvae develop inside the still-living hornworm, feeding within its body while the caterpillar continues to move—and, for a while, even eat.
Eventually, the larvae are ready to leave.
They emerge through the caterpillar’s skin, settle on its back and sides, and each spins one of those familiar white silk cocoons.
That’s the stage most gardeners notice.
But by then, the hornworm has already been serving as a nursery for quite some time.
Tobacco hornworm carrying dozens of developing braconid wasps. As disturbing as it may seem, those white cocoons are a good sign. The wasps have already begun to deal with your hornworm problem.
They’re Cocoons, Not Eggs
Those little white structures are often described as “wasp eggs,” but by the time you can see them, the egg stage is long over.
The female wasp deposited her eggs inside the hornworm days earlier. The larvae hatched there and developed within the caterpillar until they were ready to pupate. At that point, they emerged through the hornworm’s skin and immediately began spinning the small white silk cocoons you see clustered across its back and sides.
Each cocoon contains a developing wasp.
If you look closely at a fresh cluster, the cocoons are usually smooth, white, and tightly packed together. As the adult wasps mature, they eventually cut their way out, leaving behind little open-ended shells.
I happened to catch one of the larvae shortly after it had emerged from the hornworm. It looked like a tiny pale maggot, wriggling on the caterpillar’s back as it spun silk around itself. Within a surprisingly short time, it disappeared inside the familiar white capsule.
So if you see a hornworm covered in white “eggs,” you’re actually looking at the pupal stage of the parasitoid wasp—and the hornworm has already been parasitized for quite some time.
Parasitized hornworm carrying white Cotesia congregata cocoons. Note that the ends of the cocoons have been chewed open by the emerging adult wasps. Those wasps have already set forth looking for more hornworms in your garden.
Is the Hornworm Still Alive?
Disturbingly, yes.
When the white cocoons first appear, the hornworm is usually still alive. The wasp larvae have already emerged through its skin and begun pupating on the outside, but the caterpillar may continue to cling to the plant for days afterward.
It may even continue to eat—just not very much.
NC State reports that a parasitized hornworm consumes only about one-fifth as much foliage as a healthy hornworm. Their management guidelines actually account for this difference: five visibly parasitized hornworms are treated as roughly equivalent to one healthy caterpillar when estimating potential crop damage.
That tracks well with what I watched in our own tomatoes. Once parasitism became widespread, feeding nearly ground to a halt. The remaining hornworms might still move or occasionally nibble a leaf, but they were nothing like the plump, ravenous caterpillars that had been stripping plants only days earlier.
The wasps have good reason to keep their host alive for as long as possible. Cotesia congregata develops inside the caterpillar without immediately destroying it, drawing resources from the host while allowing it to continue functioning. Eventually, however, the caterpillar’s development is effectively derailed. It never becomes a moth.
Research on this relationship shows that parasitized hornworms are developmentally arrested and ultimately die after the parasitoids emerge.
But the hornworm's fate is sealed long before those cocoons emerge, and knowing the signs of an already parasitized caterpillar can help you determine which ones to pull and which to leave alone.
Two hornworms showing signs of being parasitized (altered color, sluggishness, heads pulled away from stem). Confirmation came days later when both worms sprouted dozens of white cocoons.
How to Tell a Hornworm May Already Be Parasitized
The white cocoons make parasitism unmistakable. The harder question is whether you can recognize a parasitized hornworm before the wasp larvae emerge.
After watching hundreds of hornworms in our tomatoes this summer, I began to notice a pattern. These aren’t diagnostic signs—there is enough natural variation among caterpillars that I wouldn’t condemn one based on color or behavior alone—but taken together, they became surprisingly useful clues:
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They seemed less active. Healthy hornworms are eating machines. The suspicious ones often spent long periods sitting motionless, even before their cocoons appeared.
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They weren't feeding as frequently. One of my favorite clues was the leaf directly in front of the caterpillar. An actively feeding hornworm usually had a fresh, green bite edge nearby. With many of the parasitized caterpillars, that cut edge had already browned—a little timestamp showing that nobody had eaten there for quite a while.
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They looked thinner. Healthy hornworms are almost comically plump. Many of the caterpillars that later produced cocoons seemed narrower and less well filled out.
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Their posture changed. Healthy hornworms often sit with the head close to the stem or leaf they’re feeding on. Many of the parasitized caterpillars I watched held their heads noticeably away from the plant instead, and some curled the front of the body inward (known as the sphinx state).
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Their color was "off". The healthy caterpillars in our garden tended toward a softer, minty green. Many of the parasitized ones became a much brighter, almost lime green.
That color change especially caught my attention, because it turns out the biology behind a hornworm's green skin is considerably more interesting than I expected.
Why Does a Parasitized Hornworm Look Different?
The changes I was seeing weren’t necessarily coincidence. When Cotesia congregata parasitizes a hornworm, the associated bracovirus begins interfering with the caterpillar’s normal development. It alters which proteins are being produced, disrupts processes occurring in the epidermis and cuticle, and ultimately helps arrest the hornworm’s development.
Those internal changes can show up on the outside.
In our garden, hornworms that later proved to be parasitized were often brighter green, thinner and noticeably less active than healthy caterpillars. None of those traits is proof on its own, but together they became useful clues when deciding which hornworms to remove and which ones might be worth leaving alone.
And if you have a strong enough stomach to look closely, things get even more interesting as the wasp larvae emerge. They have to break through the caterpillar’s body wall, leaving tiny wounds behind. The surrounding tissue can darken as the hornworm responds to the injury.
By then, of course, identification is no longer much of a challenge—you can usually see the larvae or the beginnings of their white cocoons. But it does offer a remarkable close-up view of just how dramatically the parasitoid has altered its host.
A hornworm carrying the next generation of Cotesia congregata wasps. Once these cocoons emerge, there's no doubt—this hornworm is finished. Though it may still move a little at this stage, it won't finish development and certainly won't turn into a moth.
Should You Remove a Hornworm Covered in White Cocoons?
No. If a hornworm is already covered in white cocoons, leave it alone.
At that point, the caterpillar is doing relatively little damage, while each of those cocoons contains a developing parasitoid wasp. Removing or crushing the hornworm now means removing the very insects that are helping bring the infestation under control.
That can be surprisingly consequential. A single parasitized hornworm may carry dozens of cocoons, each capable of producing another adult wasp that can go searching for young hornworms elsewhere in the garden.
This is also why I’ve become much more hesitant about removing caterpillars that merely look sick. The white cocoons are easy—the decision has already been made for you. But by the time I began recognizing the subtler signs of parasitism, I realized that some of the hornworms I had plucked earlier in the season may already have been carrying developing wasps.
So my rule now is fairly simple: healthy, plump caterpillar actively eating? That one may go. Sluggish, thin, oddly colored or behaving strangely? I give it some time. White cocoons? Definitely leave it.
It may not be the prettiest thing hanging from your tomato plant, but it’s essentially a nursery for the next generation of biological control.
Because of their small size, braconid wasps depend on nectar sources with shallow, accessible flowers. You can encourage their populations by planting dedicated nectar sources, like buckwheat, or by allowing herbs like dill, cilantro, fennel to persist even after they've bolted.
How to Encourage Braconid Wasps in the Garden
Once you realize how effective these tiny parasitoids can be, the obvious question is: how do you get more of them?
Fortunately, encouraging Cotesia congregata is less about adding something new to the garden and more about giving the wasps already there a chance to survive and reproduce.
Leave Parasitized Hornworms Alone
This is the simplest—and probably most important—thing you can do.
Once those white cocoons appear, the hornworm has essentially changed jobs. Instead of being primarily a tomato-eating pest, it has become a nursery for the next generation of parasitoid wasps.
And the wait is not especially long. In warm weather, the developing wasps may emerge from their cocoons within just a few days. Once they do, the females begin searching for more hornworms to parasitize.
Give Adult Wasps Something to Eat
The caterpillar feeds the developing larvae, but adult wasps need food of their own.
Adult braconid wasps feed on nectar, and access to it can dramatically increase their lifespan. In one experiment, wasps supplied only with water survived an average of about 0.6 days, while those given access to flowering buckwheat survived about 5.1 days.
That is a lot more time for a tiny wasp to spend searching for hornworms.
Buckwheat has particularly good research behind it, but the broader lesson is simple: keep small, accessible flowers blooming around the garden. Dill, cilantro and other plants with clusters of shallow flowers can provide useful nectar sources for parasitoid wasps and other beneficial insects.
Avoid Broad-Spectrum Insecticides
If you're trying to encourage natural pest control, broad-spectrum insecticides can work against you by killing parasitoids along with the pests.
That becomes especially important once you begin finding parasitized caterpillars. Spraying may eliminate a few remaining hornworms, but it can also wipe out the wasp population that is beginning to bring them under control.
Consider Leaving a Few Hosts
Braconid wasps can't reproduce without caterpillar hosts. A garden in which every hornworm is destroyed immediately doesn't give the wasps much opportunity to establish.
That doesn't mean surrendering your tomatoes—I learned the limits of that approach rather thoroughly this summer. But if you have the space, moving a few healthy hornworms to a sacrificial tomato plant can offer a reasonable middle ground: protect the plants you're depending on for a harvest while still leaving some hosts for parasitoids and, perhaps, a few sphinx moths.
Relocate Cocoons During Fall Cleanup
Late-season cocoons can contain developing wasps that overwinter and emerge the following spring.
If you find cocoon-covered hornworms while cleaning up the garden, consider moving the entire caterpillar—or cutting off the small section of stem it is resting on—and placing it somewhere undisturbed near ground level.
I wouldn't peel the individual cocoons from the caterpillar. Moving them together is simpler and leaves the delicate cocoons intact.
Heavy soil disturbance, including tillage, would also seem likely to damage cocoons that have fallen to the soil surface. But tillage also provides a means to destroy hornworm pupae. Rather than preserving an entire tomato planting solely for the wasps, relocating the cocoons you happen to find during cleanup is an easy way to give them a better chance of making it through winter.
It isn't the prettiest sight, but a parasitized hornworm is a good sign that nature is bringing things into balance. For those looking to garden with fewer chemicals (and less work) their presence might even be welcomed.
A Little Gruesome, but a Lot of Good
It's hard to look at a hornworm covered in white cocoons and not feel at least a little sorry for it. Nature isn't always gentle, and the relationship between hornworms and braconid wasps is certainly one of its more gruesome examples.
But after watching this process unfold in our tomato bed, I've come to appreciate those little white cocoons in an entirely different way.
What initially looks like another strange problem in the garden may actually be part of the solution. If you've spotted cocoons, it means wasps are already at work, quietly reducing the hornworm population without any help from us.
And perhaps that's the most useful lesson here. Not every insect in the garden needs our intervention, and sometimes the best thing we can do is recognize when nature is already handling the problem.
So the next time you find a hornworm sporting a cluster of little white cocoons, leave it right where it is. Your tomatoes may be better off for it.
Feeling better about next year's tomato prospects? Good. Those white cocoons are definitely a sign that things are coming into a better balance. Be sure to check out our collection of heirloom tomato seeds to get an early start on your next gardening adventure.
