Those small white objects on a caterpillar are almost certainly not eggs. They are tiny cocoons spun by parasitic wasp larvae that developed inside the caterpillar’s body. This is one of the most common sights in gardens, especially on tomato hornworms and other large caterpillars, and it means the caterpillar is already dying.
What the White Structures Actually Are
The white, rice-grain-sized objects clustered along a caterpillar’s back and sides are pupal cocoons belonging to a type of braconid wasp. They look remarkably like insect eggs, which is why nearly everyone who spots them assumes the caterpillar is carrying eggs. But the reality is more dramatic: each tiny oval cocoon contains a wasp larva that already spent days feeding inside the caterpillar before chewing its way out through the skin.
The most common species responsible is a small wasp called Cotesia congregata, which targets tobacco and tomato hornworms. The adult wasp is only a few millimeters long and doesn’t sting people. Female wasps use a needle-like tube to inject eggs just beneath the caterpillar’s skin, depositing around 65 eggs in a single visit, though the number can range from as few as 5 to over 160.
What Happened Inside the Caterpillar
The wasp’s eggs are microscopic, smaller than the width of a human hair, and hatch inside the caterpillar’s body within two to three days in warm conditions. The pale, grub-like larvae float freely in the caterpillar’s body fluid, feeding on internal tissues while carefully avoiding vital organs. This keeps the caterpillar alive long enough for the larvae to finish developing.
Over about 12 to 16 days from the initial egg-laying, the larvae grow from roughly half a millimeter to nearly 5 millimeters long, passing through two growth stages entirely inside the host. When they’re ready, they use their mouthparts to tear through the caterpillar’s outer skin in a back-and-forth chewing motion, emerging through the surface. Once outside, each larva spins a small white silk cocoon and begins transforming into an adult wasp. These cocoons are what you’re seeing.
What Happens to the Caterpillar
By the time the cocoons appear, the caterpillar is effectively doomed. Having dozens of larvae chew through its body wall leaves it severely weakened. Parasitized caterpillars typically stop feeding and become sluggish. Research from Wesleyan University has shown that braconid wasps can actually manipulate their host caterpillar’s feeding behavior while developing inside, altering nutrient intake in ways that benefit the growing wasp larvae at the caterpillar’s expense. The caterpillar usually dies within a few days after the cocoons appear, either from the damage itself or from secondary infection through the exit wounds.
Why You Should Leave Them Alone
If you find a caterpillar covered in white cocoons in your garden, resist the urge to remove or destroy it. Those cocoons will soon produce a new generation of adult wasps, each one capable of parasitizing more pest caterpillars. Braconid wasps are among the most effective natural pest controllers in home gardens. A single parasitized hornworm can release dozens of wasps that will go on to attack other caterpillars on your tomato, pepper, and tobacco plants.
Killing the caterpillar or picking off the cocoons eliminates beneficial insects that are doing free pest control. The caterpillar itself has already stopped causing significant damage to your plants. Let it be, and within a week or so, tiny adult wasps will emerge from the cocoons and disperse into your garden.
How to Tell Cocoons From Actual Eggs
A few details help you confirm what you’re looking at. Braconid wasp cocoons are bright white, oval or oblong, and about the size of a grain of rice. They stand upright or protrude from the caterpillar’s body, often clustered densely along the back and sides. They look fuzzy or slightly silky up close because they’re made of spun silk, not smooth like true insect eggs would be.
True parasitic fly eggs, which can also appear on caterpillars, are typically rounder, smoother, and fewer in number. Caterpillar eggs laid by butterflies or moths are found on leaves, not on other caterpillars. If the white objects are attached directly to the caterpillar’s body and number in the dozens, you’re looking at braconid wasp cocoons.
Other Caterpillars This Happens To
Tomato and tobacco hornworms are the most recognizable hosts because they’re large, common in gardens, and the white cocoons are easy to spot against their green skin. But braconid wasps parasitize a wide range of caterpillar species, including armyworms, cabbage worms, and various moth larvae. The appearance is similar across hosts: clusters of small white cocoons protruding from the caterpillar’s body, with the caterpillar itself looking limp and inactive. Different braconid wasp species target different caterpillars, but the basic life cycle, eggs laid inside, larvae feeding internally, cocoons spun on the outside, stays the same.