The caterpillar eating your parsley is almost certainly the black swallowtail, sometimes called the “parsleyworm.” These striped caterpillars feed exclusively on plants in the carrot family, and parsley is one of their favorites. They’re the larvae of a striking black-and-yellow butterfly found across eastern North America, and while they can strip a parsley plant bare, many gardeners choose to share the harvest.
How to Identify a Parsleyworm
Mature black swallowtail caterpillars have bold vertical stripes of black, white, and yellow or green. They’re often confused with monarch caterpillars, which share a similar color palette, but the two are easy to tell apart. Monarch caterpillars have a pair of black, whip-like filaments sticking out from each end of their bodies. Parsleyworms lack those filaments entirely. Instead, they have a small orange slit on the top of their head, just above three black dots, that hides a forked gland they deploy when threatened.
The other reliable clue is the plant itself. Monarch caterpillars feed exclusively on milkweed. If the caterpillar is on parsley, dill, fennel, celery, carrots, or any other member of the carrot family, it’s a black swallowtail.
What They Look Like at Different Ages
Young parsleyworms look nothing like the older ones, which catches many gardeners off guard. In their early stages (the first through fourth instars), the caterpillars are dark brown or black and covered in small white or cream-colored markings. They’re designed to mimic bird droppings, which keeps predators from bothering them. At this stage, they’re small enough to go unnoticed on your plants.
The transformation happens at the final molt. Fifth-instar caterpillars shed the bird-dropping disguise and become the bright green, black, and yellow-striped caterpillars most people recognize. They also get considerably larger and more visible, which is usually when gardeners first spot them.
Their Secret Defense Trick
If you pick up a parsleyworm, you’ll likely trigger its most distinctive feature: the osmeterium. This is a forked, bright orange gland that shoots out from behind the head like a tiny snake tongue. It releases a sharp, unpleasant smell meant to scare off birds and other predators. The organ is unique to swallowtail caterpillars.
The smell itself changes as the caterpillar grows. Younger caterpillars release a more complex mix of compounds similar to those found in fruits and spices. Older caterpillars produce a simpler secretion that smells closer to rancid butter. In species with eyespot markings, the forked osmeterium even completes a convincing snake mimicry. It’s harmless to humans, just startling and a bit smelly.
Every Plant They’ll Feed On
Parsleyworms eat plants in the carrot family (Apiaceae) and nothing else. In gardens, their preferred hosts include parsley, dill, sweet fennel, celery, caraway, and carrot tops. If you grow several of these, you may find caterpillars spread across all of them.
In the wild, they feed on Queen Anne’s lace (wild carrot), wild parsnip, golden alexander, and even poison hemlock and spotted water hemlock. This broad diet across the carrot family means adult butterflies will lay eggs wherever they find these plants, whether that’s a carefully tended herb garden or a weedy roadside.
How Long They’ll Be on Your Parsley
The caterpillar stage lasts roughly two to three weeks in warm weather, though cooler temperatures slow things down. Once a caterpillar reaches full size, it typically leaves the host plant to find a sheltered spot nearby. It spins a small silk sling to anchor itself to a branch or stem, and within about 24 to 27 hours in warm conditions, it sheds its final skin and forms a chrysalis. In summer, the butterfly emerges about 10 to 14 days later. Caterpillars that pupate in fall will overwinter in the chrysalis and emerge the following spring.
Black swallowtails can produce multiple generations per year, so new caterpillars may appear on your parsley throughout the growing season.
Managing Them in Your Garden
Because parsleyworms become beautiful pollinating butterflies, many gardeners prefer to leave them alone or relocate them rather than kill them. The simplest approach is to plant extra parsley, dill, or fennel specifically for the caterpillars. A few extra plants are cheap insurance, and you’ll be rewarded with swallowtail butterflies visiting your garden.
If you’d rather protect your harvest, handpicking is the most straightforward option. The caterpillars are large, conspicuous, and slow-moving, making them easy to remove. You can relocate them to Queen Anne’s lace or any wild carrot-family plant growing nearby. Keep in mind they can be persistent through the growing season, so you may need to check your plants regularly.
Natural parasites also help keep populations in check without any intervention. Tiny parasitic wasps lay eggs inside the caterpillars, which is one reason you rarely see truly overwhelming numbers in a single garden. If handpicking isn’t enough and your crop is at stake, products containing Bacillus thuringiensis (Bt), a naturally occurring soil bacterium, will kill the caterpillars. But for most home gardeners growing a few parsley plants, relocating caterpillars or planting a few sacrificial herbs is the more satisfying solution.