What Is Eating My Dill and Parsley? Identifying the Black Swallowtail Caterpillar

It is one of the classic late-summer garden surprises. You head out to cut herbs for dinner, part the dill or the parsley, and find it crawling with fat, boldly striped caterpillars that have eaten whole stems down to nubs. The instinct is to treat them as pests. But these particular caterpillars are worth a second look, because on carrot-family herbs in mid to late summer they are almost always the larvae of the eastern black swallowtail — one of the showiest butterflies in North America, being raised for free in your garden.

How to Identify a Black Swallowtail Caterpillar

A mature black swallowtail caterpillar, often called a "parsley worm," is unmistakable once you know it:

• Color and pattern: smooth (not fuzzy) and bright green, ringed with black bands, and each black band is studded with yellow or orange dots
• Size: up to about two inches long when full grown, plump and cylindrical
• What it is eating: members of the carrot family — dill, parsley, fennel, carrot tops, celery, caraway, and Queen Anne’s lace. If a caterpillar is on those plants, this is the leading suspect
• A hidden defense: when poked, it everts a bright orange forked gland (the osmeterium) from behind its head and gives off a strong, sharp smell to drive off predators — startling but completely harmless to you

The younger stages look very different, which trips people up. A newly hatched black swallowtail caterpillar is small, dark, and spiny with a white saddle-like band across the middle, mimicking a bird dropping so predators pass it by. It only turns into the famous green-and-black striped form as it grows and molts, so a single plant can host caterpillars that look like several different species at once.

Is It Harmful? (To You, and To Your Garden)

To you, not at all. Black swallowtail caterpillars do not sting or bite, and the orange osmeterium is just a smelly bluff — they are safe to pick up and move by hand. To your herbs, they are hungry but rarely catastrophic: a healthy dill or parsley plant usually recovers, and the caterpillars only need the plant for a week or two before moving on to pupate.

Because the reward is a native butterfly, many gardeners deliberately plant extra dill, parsley, and fennel specifically to feed them, treating a few chewed herbs as cheap rent. If you truly need to protect a plant, the gentlest option is simply to move the caterpillars to a sacrificial patch rather than kill them.

What It Turns Into

After feeding, the caterpillar wanders off, attaches itself upright with a little silk sling, and forms a chrysalis that can be green or brown. From it emerges the eastern black swallowtail: a large, mostly black butterfly with rows of yellow spots, a wash of iridescent blue on the hindwings (brightest in females), and a red-and-black eyespot near each tail. Watching the whole arc from parsley worm to butterfly is one of the easiest and most rewarding backyard nature projects there is.

Black Swallowtail Look-Alikes

A couple of other striped caterpillars get confused with the parsley worm, and the host plant is the fastest way to tell them apart:

• Monarch caterpillar: also boldly banded but in black, yellow, and white with no yellow dots, and it has fleshy black tentacles at both ends. Crucially, it feeds only on milkweed, not herbs
• Anise and other swallowtail caterpillars: very similar green-and-black striped larvae in western regions, feeding on related plants — close cousins of the black swallowtail
• Bird-dropping mimics: the tiny early stage of the black swallowtail itself, plus young caterpillars of several other species, all use the same dark-with-white-saddle disguise

Not sure whether the striped caterpillar mowing down your dill is a future black swallowtail or something else entirely? Snap a photo and let Spotter identify it, so you know whether to move it to a spare plant or just let the butterfly factory run.