Eastern Black Swallowtail Caterpillar Identification Guide
A smooth green caterpillar banded in black with yellow-orange spots, commonly found on parsley, dill, and carrot family plants.
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Key Visual Features
The eastern black swallowtail caterpillar changes appearance noticeably as it grows, reaching about 2 inches at full size.
- Mature coloring: Bright green body crossed by black bands, each black band dotted with small yellow-orange spots
- Young coloring: Early instars look very different — mostly black and white, resembling a bird dropping, which offers protection while small
- Body shape: Smooth, cylindrical, without spines or long hairs
- Osmeterium: A forked, fleshy orange organ hidden behind the head that can be briefly extended, giving off a scent when the caterpillar is disturbed
- Legs: Three pairs of small true legs near the head and several pairs of prolegs along the body
Where and When You'll See It
This caterpillar feeds almost exclusively on plants in the carrot family, including parsley, dill, fennel, carrot tops, and Queen Anne's lace, making garden herb patches a common place to find it. It is most often seen from late spring through early fall, with multiple generations possible depending on region. Look closely along stems and leaf edges of host plants, since the caterpillar's banded pattern blends reasonably well with the fine, feathery foliage of its favorite host plants.
Similar-Looking Bugs
- Tiger swallowtail caterpillar: Green with large eyespots near the head rather than black bands with yellow-orange spots, and feeds on trees like cherry and tulip tree rather than carrot-family herbs
- Spicebush swallowtail caterpillar: Also has eyespots rather than banding, and feeds on spicebush and sassafras
- Monarch caterpillar: Has bold black, white, and yellow stripes and a pair of fleshy black filaments at each end, quite different from the black-banded pattern here, and feeds only on milkweed
Quick ID Checklist
- Green body with black bands dotted with yellow-orange spots (mature stage)
- Young caterpillars resemble a bird dropping (black and white)
- Smooth body, no spines
- Hidden orange forked organ behind the head that can extend when disturbed
- Found on parsley, dill, fennel, or Queen Anne's lace
Frequently asked questions
Why do young black swallowtail caterpillars look different from mature ones?
Young caterpillars are mostly black and white, resembling a bird dropping for camouflage, while mature caterpillars turn green with black bands and yellow-orange spots as they grow.
What plants attract black swallowtail caterpillars?
They feed on plants in the carrot family, most commonly parsley, dill, fennel, carrot foliage, and Queen Anne's lace.
What is the orange forked structure behind its head?
It is called an osmeterium, a fleshy organ that the caterpillar can briefly extend when disturbed; it is a normal identification feature of swallowtail caterpillars.
How can I tell a black swallowtail caterpillar from a tiger swallowtail caterpillar?
The black swallowtail has black bands with yellow-orange spots on a green body, while the tiger swallowtail has large yellow-and-black eyespots near its head and no banding pattern.