Swallowtail Caterpillars by Species: Identification Chart
Identify a swallowtail caterpillar by combining its host plant, body shape, color pattern, and defensive features. A green-and-black caterpillar on dill is usually a black swallowtail, a green caterpillar with large false eyes on spicebush is a spicebush swallowtail, and a large green eyespot caterpillar on a tree may be a tiger swallowtail. Young stages can look very different from mature ones, so never rely on one color alone.
How Do You Use This Identification Guide?
First identify the plant. Most swallowtail caterpillars have a narrow host range, and a host plant is often more useful than a pattern description. Then check whether the caterpillar is smooth, spiny, striped, or marked with false eyespots, and note its size and whether it has a small horn-like organ behind the head.
Finally, compare it with local species. The same butterfly may look different in early and late instars, and several species have green larvae. A clear photograph that includes the caterpillar and its leaf is the best record for a regional extension office or naturalist group.
Black Swallowtail Caterpillar
The mature black swallowtail caterpillar is yellow-green with black bands, each band dotted with orange spots. It is usually found on parsley, dill, fennel, carrot, celery, or other parsley-family plants. University of Minnesota gives a mature length of up to about two inches.
Young black swallowtails look very different. They are dark with pale markings and short spines, resembling bird droppings. As they molt, the spiny look gives way to the familiar green striped form. If disturbed, the caterpillar can push out a yellow-orange forked organ called an osmeterium that has a strong odor.
Eastern Tiger Swallowtail Caterpillar
Eastern tiger swallowtail caterpillars are smooth green larvae with two large false eyespots near the head. The eyespots make the front end look larger and more snake-like to a predator. They feed on trees such as tuliptree, cherry, birch, ash, and others, depending on local conditions.
They often rest on a leaf or in a leaf shelter, so the green body can disappear into foliage. The caterpillar has the typical swallowtail osmeterium, although it may not show it unless disturbed. Avoid poking it for a photograph; the defensive display costs the animal energy and exposes it to predators.
Spicebush Swallowtail Caterpillar
A spicebush swallowtail caterpillar is usually bright green when mature, with very large yellowish false eyespots edged in dark color near the front. It often makes a folded leaf shelter on spicebush or sassafras. The combination of oversized eyespots and host leaf is distinctive.
Earlier instars can be brown or mottled and resemble bird droppings. The switch to green and eyespots happens as the caterpillar grows. A leaf folded with silk is a clue, but do not unfold it forcefully. The sheltered larva may be resting or preparing to molt.
Zebra Swallowtail Caterpillar
Zebra swallowtail caterpillars are associated with pawpaw. They are pale green to yellowish with dark transverse bands and small projections, producing a pattern quite different from the smooth green tiger or spicebush larvae. The pawpaw host is the strongest clue.
Because pawpaw is so closely tied to this butterfly, a caterpillar on another common garden plant is probably not a zebra swallowtail. Learn the local range before making an identification, because zebra swallowtails do not occur throughout all of North America.
Pipevine and Giant Swallowtail Caterpillars
Pipevine swallowtail caterpillars feed on pipevines and are often dark, with rows of fleshy projections. Their look is more dramatic than a black swallowtail’s neat bands. The plant is essential because other dark spiny caterpillars exist.
Giant swallowtail caterpillars on citrus relatives or prickly ash are mottled brown, green, and white, famously resembling bird droppings. This camouflage is an early warning not to judge a caterpillar by whether it looks attractive. What matters is its host and its role in the life cycle.
What Is an Osmeterium?
Most swallowtail larvae have an osmeterium, a soft forked organ tucked behind the head. When threatened, the caterpillar can extend it and release an odor. Illinois Extension describes the black swallowtail’s as a horn-like yellow scent gland believed to discourage ants and other predators.
It is not a stinger and does not harm people. Leave the caterpillar alone after seeing the display. Repeated prodding for a reaction is not a good way to identify a living animal.
What Should You Do With a Swallowtail Caterpillar?
Usually, leave it on its host plant. Minor leaf loss is expected and supports a butterfly that may later visit flowers. Move it only if a plant must be harvested or treated, and then transfer it to the same host species nearby rather than to a random ornamental.
Avoid spraying caterpillar hosts. Minnesota Extension points out that even products thought of as gentle can kill caterpillars if applied to the insect or the leaf it eats. Identification is a better first response than control.
What Is the Simple Answer?
Use host plant first, then pattern. Black swallowtails belong on parsley-family plants, tiger swallowtails on trees, spicebush swallowtails on spicebush or sassafras, and zebra swallowtails on pawpaw. Look for eyespots, bands, and an osmeterium as supporting clues.
To see how these larvae become butterflies, read the swallowtail life-cycle guide. A caterpillar’s changing appearance is part of a normal series of molts.
