Eastern Tiger vs. Black Swallowtail Caterpillars
Black swallowtail and eastern tiger swallowtail caterpillars are easiest to separate by the plant they are eating and by their mature markings. Black swallowtails occur on parsley, dill, fennel, carrot, and relatives, with black bands and orange dots; eastern tiger swallowtails occur on trees such as tuliptree and cherry, with a smooth green body and large false eyespots. Both are swallowtail larvae, but they are not interchangeable garden visitors.
Start With the Host Plant
A caterpillar on dill, parsley, fennel, carrot, or Queen Anne’s lace is likely a black swallowtail. A caterpillar on tuliptree, wild black cherry, birch, ash, or other suitable tree is more likely an eastern tiger swallowtail. Penn State’s host list makes this distinction clear.
Host plant is more dependable than a quick glance at color. Caterpillars change after each molt, and leaves change the light. Photograph the insect with its plant before moving or identifying it.
What Does a Black Swallowtail Caterpillar Look Like?
Young black swallowtails are dark with pale patches and small spines. Mature caterpillars are yellow-green with black crossbands, each marked with orange or yellow dots. They are fairly smooth once mature and may reach around two inches.
When disturbed, they push out an orange forked osmeterium from behind the head. It smells unpleasant to predators. It is not a stinger and is not a reason to remove the caterpillar from an herb plant.
What Does an Eastern Tiger Swallowtail Caterpillar Look Like?
Eastern tiger swallowtail caterpillars are green and smooth when mature, with large paired false eyespots near the head. The eyespots make the front end look more like the head of a larger animal. The body pattern lacks the bold black-and-orange banding of a black swallowtail.
They may rest in a leaf shelter or blend closely with tree foliage. A big green caterpillar on a tuliptree is a strong tiger swallowtail clue, while one on parsley is not.
What Features Do They Share?
Both species have a typical butterfly life cycle: egg, caterpillar, chrysalis, and adult. Both molt as they grow and both have an osmeterium. They also depend on unsprayed host foliage and are vulnerable to pesticides even when the product is marketed as a gentle garden treatment.
Both are part of the same swallowtail family, which explains the shared defensive organ. Their specialized diets, markings, and eventual adult wing patterns are what distinguish them.
Should You Move Either Caterpillar?
Leave it on its host plant whenever possible. If a parsley pot must be harvested, transfer a black swallowtail to another parsley-family plant. If a tree branch needs pruning, move a tiger swallowtail only to a similar host nearby. A random leafy plant is not adequate food.
Handle gently only if necessary, and avoid disturbing a caterpillar that is resting before a molt or hanging in preparation to pupate. Most plant feeding is tolerable and worth the chance to see an adult butterfly later.
What About Lookalikes?
Several caterpillars share green coloring, so confirm the host before naming one. A hornworm may have a rear horn, while swallowtail larvae have the defensive organ tucked behind the head. A photograph that includes the plant, the caterpillar’s side, and its head end prevents most mistakes. If a garden plant is truly suffering, identify the larva before taking action.
What Is the Simple Answer?
Black swallowtail larvae belong on parsley-family plants and become striped green caterpillars. Eastern tiger swallowtail larvae belong on trees and become smooth green caterpillars with big false eyes. Plant and pattern together give a confident identification.
For the wider change from caterpillar to winged adult, see the swallowtail life-cycle guide. One species on the wrong plant is usually not a match.
What Should You Record for a Difficult Caterpillar?
Photograph the animal from above and from the side without moving it to a new plant. Include the head, the enlarged front section, the rear markings, and a leaf that can be identified. Record whether any spots are flat color or raised structures. A ruler beside the leaf is safer than placing one against the caterpillar. If the osmeterium appears after a natural disturbance, note its color and shape without repeatedly provoking the animal. Location, date, host, and growth stage often settle a comparison that one dramatic close-up cannot.
