Caterpillars in Pennsylvania: Identification and Poisonous Species

Pennsylvania’s forests run from oak-covered Piedmont ridges in the southeast to the vast Allegheny Plateau in the northwest, and that range of habitat supports a rich variety of caterpillars. You’ll find woolly bears crossing trails on October mornings, spicebush swallowtail caterpillars tucked inside rolled sassafras leaves along creek edges, and spongy moth caterpillars scattered across oak branches during outbreak summers. Most won’t hurt you. Three species in Pennsylvania deliver venomous stings on contact, and the most common one, the hickory tussock moth caterpillar, turns up in backyards across every county in the state. This guide covers 9 caterpillars in Pennsylvania you’re most likely to encounter, with the stinging species clearly marked so you know which ones to leave alone.

Common caterpillars in Pennsylvania include the monarch on milkweed, the woolly bear crossing roads in fall, the eastern tent caterpillar building silk tents in cherry trees each spring, and the spongy moth on oaks. Three species sting on contact: the hickory tussock moth caterpillar, the saddleback caterpillar, and the IO moth caterpillar. The hickory tussock moth caterpillar is the most frequently reported stinger in Pennsylvania: it looks fuzzy and harmless, but contact with its black tufts causes a burning rash.

Common Caterpillars in Pennsylvania to Know

Pennsylvania sits within 4 physiographic provinces: the Piedmont in the southeast, the Great Valley and Ridge and Valley in the central portion, and the Allegheny Plateau covering the north and west. Each province supports a distinct mix of host plants, which shapes which caterpillars you’re likely to find. Roughly 159 caterpillar species have been documented in the state. The spongy moth (Lymantria dispar), formerly called the gypsy moth until the Entomological Society of America renamed it in 2022, is Pennsylvania’s most damaging caterpillar pest: in peak outbreak years it has defoliated millions of acres of Pennsylvania oak and aspen forest, with the outbreak corridor running across the entire southern half of the state through the 1970s and 1990s. Three species in Pennsylvania deliver venomous stings: the hickory tussock moth caterpillar, the saddleback caterpillar, and the IO moth caterpillar. Caterpillar activity runs from April through September, with woolly bears crossing roads in October as they search for overwintering sites under bark and leaf litter.

1. Monarch Caterpillar (Danaus plexippus)

Bold black, white, and yellow rings circling the whole body, with two black tentacles at each end. About 2 to 2.5 inches long at full size. Found only on milkweed plants (Asclepias species), which is the fastest way to confirm the ID. If the caterpillar is on milkweed, you’re almost certainly looking at a monarch.

Pennsylvania sits along the eastern monarch migration flyway. Milkweed corridors in the Piedmont and central valleys see steady traffic from late May through September. The chemicals in milkweed make monarchs toxic to most predators, which is why the bold coloring works as a warning. Common in gardens, roadsides, and old fields wherever milkweed grows. Doesn’t sting.

2. Banded Woolly Bear (Pyrrharctia isabella)

Fuzzy with a rusty-brown middle band and black at both ends. About 2 inches long. The hairs don’t sting, though they can irritate sensitive skin. Best known for crossing roads and trails in September and October, searching for somewhere sheltered to overwinter.

Woolly bears genuinely do survive Pennsylvania winters: they freeze solid under bark or leaf litter and thaw when temperatures rise in spring. The caterpillar’s cold-hardiness comes from glycerol it produces before winter, which acts as antifreeze in its cells. The folk tale linking brown-band width to winter severity has no scientific backing. Band width reflects age and food availability, not forecast. It turns into the Isabella tiger moth in spring.

3. Eastern Tent Caterpillar (Malacosoma americanum)

Hairy and blackish with a white stripe down the back, blue spots along the sides, and a row of oval yellow marks. Young caterpillars build communal silk tents in the branch forks of wild cherry, apple, and crabapple trees, starting in late March or early April in Pennsylvania. They feed during the day and return to the tent at night.

By May most caterpillars disperse as solitary wanderers, which is when you typically find them on sidewalks and fences. Heavy infestations can strip trees bare, but established trees push out new leaves within a few weeks. One of the most recognizable spring caterpillars in Pennsylvania, common statewide. Doesn’t sting.

4. Black Swallowtail Caterpillar (Papilio polyxenes)

Bright green with black bands, each band dotted with yellow spots. Disturb one and it extends an orange forked organ (the osmeterium) from behind the head and releases a sharp, pungent smell. Found on plants in the carrot family: parsley, fennel, dill, Queen Anne’s lace, and wild carrot.

If you found it in a herb garden or along a weedy roadside with carrot-family plants nearby, this is almost certainly what you have. Common across Pennsylvania from May through October. Doesn’t sting.

5. Spicebush Swallowtail Caterpillar (Papilio troilus)

Plump and green with two large false eyespots on the thorax: yellow with a blue center and a dark pupil. The eyespots make it look like a small snake peering back at you, which is exactly the point. Early instars are dark brown with a white saddle mark, mimicking bird droppings.

The caterpillar rolls a leaf over itself and rests inside during the day, so you find it by looking for curled leaves rather than exposed caterpillars. Found almost exclusively on spicebush and sassafras in woodland edges and creek-side thickets throughout Pennsylvania’s Piedmont and forested ridge counties. Doesn’t sting.

6. Spongy Moth Caterpillar (Lymantria dispar)

Pennsylvania’s most notorious forest pest. Young caterpillars are dark and hairy; by the final instars they show 5 pairs of blue spots on the front portion of the body followed by 6 pairs of red spots toward the rear. About 2 to 2.5 inches long when fully grown.

The hairs can cause skin irritation in sensitive individuals, but the spongy moth isn’t a true stinging caterpillar. The real problem is scale: during outbreak years, caterpillars strip entire hillsides of oak, aspen, and apple foliage by June, leaving bare canopies that look like late February. Common statewide from April through June on oaks, maples, and many other hardwoods.

Stinging Caterpillars in Pennsylvania

Three caterpillars in Pennsylvania deliver a venomous sting on contact. All three cause burning, swelling, and hives that can last several hours. The identifying rule that covers two of them: branching, tree-like spines on a caterpillar almost always mean it stings.

For a broader look at spiny species across North America, see Spiky Caterpillars: How to Identify and Avoid Stings.

7. Hickory Tussock Moth Caterpillar (Lophocampa caryae)

White and fuzzy with a row of black tufts running down the back and a clump of longer black hairs at each end. About 1.5 to 2 inches long. Common on hickory, walnut, oak, beech, birch, and many other hardwood trees across Pennsylvania. This is the stinging caterpillar you’re most likely to encounter in a Pennsylvania backyard.

The white hairs look soft, but the black tufts contain hollow setae connected to venom glands. Contact causes a burning rash and raised hives that typically last several hours. Children are especially prone to picking them up because they look fluffy and interesting. Penn State Extension has fielded repeated reports of tussock moth stings at Pennsylvania schools and summer camps. If stung, use tape to lift out embedded hairs, wash with soap and water, and apply ice. See a doctor if symptoms spread beyond the sting site.

8. Saddleback Caterpillar (Acharia stimulea)

One of the most distinctive caterpillars in Pennsylvania: dark brown at both ends with a bright green midsection, and a brown oval “saddle” outlined in white sitting squarely in the center. Four knob-like protrusions at the corners of the green section carry the stinging spines. The saddle marking works as a warning, and it’s reliable.

Touching the spines causes immediate burning, a raised welt, and a rash lasting several hours. Found on oaks, maples, cherries, elms, and many garden plants. Present throughout Pennsylvania from July through September.

9. IO Moth Caterpillar (Automeris io)

Lime green with a white stripe outlined in reddish-brown along each side. The venomous spines are the same lime green as the body, which makes them easy to miss against foliage. Touching the spines causes immediate burning, swelling, and hives lasting several hours.

Young IO moth caterpillars are bright orange and cluster in groups: those clusters are easier to spot. By the final instar they’ve shifted to the lime-green form that delivers the sting. Found on oaks, maples, willows, and other hardwoods throughout Pennsylvania. Active from July through September. A plump lime-green caterpillar with bristly tufts on hardwood leaves in summer deserves caution until you’ve confirmed the ID.

How to Identify Caterpillars in Pennsylvania by Photo

For most caterpillars in Pennsylvania, four clues narrow it down quickly:

Color and pattern: solid, striped, banded, or spotted
Texture: smooth skin, hairy or fuzzy, or spiny
Host plant: what plant was it on? Monarch on milkweed. Black swallowtail on parsley or fennel. Spicebush swallowtail on spicebush or sassafras. Host plant alone closes most IDs.
Season: spring tent caterpillars and spongy moths (April through June), summer stingers (July through September), fall woolly bears (September through October)

If those four clues leave you with multiple options, a photo does the rest. Animal Identifier lets you photograph the caterpillar and returns the species name in seconds. After the ID you can ask follow-up questions in the same session, whether you want to know if it stings or what it’ll turn into. It covers insects including caterpillars statewide across Pennsylvania’s varied terrain.

For more on the state caterpillar series, see Caterpillar Identification in Michigan and white fuzzy caterpillar identification for the hickory tussock moth and flannel moth species that also appear across Pennsylvania’s hardwood forests. For the black-and-yellow species common in Pennsylvania gardens and roadsides, black and yellow caterpillars covers the full roster.

Frequently Asked Questions

Are there poisonous caterpillars in Pennsylvania?

Not in the way most people mean. No caterpillar in Pennsylvania is poisonous to touch in the sense of being toxic through the skin, and none will hurt you if you leave it alone. What Pennsylvania does have is a handful of stinging species — caterpillars with hollow spines or barbed hairs that inject an irritant when pressed against skin. The reaction is a burning welt, not a poisoning. A few species are genuinely toxic if eaten, which matters mainly for pets and small children. The species to recognize are listed below.

Are there stinging caterpillars in Pennsylvania?

Three caterpillars in Pennsylvania sting on contact: the hickory tussock moth caterpillar, the saddleback caterpillar, and the IO moth caterpillar. The hickory tussock moth caterpillar is the most common: its black tufts contain hollow hairs connected to venom glands, and it lives on oaks, hickories, and other hardwoods statewide. All three cause burning, hives, and a raised rash lasting several hours. If stung, use tape to remove embedded hairs or spines, wash with soap and water, apply ice, and see a doctor if symptoms spread.

What is the fuzzy caterpillar crossing the road in fall in Pennsylvania?

The banded woolly bear (Pyrrharctia isabella): fuzzy with a rusty-brown middle band and black at both ends, about 2 inches long, crossing roads in September and October to find a winter shelter. Their hairs don’t sting, though they can cause mild skin irritation. Woolly bears survive Pennsylvania winters by freezing solid under bark or leaf litter, then thawing and resuming activity in spring when temperatures rise.

What caterpillar builds silk tents in Pennsylvania trees each spring?

Eastern tent caterpillars (Malacosoma americanum) build communal silk tents in the branch forks of wild cherry, apple, and crabapple trees starting in late March or early April. The caterpillars leave the tent to feed during the day and return at night. Heavy infestations strip branches bare, but established trees typically push out new foliage within a few weeks of the caterpillars dispersing in May.

What is the spongy moth caterpillar and why is it a problem in Pennsylvania?

The spongy moth caterpillar (Lymantria dispar), renamed from “gypsy moth” by the Entomological Society of America in 2022, is Pennsylvania’s most damaging caterpillar pest. Identifiable by 5 pairs of blue spots on the front of the body followed by 6 pairs of red spots toward the rear. During outbreak years it defoliates entire hillsides of oak and aspen by June. Its hairs can irritate skin but it doesn’t sting the way tussock moth or saddleback caterpillars do. The Pennsylvania Department of Agriculture runs active monitoring and management programs across the state.

When are caterpillars most active in Pennsylvania?

Pennsylvania caterpillar activity runs in two main windows. The first is April through June, when spring-hatching broods feed on fresh foliage — eastern tent caterpillars, spongy moths, and early butterfly broods are all active then. The second runs July through September, when late-season broods fatten before winter, including all three stinging species. Woolly bears cross roads in October as they search for overwintering sites under bark and leaf litter.

If you find a caterpillar in Pennsylvania you can’t place, photograph it and open Animal Identifier. Point the camera, snap the picture, and you’ll have the species and details in seconds. You can ask follow-up questions in the same session, whether you want to know if it stings or what butterfly or moth it becomes. Download it free on iOS or Android and start with 2 identifications a day at no charge.