Pennsylvania is home to dozens of caterpillar species, from harmless leaf-eaters to a handful that can deliver a painful sting. Whether you spotted something strange on a tree in your yard or you’re trying to figure out if a fuzzy caterpillar is safe to touch, here’s what you need to know about the most common and notable species across the state.
Common Caterpillars You’ll See on Trees
Several caterpillar species show up reliably across Pennsylvania, especially in spring and early summer when deciduous trees are leafing out. Learning a few key markings makes identification straightforward.
Spongy moth caterpillars are among the most widespread. Young ones have black dots running down their backs. As they grow, they develop five pairs of blue spots behind the head followed by six pairs of red spots. They feed on hundreds of plant species, but oak, apple, birch, poplar, and willow are favorites. These caterpillars are a major forest pest: in 2023 alone, spongy moths defoliated nearly 442,000 acres of Pennsylvania forest, down from about 850,000 acres the year before.
Eastern tent caterpillars are brown and hairy with a distinctive white stripe down the back and patches of blue and orange. They build conspicuous silk “tents” in the forks of branches, almost always on cherry, crabapple, or apple trees. Wild black cherry is their top choice. Outbreaks are periodic but short-lived, and healthy trees recover without intervention even after complete defoliation. Because these caterpillars hatch early in spring, they sometimes consume all the fresh buds, making it look like a tree never leafed out at all.
Forest tent caterpillars look similar but have white keyhole-shaped marks down the back instead of a solid stripe. They prefer sugar maple, oaks, birch, and black gum. Despite the name, they don’t actually build tents.
Cankerworms (also called inchworms or loopers) move by anchoring their rear end and stretching forward in that familiar looping motion. They range from light green with pale stripes to nearly solid black. You’ll find them on ash, beech, black cherry, maple, and oak.
Stinging Caterpillars to Avoid
A small number of caterpillars in Pennsylvania carry hollow, quill-like hairs connected to venom sacs. When you brush against them, the hairs pierce your skin and release poison. Reactions range from mild itching to pain, swelling, blistering, and in rare cases intestinal problems. The key rule: if a caterpillar has prominent spines, horns, or an unusually dense coat of fur, don’t touch it.
Hickory tussock moth caterpillars are one of the most commonly encountered stinging species in Pennsylvania. They’re white and fuzzy with a row of black tufts down the back. Their barbed hairs detach easily and can embed in skin, eyes, or mucous membranes. Most people develop a localized rash and itching that resolves on its own within 48 hours, though allergic reactions are possible.
Io moth caterpillars are bright green to yellow, about two inches long, with a narrow red line bordered by white along each side. They’re covered in branched, black-tipped yellow spines that pack a real punch.
Buck moth caterpillars are brown to purplish-black with yellow spots and branched black spines tipped in red or black. Also about two inches long, they’re found on oaks.
Saddleback caterpillars have a striking look: brown on both ends, green in the middle with a purple spot centered on the green “saddle.” They sport prominent horns on the front and rear, and their sting can cause severe irritation.
Puss caterpillars are arguably the most dangerous. They look deceptively soft, covered in a dense coat of brown woolly hair that hides short venomous spines underneath. About an inch long, they feed on various trees and shrubs. Their stings are typically more severe than those of other caterpillar species, and the pain intensifies with larger caterpillars.
Less common but still present are hag moth caterpillars (brown, resembling a dried leaf, with nine pairs of fleshy lobes bearing stinging hairs), stinging rose caterpillars (yellow to red with black and blue stripes and spiny yellow horns), and flannel moth caterpillars (furry and similar in appearance to the puss caterpillar, with venomous spines hidden beneath longer harmless hairs).
When Caterpillars Are Most Active
Pennsylvania’s caterpillar activity follows two main waves. The spring wave, roughly April through June, brings the biggest populations. Spongy moths, eastern tent caterpillars, forest tent caterpillars, and cankerworms all emerge during this window, timed to feed on fresh leaves. This is also when you’re most likely to notice defoliation on backyard and forest trees.
A second wave arrives in late summer and early fall. Hickory tussock moth caterpillars, io moths, and saddleback caterpillars are more common during this period. The familiar woolly bear caterpillar, banded in black and brown, shows up in early fall as it crawls across sidewalks and driveways searching for a sheltered spot to overwinter. It emerges the following spring as the Isabella tiger moth.
Tree Damage and What to Expect
Most caterpillar damage in Pennsylvania is cosmetic. Even when eastern tent caterpillars completely strip a tree, healthy trees typically rebound and push out a second flush of leaves. Penn State Extension describes the damage as “aesthetic rather than a health threat to most trees,” and outbreaks tend to be periodic and short-lived.
Spongy moths are the bigger concern. Their ability to feed on hundreds of tree species means outbreaks can affect vast stretches of forest. Repeated years of heavy defoliation can weaken and eventually kill trees, particularly oaks already stressed by drought. Pennsylvania’s Department of Conservation and Natural Resources conducts annual aerial spraying programs in response, and the decline from 850,000 defoliated acres in 2022 to about 442,000 in 2023 reflects both natural population cycling and those suppression efforts.
If caterpillars are stripping a single ornamental tree in your yard, you can physically remove the egg masses or silk tents in early spring before larvae disperse. For eastern tent caterpillars, pulling down the web tents on a cool morning (when caterpillars cluster inside) and disposing of them is effective and simple.
What to Do if You Get Stung
If you’ve brushed against a stinging caterpillar, the first step is removing the tiny hairs from your skin. Press a strip of adhesive tape (duct tape or packing tape works well) sticky-side down onto the affected area and peel it off. This pulls out embedded hairs that washing alone won’t remove. Repeat several times with fresh strips.
After the hairs are out, apply aloe vera or a hydrocortisone cream to reduce inflammation. An oral antihistamine can help if the rash spreads or itching worsens. Most reactions resolve within a day or two with this basic care.
Severe reactions are uncommon but possible. Significant swelling beyond the sting site, difficulty breathing, or symptoms that keep getting worse after home treatment warrant prompt medical attention. Puss caterpillar stings in particular can produce intense, radiating pain that catches people off guard given how harmless the caterpillar looks.