Stinging caterpillars turn into moths. Every species of stinging caterpillar in North America is the larval stage of a moth, not a butterfly. The adults are typically fuzzy, stout-bodied, and completely harmless, having lost their venomous spines during metamorphosis. Most people who encounter the painful caterpillar would never connect it to the unremarkable moth it becomes.
Why Caterpillars Sting but Moths Don’t
Stinging caterpillars have hollow, quill-like hairs connected to small poison sacs beneath their skin. When you brush against these hairs, the tips break off and pierce your skin, releasing venom directly into the wound. This is purely a defensive system for a slow, vulnerable larva that can’t fly away from predators.
Once a caterpillar enters its cocoon and undergoes metamorphosis, those venomous spines are broken down along with the rest of the larval body. The adult moth that emerges has soft, hair-covered wings and legs but no venom apparatus whatsoever. Some species do weave their old stinging hairs into the silk of their cocoons, which means the cocoon itself can still cause a sting if you handle it carelessly.
Puss Caterpillar to Southern Flannel Moth
The puss caterpillar is the most notorious stinging caterpillar in the United States. It looks like a small tuft of fur, usually tan or grayish-brown, and its sting can cause extreme pain, a grid-like rash, and occasionally systemic symptoms like nausea. It becomes the southern flannel moth, a heavy-bodied, fluffy moth with a reddish-brown head and thorax and broad wings that blend from orange-brown near the body to pale yellow or creamy white at the tips. The wings have a distinctive wrinkled, woolly texture from wavy hairs across their surface. Its wingspan is roughly one inch.
Southern flannel moths are found throughout the southeastern U.S., from New Jersey south to Florida and west to Texas and Oklahoma. In southern states like Florida and Texas, they produce two or more generations per year, with adults flying year-round. Farther north, adults are active from April through September. Peak caterpillar season is September in the Southeast and October in south-central states like Texas, which is when most stings occur.
Saddleback Caterpillar to Saddleback Moth
The saddleback caterpillar is one of the most visually striking stinging species: bright green with a prominent brown, saddle-shaped marking on its back and fleshy horn-like projections at both ends. Clusters of venomous spines line its sides. The adult it produces, the saddleback caterpillar moth, is the largest of the slug moth family in North America. It’s a stout, dark brown moth with broad reddish-brown forewings marked by small white dots near the base and farther out toward the wing tips. Under certain light, silvery-blue scales along the middle of the wing give it a subtle sheen. It’s an attractive moth if you look closely, but nothing about it hints at the vivid, weaponized caterpillar it once was.
Io Caterpillar to Io Moth
The Io caterpillar is pale green with a red-and-white stripe running along each side, covered in clusters of branching, venomous green spines. It feeds on a wide range of trees and shrubs and delivers a sting comparable to the puss caterpillar’s. The adult Io moth is one of the more spectacular transformations on this list. It’s a large moth with a wingspan of 2 to 3.5 inches. Males are bright yellow, females reddish-brown, and both carry large eyespots on their hindwings: black or blue circles with a white center, surrounded by a grayish iris.
These eyespots serve as the adult’s defense mechanism. At rest, the forewings cover the hindwings completely, making the moth look plain. When disturbed, it snaps its wings open to flash the eyespots, creating a sudden “face” that startles birds and other predators. It’s a striking visual trick that replaces the venomous spines the caterpillar relied on.
Buck Moth Caterpillar to Buck Moth
Buck moth caterpillars are dark-bodied, covered in branching spines, and commonly found feeding on oak trees. Their sting is sharp and can cause a lingering welt. The adult buck moth has a wingspan of 2 to 3 inches and a look quite different from the flannel or slug moths. Its wings are black and semi-transparent with a broad white band across each one, plus prominent eye markings on the forewings. The body is stout and hairy with a grayish-black abdomen tipped in reddish-orange.
Buck moths have an unusual seasonal schedule. Unlike most moths, adults emerge in the fall to mate and lay eggs. The eggs overwinter and hatch in spring, so caterpillar encounters happen primarily from late spring through early summer.
How They Build Their Cocoons
When a stinging caterpillar is ready to transform, it typically crawls down from its feeding site toward the base of its host plant or into leaf litter, seeking a sheltered crevice. The larva spins brown silk around itself, forming a hardened outer shell roughly 5/8 inch long. Pupation begins inside this cocoon after about five days. Some species, particularly puss caterpillars, incorporate their old venomous hairs into the cocoon wall. This means you can get stung by picking up what looks like an empty seed pod or dried bit of debris on a tree trunk.
What to Do if You Get Stung
If you brush against a stinging caterpillar, the tiny spines often stay embedded in your skin and continue releasing venom. The first step is removing them. Press adhesive tape (packing tape works well) firmly against the affected area and peel it away to pull out the hairs. Repeat with fresh strips until no more spines are visible.
After removing the spines, wash the area with soap and water or baking soda mixed with water, which helps neutralize remaining traces of venom and reduce inflammation. Applying an ice pack can bring down swelling and ease the burning pain. For the rash and itching that follow, over-the-counter corticosteroid cream or an antihistamine can help. Most stings resolve within a few hours, though puss caterpillar stings sometimes cause pain that radiates up the arm or leg and can last a full day or more.