The asp caterpillar, also called the puss caterpillar, delivers one of the most painful insect stings in North America. Found throughout Texas on oak and elm trees, this fuzzy, innocent-looking creature hides venomous spines beneath its soft hair that can cause intense burning pain, skin welts, and occasionally serious allergic reactions. If you’ve just been stung or want to know what to watch for, here’s what you need to know.
What an Asp Caterpillar Looks Like
Asp caterpillars grow to about one inch long, though fully mature larvae can reach 1.4 inches. They’re completely covered in thick, soft fur that ranges from tan to grayish-white, tapering toward a small tail at the back end. The fur gives them a rounded, almost cuddly appearance, which is exactly the problem. Their nickname “puss caterpillar” comes from their resemblance to a small cat. In parts of Texas, people also call them “perrito” (Spanish for puppy), “possum bugs,” or “woolly slugs.”
You can’t see the head or legs from above. What you also can’t see are the shorter, sharp spines hidden among the longer body hairs. These spines are the venom delivery system, and they discharge their payload the moment they touch skin. Young caterpillars start out yellow and turn pale greenish-white to white as they grow through their later stages.
Where and When You’ll Encounter Them
Asp caterpillars are commonly found in oak and elm trees, but they also congregate in shrubbery and ornamental landscaping. In Texas, fall is the peak season, with large numbers appearing in yards, parks, and around schools. They show up nearly every year in high enough numbers to cause concern, particularly in southern Texas. Most stings happen when someone brushes against a caterpillar on a tree branch, leans against a trunk, or picks one up out of curiosity. Children are especially at risk because the caterpillars look like something worth touching.
How the Sting Works
The venom hiding in those short spines is more sophisticated than you’d expect from a caterpillar. Researchers at the University of Queensland discovered that asp caterpillar venom contains toxins that punch holes in cells the same way bacterial toxins from E. coli and Salmonella do. The venom proteins bind to the surface of cells and assemble into donut-shaped structures that pierce through cell walls. It’s a mechanism similar to box jellyfish venom, and it evolved from genes transferred from bacteria more than 400 million years ago.
This helps explain why the pain is so disproportionate to the size of the creature. People who have been stung commonly describe it as feeling like touching burning coal or being hit with blunt force trauma. The sting is severe enough to send some people to the hospital.
What the Sting Feels Like
The pain hits immediately and intensely. Within seconds of contact, you’ll feel a sharp, burning sensation at the sting site. A characteristic grid-like pattern of red welts often appears on the skin, following the lines where the spines made contact. The area swells quickly and can blister.
Beyond localized pain, some people experience symptoms that spread beyond the sting site: radiating pain up the arm or leg, swelling of nearby lymph nodes, nausea, headache, or a general feeling of being unwell. In rare cases, people develop allergic reactions with generalized itching or difficulty breathing. These are the situations that require immediate medical attention.
What to Do Right After a Sting
If the caterpillar is still on your skin, remove it with a stick or other tool. Do not touch it with bare hands, as you’ll just add more spine punctures. Then follow these steps:
- Remove the spines. Press tape (cellophane or duct tape) sticky-side down onto the sting site, then peel it off. This pulls embedded spines out of the skin and stops them from releasing more venom. Repeat several times with fresh tape.
- Wash the area. Use soap and water, then dry the site thoroughly. Some sources recommend using a hair dryer on a low setting.
- Apply ice. Wrap an ice pack or ice cube in a thin cloth and hold it over the sting. This helps with both pain and swelling.
- Apply rubbing alcohol or a baking soda paste. Either isopropyl alcohol or a baking soda and water slurry can help soothe the burning. Calamine lotion is another option.
An oral antihistamine can help relieve itching and burning. If baking soda paste doesn’t provide enough relief, a hydrocortisone cream is a reasonable next step. Antihistamine creams are a third option, though they don’t always work and can cause skin reactions in some people.
Signs That Need Medical Attention
Most asp stings, while extremely painful, resolve on their own within a few hours to a day. But certain reactions cross into dangerous territory. Generalized itching that spreads far from the sting site, difficulty breathing, chest tightness, or significant swelling of the face or throat are signs of a serious allergic reaction that needs immediate emergency care.
If the caterpillar’s spines get into your eyes, see a specialist right away. Eye injuries from venomous spines carry their own set of risks and need professional treatment. If the sting area develops bad blistering, contact your healthcare provider. It’s also worth checking whether your tetanus shots are current, particularly if the sting broke the skin significantly.
Preventing Asp Stings
Awareness is the best protection, especially during fall in Texas. Before leaning against a tree, sitting on outdoor furniture beneath oaks or elms, or letting children climb trees, check the bark and branches for caterpillars. Wear gloves and long sleeves when doing yard work around trees where you’ve seen them before. If you spot asp caterpillars congregating in shrubs near your home or your children’s school, keep a safe distance and consider contacting a pest management professional rather than handling removal yourself.
Even dead or shed caterpillar skins can sting. The spines retain their venom after the caterpillar has moved on or died, so avoid touching any fuzzy remains you find on the ground or stuck to tree bark.