A flannel moth caterpillar sting causes immediate, intense pain that can radiate well beyond the contact site. These caterpillars, most commonly the southern flannel moth larva (also called the puss caterpillar or “asp”), look deceptively soft and harmless. Their fur-like covering hides rows of hollow venomous spines that puncture skin on contact, delivering venom that makes this one of the most painful caterpillar stings in North America.
How the Sting Works
The puss caterpillar’s long, silky hairs are not the problem. Beneath that soft exterior lie shorter, stouter venom spines, each only about 30 micrometers wide, tapering to dagger-like tips less than 5 micrometers thick. These spines are hollow for most of their length, acting as tiny hypodermic needles. Each one connects to a bulb-shaped reservoir at its base, which is fed by a canal running down through the caterpillar’s skin to a dense mass of secretory cells that produce the venom. A 2023 study published in PNAS found that the venom genes in these caterpillars were actually acquired through horizontal gene transfer from bacteria, making their sting biochemically unusual among insects.
When you brush against the caterpillar, even lightly, the spines snap off and embed in your skin. Because there are many spines arranged in pairs along the caterpillar’s underside, a single brief contact can leave dozens of tiny venom-filled barbs in the skin at once.
What the Sting Looks and Feels Like
The pain begins within seconds and is often described as throbbing or burning, sometimes compared to a wasp sting but more radiating. It typically intensifies over the first five to ten minutes. Some people report pain spreading up the arm or leg from the sting site, following the path of nearby lymph nodes.
The skin itself develops a distinctive pattern. Because the spines are arranged in rows along the caterpillar’s body, the resulting rash often appears as a grid-like or “tram track” pattern of small hemorrhagic bumps. In some cases, the rash outlines the exact shape of the caterpillar where it made contact. The area around the sting becomes red and swollen, and a raised, itchy rash can persist for days.
Most stings cause only local symptoms: pain, swelling, redness, and itching. But more significant reactions can include nausea, headache, abdominal pain, muscle cramps, and in rare cases, difficulty breathing or drops in blood pressure. These systemic reactions are uncommon but more likely with multiple stings or in sensitive individuals, especially children.
First Aid for a Flannel Moth Caterpillar Sting
If the caterpillar is still on your skin, remove it with a stick or similar tool. Do not touch it with bare hands, as this will drive more spines into your skin. The priority after that is removing as many embedded spines as possible, since they continue releasing venom as long as they’re in the skin.
The most effective removal method is pressing a strip of adhesive tape firmly over the sting area and peeling it off. Repeat this several times with fresh strips of tape. This pulls out spines that are too small to see or pick out individually. After that:
- Wash the area with soap and water, then dry it with a hair dryer on a low setting rather than rubbing with a towel (which can push remaining spines deeper).
- Apply rubbing alcohol (isopropyl alcohol) to the site to help neutralize residual irritation.
- Ice the area with an ice pack or ice cube wrapped in a thin cloth to reduce swelling and numb the pain.
- Apply a baking soda paste or calamine lotion for additional soothing relief.
Over-the-counter antihistamines can help with swelling and itching. Pain relievers can take the edge off, though many people find the worst pain subsides within an hour or two even without medication. The rash and tenderness may last several days.
When Stings Become Serious
Severe reactions are treated similarly to serious insect stings. If you experience chest tightness, difficulty breathing, widespread hives, dizziness, or swelling far from the sting site, that warrants emergency care. Children are at higher risk for stronger reactions due to their smaller body size relative to the amount of venom delivered. Multiple stings from handling the caterpillar also increase the likelihood of systemic symptoms.
Where and When You’re Most Likely to Encounter Them
The southern flannel moth ranges from New Jersey south to Florida and west to Arkansas and Texas. Texas sees the highest concentrations, particularly from Dallas southward through the western central part of the state. Florida is another hotspot. The caterpillars feed on a wide variety of trees and shrubs, with oaks and elms being especially common hosts. In Florida, they’re frequently found on both native elms and ornamental Chinese elms.
There are two distinct peak seasons each year: one in early summer and another in fall. In the Deep South, a partial third brood is possible. The fall brood tends to generate the most sting reports, likely because the caterpillars are larger and more numerous by then, and because they sometimes drop from trees onto people below.
Avoiding Stings
The biggest risk factor is simply not recognizing what you’re looking at. Puss caterpillars resemble small tufts of cotton or fur clinging to leaves and branches, and children are naturally drawn to touch them. Teaching kids to never handle hairy caterpillars is the single most effective prevention measure.
If you’re gardening or doing yard work in areas where these caterpillars are common, wear gloves and a hat. They often feed on the undersides of leaves, making them easy to grab accidentally when pruning or raking. Check outdoor furniture, playground equipment, and the undersides of railings during peak season. If you find caterpillars on trees near high-traffic areas, you can knock them off with a stick and crush them, or monitor the plants regularly and remove caterpillars as they appear. In severe infestations near schools or play areas, removing or relocating known host plants is sometimes the most practical long-term solution.