Puss Caterpillar / Asp — Fact Sheet

Scientific name: Megalopyge opercularis (J. E. Smith, 1797) Common names: Puss caterpillar, asp, Italian asp, woolly slug, opossum bug, tree asp, perrito; adult moth: Southern flannel moth Family: Megalopygidae (flannel moths) Status in the San Antonio / Boerne corridor: Native and abundant. This is actually the most medically significant stinging caterpillar in North America. According to Texas A&M AgriLife Extension, you'll find them peaking in numbers during the fall.

At a glance #

Caterpillar size	25–35 mm (1" to 1.5")
Color	Pale gray to reddish-brown to golden-yellow; covered in dense fur-like hair
Distinctive shape	Teardrop or "tuft of fur" appearance; long "tail" of hair extending past body
Sting mechanism	Hollow venomous spines hidden under the fur, contact-activated
Adult moth	Fuzzy, dull orange to lemon yellow, with hairy legs and black "fuzzy boots"
Active period in Texas	Two generations: peak July, larger peak October–November
Host plants in Hill Country	Oak, pecan, elm, hackberry; also roses, ivy, dwarf yaupon

Why this fact sheet exists in a stinging insect content set #

Puss caterpillars are the larvae of a moth, not insects or arachnids, but part of the Lepidoptera order. I've seen a rise in questions about them when it comes to "stinging pests in Texas" because:

• The asp is the most common cause of severe caterpillar stings in North America. A 2017 study published in PubMed recorded **3,484 Megalopyge opercularis caterpillar stings reported to Texas Poison Control centers between 2000 and 2016** — and that’s just the cases that bothered to call Poison Control. The actual number of stings is likely much higher.
• Pediatric school nurses, pediatricians, and Hill Country veterinarians all consider this a recurring fall health issue. Asp envenomation comes up regularly in San Antonio pediatric practices during October and November.
• Asps are abundant on the same trees that dominate Hill Country residential landscaping — live oak, post oak, pecan, elm, hackberry. Even customers who usually don’t worry about wasps can find themselves dealing with asps every fall.
• The pain is genuinely severe. Of all the stinging insects and critters in this fact sheet series, the puss caterpillar likely causes the most intense pain that a typical Texas resident might encounter.

Caterpillars can be a nuisance, especially those that turn into stinging insects. I've seen them showing up in yards around San Antonio and the Texas Hill Country. One common type is the io moth caterpillar (Automeris io), which can be found from August to October. These caterpillars are about 3 to 4 inches long and are often mistaken for harmless ones. They can deliver a painful sting, so it's best to keep your distance. Prevention is key: keep your yard tidy, trim plants regularly, and remove any fallen fruit, which can attract them. If you spot these pests, don’t hesitate to reach out. About half our calls for caterpillar issues turn out to be related to their stinging adult form.

Identification #

If you spot something that resembles a small tuft of fur or a tiny toupee crawling on a tree branch or wall in Central Texas, stay away from it.

Diagnostic features of the larva (the caterpillar form):

• Teardrop or oval shape, about 1 to 1.5 inches long.
• Completely covered in dense, soft-looking fur-like hair that hides what's underneath.
• Color can vary widely — pale gray, golden yellow, light tan, reddish-brown, or grayish silver.
• Has a distinctive "tail" of long hair that sticks out beyond the back of the body.
• The hair looks genuinely silky and seems harmless — kids are especially attracted to it.
• Often described as looking like a "Persian cat hair clump" or a "small toupee."

The adult moth (Southern flannel moth):

• Fuzzy body, dull orange to lemon yellow.
• Hairy legs with characteristic black or dark "fuzzy feet."
• Wingspan about 25 mm (1").
• Active in late spring and fall.
• Adult moths do not sting.

The cocoon: This is a unique, oval-shaped structure that sticks to the trunk or branch of a host tree. It matches the color of the larva because the larval hairs are mixed into the silk. One end features a trap-door operculum that the adult pushes open to get out. The top side looks a bit like a leaf scar. You can sometimes spot these cocoons on tree bark in winter, and they aren't dangerous to handle. The venom is in the larval spines, which are mostly shed during pupation, but the removed larval hair might still cause some irritation.

The species name opercularis comes from the operculum, which is the trap door on the cocoon.

The venom system #

The venom delivery mechanism is what makes this caterpillar dangerous. Critical to understand:

The visible "fur" is harmless. Those soft hairs you see are called plumose setae. They don’t have venom. Think of them as insulation and a way to blend in.

Hidden among the soft hairs are venomous spines — sharp, hollow structures that connect to venom-secreting cells beneath the outer layer. These spines cluster together and are usually hard to see since they’re surrounded by longer, fluffy hairs known as plumose setae.

When a caterpillar touches your skin, its spines can break off and release venom right into the dermis, much like a hypodermic needle. The longer you come into contact with it, the more spines can penetrate, which means more venom is delivered.

Walker et al. published a detailed paper in 2023 in PNAS titled "Horizontal gene transfer underlies the painful stings of asp caterpillars." Here are the key findings:

• Megalopyge venoms are mostly made up of large aerolysin-like pore-forming toxins, which researchers have named "megalysins".
• These toxins activate mammalian sensory neurons by punching holes in nerve cell membranes — they really do permeabilize the membrane.
• The venom system here is quite different from other venomous caterpillar families (Limacodidae), hinting at a independent evolutionary origin for envenomation in megalopygids.
• Most surprisingly, the megalysin toxins were obtained through horizontal gene transfer from bacteria at some point in their evolution. The venom toxins of the asp caterpillar didn't come from caterpillar genes — they were borrowed from microbes. This is one of the more fascinating examples of horizontal gene transfer in animal venom evolution.

The sting from an asp caterpillar leaves a telltale mark on the skin. You’ll see a "grid-like hemorrhagic papular eruption" — that’s a fancy way of saying it looks like a rectangle made up of small dark red spots. According to Purdue Extension, this pattern matches the way the caterpillar’s spines are arranged. I've seen dermatologists who know their stuff can often recognize asp stings just by looking at the skin.

What an asp sting actually feels like #

This is where things get intense. Descriptions of Megalopyge opercularis stings from scientific studies really paint a picture of the pain involved.

• Feels like hot coals on your skin.
• It’s like getting slammed by a baseball bat.
• Comparable to the pain of a broken bone or blunt-force trauma.
• It’s white hot.
• Justin Schmidt's The Sting of the Wild calls asp pain "spectacularly intense."

The pain hits you right away or builds up within five minutes after getting stung. People often describe it as throbbing, feeling deep rather than just on the surface, and importantly — it radiates. If you get stung on your forearm, the pain can travel up your arm and might settle in your armpit (it's common to see swelling in the nearby lymph nodes). A sting on the leg can shoot pain down to the groin.

Pain usually hits its peak within an hour and then fades over the next few hours. If the venom dose is higher, you might feel symptoms for a few days. Those grid-like spots you see on the skin usually go away within a day, but some darker bruise-like marks can stick around for longer.

Systemic symptoms — up to 1 in 3 victims #

Asp stings can lead to serious systemic effects in a notable number of people. Medical studies show that some envenomations result in various systemic reactions.

• Headache
• Nausea and vomiting
• Severe abdominal pain (I've seen cases where asp stings were mistaken for needing surgery)
• Swollen lymph nodes and inflammation
• Muscle spasms
• Faintness and dizziness
• Rapid heartbeat
• Symptoms that resemble shock
• Trouble breathing
• Difficulty swallowing
• Sweating from the welts
• Convulsions (rare but can happen)

About one-third of people can have some systemic reactions. Most of the time, these clear up on their own without needing treatment, but serious cases should see a doctor right away. Allergic reactions are rare, but they can happen in some individuals.

There aren’t any documented deaths from asp stings in modern medical literature, but the severity of reactions can be serious enough that researchers think fatalities could happen, especially in young children, the elderly, or those with weakened immune systems. Sometimes, hospitalization is needed just for pain relief or to manage symptoms.

First aid #

The recommended initial response from medical literature:

• Remove embedded spines using tape. Stick some cellophane tape on the sting site, press down hard, then peel it off. The spines will come off with the tape. You might need to do this a few times.
• Wash the area with soap and water. Use a hair dryer on a low setting to dry the area afterward.
• Apply ice or cold compress to help reduce pain.
• Oral antihistamine like Benadryl can help with allergic reactions or itching.
• Hydrocortisone cream can help with swelling and inflammation.
• Isopropyl alcohol on the sting site has worked for some people.
• Aspirin is reportedly NOT effective — doctors might give you stronger pain meds if the pain is really bad.
• Avoid scratching to lower the risk of a secondary infection.

Seek emergency medical attention if:

• Severe pain that doesn't go away with basic treatment.
• Systemic symptoms like nausea, trouble breathing, abdominal pain, or chest pain.
• Signs of an allergic reaction, including generalized itching, hives, or swelling that spreads beyond the sting site.
• Stings to the face, neck, or eyes.
• Kids who show serious envenomation symptoms.
• Multiple stings at once.

Biology and lifecycle #

Two generations per year #

In Texas, Megalopyge opercularis has two generations each year. The Texas Poison Center sting data from a study done between 2000 and 2016 shows a clear pattern over time:

• First generation: peak in July (12% of annual stings)
• Second generation: peak in October and November (a striking 59% of annual stings)

The fall generation of stinging insects is way more abundant than the summer generation. This seasonal pattern explains the rise in asp-related calls and ER visits we see in October and November across Texas.

Why fall is so much worse #

The fall asp peak overlaps with several factors that increase human contact:

• Cooler temperatures get folks outside more often.
• Kids are back to school, and outdoor activities ramp up in the fall.
• Halloween decorations go up, and yard work picks up pace.
• With milder weather, children are outdoors playing more.
• Pets are enjoying the outdoors more, too.
• Caterpillars are out in greater numbers thanks to a successful summer breeding season.

Lifecycle stages #

Adult flight: You'll see the first generation flying in late spring and early summer, while the second generation shows up in late summer and fall. These adults don’t live long—just a few days to a couple of weeks.

Egg laying: Female stinging insects can lay several hundred eggs on their preferred host trees.

Larval stage: The caterpillar munches on the leaves of host trees. It goes through several instars over weeks to months. This is when it poses the biggest threat.

Pupation: The final instar caterpillar spins a unique operculate cocoon on the trunk or branch of its host tree. Cocoons stay on tree trunks through the winter for the second generation of pupae.

Eclosion: The adult moth breaks free from the cocoon by pushing open the operculum. When it first emerges, the moth is a dull orange or yellow and has a fuzzy appearance.

Host plants #

In our service area, the primary host plants are:

• **Live oak (Quercus virginiana)** — this tree is a staple for providing shade in the Hill Country.
• **Post oak (Quercus stellata)** — another common oak variety you’ll see around.
• Other Texas oaks — there are several species worth noting, each with its own charm.
• **Pecan (Carya illinoinensis)** — a popular choice in many landscapes around here.
• **Elm (Ulmus)** — especially the cedar elm, which thrives in our climate.
• **Hackberry (Celtis)** — these are resilient trees that do well in our area.
• Yaupon and dwarf yaupon holly — great for adding greenery and structure.
• Roses in landscape beds — they add beauty and fragrance to any yard.
• English ivy and other ground covers — useful for filling in spaces and controlling erosion.

This list highlights the main trees you'll find along the San Antonio to Boerne corridor. Live oak and pecan trees make up most of the mature shade trees in neighborhoods from Alamo Heights to Comfort. If you have mature live oaks on your property, you may have potential asp exposure.

Local context — San Antonio and the Hill Country #

Asp stings are a recurring fall medical event in the entire San Antonio / Hill Country region. Specific local patterns:

Schools and pediatric facilities: Boerne ISD, Comal ISD, Northside ISD, Alamo Heights ISD, and SCUCISD all have protocols in place for asp stings. In October and November, school nurses see multiple visits each week related to asp stings. The trees around playgrounds, especially live oak shade trees, are common spots where kids get stung.

Hill Country residential landscapes: Stone Oak, Sonterra, Encino Park, Fair Oaks Ranch, Bulverde, Spring Branch — if you've got mature live oaks in your yard, expect some ant pressure each fall. Homes with lots of landscaping and oak trees seem to get hit the hardest.

Boerne and Bergheim: The historic district and nearby neighborhoods are filled with mature oak trees. We see high populations of asp every year.

Helotes and Government Canyon: The area has a lot of native oak and cedar trees. This creates a rich habitat, but it also means there's high pressure from asp (Aspen leaf miner) in the wooded subdivisions.

Outdoor venues: Pool decks shaded by live oaks, outdoor dining areas, playgrounds, and picnic shelters in city parks. I've seen asps drop or get knocked from the branches above, landing on unsuspecting folks below.

Pets: In the fall, dogs can stumble upon asps that have fallen from trees onto the grass. We've seen this happen around San Antonio and nearby areas. When dogs get stung, vets usually find stings on their paws or muzzles. Treating these stings is similar to what you'd do for humans: carefully remove the spines with tape if you can, offer support, give antihistamines, and keep an eye out for any serious reactions.

The "caterpillar dropping from the live oak" scenario #

In our service area, I've seen a common fall scenario: a homeowner relaxing under a mature live oak in October. Suddenly, a caterpillar drops onto their bare skin—maybe their forearm, neck, or ankle. They instinctively brush it off, and that’s when trouble starts. The spines from the caterpillar embed deep into their skin. Pain kicks in within minutes. Before they know it, they’ve got a dozen or more spines stuck in there, and the pain is radiating and severe.

This situation accounts for a significant number of fall ER visits due to asp stings in San Antonio hospitals.

Risk to humans and pets #

Moderate to high. A single sting can be extremely painful and can cause systemic symptoms in a significant number of victims. Kids and those with allergies face a greater risk of severe reactions.

Asps aren’t aggressive. They won’t chase you down, and they only sting if you come into contact with them. So, if you're wandering through areas where asps live, working on or under trees they favor, or sitting in outdoor furniture beneath those trees during their peak season, you might be at a higher risk of getting stung.

Between 2000 and 2016, Texas Poison Centers reported 3,484 stings, averaging around 200 stings each year. This number only counts the calls made to Poison Control. According to CDC, the actual number of asp envenomations in Texas is probably in the thousands each year.

Treatment approach #

Asp control is genuinely challenging because:

• Caterpillars munch away in the canopy of mature trees.
• Treating them means getting to those high leaves.
• Aerial spraying doesn't work well for homes.
• You’ve got a short window for effective treatment – only during the caterpillar stage.
• Asp populations can swing wildly from year to year.
• Hard sprays can end up killing off helpful pollinators.

Our approach for properties with confirmed asp problems:

Identification and assessment:

• Visual inspection of host trees during peak season, which is usually late spring to early summer.
• Identify which trees on your property have active infestations.
• Document the number of caterpillars to help decide on treatment.
• Survey ground and turf areas for fallen caterpillars.

Targeted treatment:

• For lower branches, shrubs, and vines, we use caterpillar-specific products like Bacillus thuringiensis (Bt). It targets caterpillars while being gentle on pollinators.
• For taller trees, systemic insecticides applied through trunk injection or soil drench can work well during the early larval stage. Just keep in mind, this method is more invasive.
• We recommend pyrethroid-based contact sprays for accessible foliage, but make sure to avoid applying them during bloom periods.

Habitat and exposure reduction:

• Trim low branches that hang over outdoor seating, walkways, and play areas.
• Teach everyone in your household how to identify stinging insects and avoid them.
• Remove ground litter under trees during peak season to minimize contact with fallen caterpillars.
• Move outdoor furniture away from the worst-affected areas during October and November.

Education-first approach for low-density populations:

• Many homes see a few asps each year, and that usually doesn't call for chemical treatments.
• Teaching kids how to identify them and promoting awareness can often be enough to avoid issues.
• We hand out laminated identification cards for school-age kids in our client households.

What NOT to do:

• Avoid crushing or smashing caterpillars with your bare hands. It seems obvious, but it’s worth mentioning.
• Don't use a hose or pressure washer to knock asps from the canopy. It sends them flying onto people, pets, and surfaces below.
• Don’t think that cleaning up dead caterpillars eliminates the risk. Shed hairs in the area can still cause irritation for days.

Odd, funny, and genuinely true #

• 3,484 asp stings were reported to Texas Poison Centers between 2000 and 2016. Texas alone, just the calls that reached Poison Control, just one species of caterpillar. The actual total envenomation count is far higher.
• The pain has been described in peer-reviewed medical literature as "like hot coals applied to the skin" and "like being hit by a baseball bat." These are not journalist embellishments — they are direct quotes from clinical case reports in Annals of Emergency Medicine and Southern Medical Journal.
• The asp's venom toxins came from bacteria via horizontal gene transfer. A 2023 PNAS paper by Walker et al. established that the megalysin pore-forming toxins in Megalopyge venom are derived from aerolysin-like proteins that were originally bacterial. At some point in the evolutionary past, asp ancestors acquired these genes from microbial sources — a remarkable example of a venomous animal "borrowing" its venom from a different domain of life.
• Asp envenomation is one of two animal toxin systems known to use aerolysin-type pore-forming toxins. The other is found in certain Doratifera slug caterpillars in Australia. Both involve horizontal gene transfer events from bacteria, but the gene transfers were independent — convergent evolution at the molecular level.
• The sting can radiate up an entire limb. A documented characteristic of asp envenomation is regional lymphatic involvement: a sting on the forearm produces pain in the armpit, a sting on the calf produces groin pain. This radiation is part of why asp stings can be confused with cardiac events or surgical emergencies — the symptom pattern is unusual.
• Asp stings have been mistaken for acute abdominal emergencies. A 1996 case report in Southern Medical Journal describes a patient who presented with severe abdominal pain that was eventually traced to an asp sting on the chest, with referred pain mimicking surgical pathology.
• The grid-like skin lesion is diagnostic. The hemorrhagic papules that form in a roughly rectangular pattern correspond to the cluster arrangement of venom spines on the caterpillar's body. Dermatologists in Texas can often identify asp envenomation from the lesion pattern alone.
• The Italian "asp" name is a misnomer. The caterpillar has nothing to do with the asp (a type of snake) — the name is folk usage. Other folk names include "perrito" (Spanish for "little dog"), "woolly slug," "opossum bug," and "Italian asp." The Texas usage of "asp" predominates locally and has been entrenched for over a century.
• The Southern flannel moth (the adult form) is genuinely beautiful. Dull orange or lemon yellow, fuzzy body, characteristic dark "fuzzy feet" that look like little black socks. Photographs of the adult are sometimes used on Texas wildlife calendars. The moth itself is harmless.
• Cocoons survive winter on tree trunks. Empty cocoons can persist on bark for months after the adult emerges. The intact cocoon (containing a developing pupa) is the overwintering stage — meaning asps survive Hill Country winters as pupae glued to oak and pecan tree trunks, ready to emerge as adults in spring.
• Researchers studying asp venom milk individual caterpillars. The standard technique involves placing parafilm against the spines and collecting the venom droplets that form. Photographs of these venom droplets show the characteristic grid-like clustering — the same pattern that produces the diagnostic skin lesion in humans.
• The puss caterpillar is a member of the same broader Lepidoptera group (Zygaenoidea) as several other stinging caterpillar families — including the saddleback caterpillar, Io moth caterpillar, and slug caterpillars. Each family independently evolved venomous defense, with different toxin systems. Megalopygids are unique in their use of aerolysin-derived megalysins.
• They cause two generations per year, and the second is six times more dangerous. That fall peak — 59% of annual stings concentrated in October and November — is the single most predictable seasonal stinging-pest event in Central Texas. If you are doing pest education content for a fall-season audience, the asp is the headline topic.
• The 1922 paper by Nathan Foot in the Journal of Experimental Medicine — "Pathology of the Dermatitis caused by Megalopyge opercularis, a Texan caterpillar" — was one of the earliest medical-literature treatments of the species and established baseline understanding of the envenomation pathology. Texas asps have been a documented medical concern for over a century.

FAQ hooks (for LuperIQ / SEO) #

• An asp caterpillar, or Megalopyge opercularis, is a fuzzy caterpillar known for its painful sting and distinctive appearance. They’re often found in Texas.
• If you get stung by a fuzzy caterpillar, wash the area with soap and water, apply ice to reduce swelling, and take an antihistamine for itching. If symptoms worsen, seek medical help.
• Yes, puss caterpillars, or asp caterpillars, are found in Texas, especially in the eastern parts.
• An asp sting can be very painful and may cause severe reactions in some people, but it’s rarely life-threatening.
• Asps are typically active from late summer to fall, around August to November, so that’s when you’ll likely encounter them.
• Asp caterpillars are often found on trees like oaks, elms, and other hardwoods, which they use for food and shelter.
• To get rid of asps in your yard, carefully remove them by hand while wearing gloves, and keep your yard tidy to deter them.
• While asp stings can be serious, they are unlikely to kill children or pets, though reactions can vary, so keep an eye on them if stung.

We’ve consulted a variety of sources for this information. These include the Wikipedia entry for Megalopyge opercularis, Texas A&M AgriLife Extension’s fact sheet on asps and stinging caterpillars, and the Merck Manual Professional Edition on puss moth caterpillar stings. The MSD Manual Consumer Version also provided insights. A 2023 study by Walker et al. in PNAS discussed the painful stings of asp caterpillars due to horizontal gene transfer. We looked at the Forrester 2018 study published in PubMed which analyzed Texas Poison Center data on 3,484 stings from 2000 to 2016. The Eagleman 2008 case report on asp caterpillar envenomation (Clinical Toxicology) was another resource. Foot's foundational paper from 1922 in the Journal of Experimental Medicine contributed to our understanding as well. Finally, we reviewed first-aid information from Bugs in the News by entomologist Jerry Cates. Pain descriptions come from patient accounts in studies by McMillan and Purcell (1964), Pinson and Morgan (1991), and Stipetic et al. (1999).

Frequently asked questions #

How can I identify a Puss Caterpillar? #

Puss Caterpillars are fuzzy and often resemble a tuft of fur. They are typically yellow to brown in color with long, hair-like projections. Their body can be about 1 to 3 inches long, and they can be mistaken for a harmless caterpillar, but their stinging hairs can cause severe reactions.

What should I do if I get stung by a Puss Caterpillar? #

If you’re stung, the first step is to wash the area with soap and water to reduce the risk of infection. You can apply ice to alleviate swelling and pain, and over-the-counter pain relievers may help. If you experience severe symptoms or an allergic reaction, seek medical attention immediately.

When are Puss Caterpillars most active in San Antonio? #

In San Antonio and the Texas Hill Country, Puss Caterpillars are most commonly found from late summer to early fall. They are often seen on trees, especially oak and citrus, where they feed and can sometimes be found resting.

How do you treat an infestation of Puss Caterpillars? #

We typically start with a thorough inspection to locate any nests or areas where the caterpillars are present. Treatment may involve targeted insecticides and removal of the caterpillars by hand, but we always prioritize safety and follow up to prevent future infestations.

Are Puss Caterpillars common in residential areas of San Antonio? #

Yes, Puss Caterpillars can be found in residential neighborhoods, particularly where there are host plants like oaks and citrus trees. Homeowners should be vigilant, especially in late summer and fall, to avoid encounters with these potentially dangerous pests.

Last reviewed April 25, 2026 by Travis Lambert (Owner).