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, abundant, the most medically significant stinging caterpillar in North America — peak abundance in 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 larval form of a moth — neither insect nor arachnid in the broader popular sense, but Lepidoptera. They are absolutely included in customer queries about "stinging pests in Texas" because:

• The asp is the most common cause of severe caterpillar stings in North America. A 2017 PubMed-published study documented **3,484 Megalopyge opercularis caterpillar stings reported to Texas Poison Control centers between 2000 and 2016** — and this represents only the cases that called Poison Control. Total stings are dramatically higher.
• Pediatric school nurses, pediatricians, and Hill Country veterinarians all consider this a recurring fall health issue. Asp envenomation is a routine topic in San Antonio pediatric practices in October and November.
• Asps are abundant on the same trees that dominate Hill Country residential landscaping — live oak, post oak, pecan, elm, hackberry. Customers who never need to think about wasps are exposed to asps every fall.
• The pain is genuinely severe. Of the stinging things in this entire fact sheet series, the puss caterpillar may produce the most intense single-incident pain that a typical Texas resident is likely to experience.

So this is a stinging-pest fact sheet about a caterpillar. The treatment, identification, and prevention work are squarely in the pest-management space.

Identification

If you see what looks like a small tuft of fur or a tiny toupee crawling on a tree branch or wall in Central Texas, do not touch 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 obscures the body underneath
• Color highly variable — pale gray, golden yellow, light tan, reddish-brown, or grayish silver
• Distinctive "tail" of long hair extending past the rear of the body
• Hair appearance is genuinely silky and looks completely harmless — children especially are drawn to it
• Often described as resembling a "Persian cat hair clump" or "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: A unique distinctive structure attached to the trunk or branch of a host tree, oval-shaped, with the same color as the larva because the larval hairs are incorporated into the silk. There is a trap-door operculum at one end that the emerging adult pushes open to exit. The dorsal surface has a structure resembling a leaf scar. These cocoons can sometimes be found on tree bark in winter and are themselves not dangerous to handle (the venom is in the larval spines, which are largely shed during pupation), though removed larval hair may still cause irritation.

The species name opercularis refers to the cocoon's operculum trap door.

The venom system

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

The visible "fur" is harmless. The soft hairs you can see are called plumose setae and have no venom. They are essentially insulation and camouflage.

Hidden among the soft hairs are venomous spines — sharp hollow structures connected via canals to venom-secreting cells beneath the cuticle. These spines are arranged in clusters and are normally invisible because they are surrounded by the longer plumose setae.

When the caterpillar contacts skin, the spines break off in the skin, releasing venom directly through the broken spine into the dermis. The mechanism is essentially identical in concept to a hypodermic needle. The longer the contact, the more spines penetrate, the more venom is delivered.

The chemistry was characterized in detail by Walker et al. in a 2023 paper published in PNAS ("Horizontal gene transfer underlies the painful stings of asp caterpillars"). Key findings:

• Megalopyge venoms consist primarily of large aerolysin-like pore-forming toxins, which the researchers named "megalysins"
• These toxins activate mammalian sensory neurons via membrane permeabilization — they literally punch pores in nerve cell membranes
• The venom system is markedly different from those of other venomous caterpillar families (Limacodidae), suggesting independent evolutionary origin of envenomation in megalopygids
• Most strikingly: the megalysin toxins were acquired through horizontal gene transfer from bacteria at some point in the evolutionary past. The asp caterpillar venom toxins were not originally derived from caterpillar genes — they were borrowed from microbial origins. This is one of the more remarkable cases of horizontal gene transfer underlying animal venom evolution.

The lesion the venom produces is itself diagnostic. Asp stings characteristically produce a "grid-like hemorrhagic papular eruption" — a roughly rectangular pattern of small dark red spots in the skin, corresponding to the spine cluster pattern of the caterpillar. Dermatologists familiar with the species can identify asp stings by the lesion pattern alone.

What an asp sting actually feels like

This is the section where the medical literature gets unusually vivid. Quoted descriptions of Megalopyge opercularis envenomation pain from peer-reviewed sources include:

• "Like hot coals applied to the skin"
• "Like being hit by a baseball bat"
• "Similar to a broken bone or blunt-force trauma"
• "White hot"
• Justin Schmidt's The Sting of the Wild describes asp pain as "spectacularly intense"

The pain is immediate or develops within five minutes of contact. It is described as throbbing rather than burning, deep rather than surface, and crucially — it radiates. A sting on the forearm produces pain that spreads up the limb and may localize in the armpit (lymphadenopathy of the regional lymph nodes is common). A sting on the leg radiates to the groin.

Pain intensity peaks within an hour and typically subsides within several hours. With higher venom doses, symptoms can persist for several days. The grid-like hemorrhagic spots on the skin typically fade within a day, though darker bruise-like marks can persist longer.

Systemic symptoms — up to 1 in 3 victims

Asp stings cause systemic effects in a substantial minority of victims. The medical literature documents the following systemic reactions, occurring in some fraction of envenomations:

• Headache
• Nausea, vomiting
• Acute abdominal distress (so severe in some cases that asp stings have been mistaken for surgical emergencies)
• Lymphadenopathy and lymphadenitis
• Muscle spasms
• Faintness, vertigo
• Tachycardia
• Shock-like symptoms
• Respiratory distress
• Difficulty swallowing
• Sweating from the welts
• Convulsions (rare)

Up to about one-third of patients experience some systemic involvement. Most resolve without specific treatment, but severe cases warrant emergency medical evaluation. Hypersensitivity reactions appear to be uncommon but can occur in allergic individuals.

Deaths from asp stings are not documented in the modern medical literature, but the severity of reactions in some patients has led researchers to suggest that fatal outcomes are biologically plausible, particularly in young children, the elderly, or immunocompromised individuals. Hospitalization is occasionally required for pain management or symptom control.

First aid

The recommended initial response from medical literature:

• Remove embedded spines using tape. Apply cellophane tape to the sting site, press firmly, then peel off. The spines stick to the tape and are removed. Repeat several times.
• Wash the area with soap and water. A hair dryer on low setting can be used to dry the area.
• Apply ice or cold compress for pain reduction.
• Oral antihistamine (Benadryl) for allergic symptoms or itching.
• Hydrocortisone cream for swelling and inflammation.
• Isopropyl alcohol applied to the sting site has been reported helpful by some patients.
• Aspirin is reportedly NOT effective — physicians may administer stronger pain medication for severe pain.
• Avoid scratching to prevent secondary infection.

Seek emergency medical attention if:

• Severe pain unresponsive to initial measures
• Systemic symptoms (nausea, breathing difficulty, abdominal pain, chest pain)
• Signs of allergic reaction (generalized itching, hives, swelling beyond sting area)
• Stings to face, neck, eyes
• Pediatric patients with significant envenomation
• Multiple stings

Biology and lifecycle

Two generations per year

In Texas, Megalopyge opercularis completes two generations annually. The Texas Poison Center sting data from the 2000–2016 study reveals the temporal pattern clearly:

• 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 is dramatically more abundant than the summer generation. This is the seasonal pattern that drives the consistent October–November surge in asp-related calls and ER visits across Texas.

Why fall is so much worse

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

• Cooler temperatures bring people outdoors
• School recess and outdoor activities pick up in fall
• Halloween decorations and yard work increase
• Children spend more time playing outside as the weather moderates
• Pets are outside more
• The caterpillars themselves are more numerous due to summer breeding success

Lifecycle stages

Adult flight: Late spring/early summer for the first generation; late summer/fall for the second generation. Adults are short-lived (days to weeks).

Egg laying: Adult females lay several hundred eggs on favored host trees.

Larval stage: The caterpillar feeds on host tree foliage. Multiple instars over weeks to months. This is the dangerous stage.

Pupation: Final instar caterpillar spins the distinctive operculate cocoon on host tree trunk or branch. Cocoons overwinter on tree trunks for the second-generation pupae.

Eclosion: Adult moth emerges from the cocoon by pushing open the operculum. The emerging moth is dull-orange or yellow and fuzzy.

Host plants

In our service area, the primary host plants are:

• **Live oak (Quercus virginiana)** — the dominant Hill Country shade tree
• **Post oak (Quercus stellata)**
• Other Texas oaks
• **Pecan (Carya illinoinensis)** — common landscape tree
• **Elm (Ulmus)** — particularly cedar elm
• **Hackberry (Celtis)**
• Yaupon and dwarf yaupon holly
• Roses in landscape beds
• English ivy and other ground covers

This list essentially covers the dominant tree canopy of the entire San Antonio to Boerne corridor. Live oak and pecan together account for the majority of mature shade trees in residential landscapes from Alamo Heights to Comfort. Every property with mature live oaks has 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 established protocols for asp stings. School nurses' offices in October and November field multiple asp-related visits weekly. Playground perimeter trees (especially live oak shade trees) are predictable exposure sources.

Hill Country residential landscapes: Stone Oak, Sonterra, Encino Park, Fair Oaks Ranch, Bulverde, Spring Branch — every neighborhood with mature live oaks has asp pressure each fall. Custom homes with extensive landscaping featuring oaks are particularly affected.

Boerne and Bergheim: Mature oak canopy throughout the historic district and surrounding residential areas. Asp populations are high every year.

Helotes and Government Canyon: Native oak/cedar habitat with high asp pressure on the wooded subdivisions.

Outdoor venues: Pool decks under live oak shade, outdoor restaurant seating, playgrounds, picnic shelters in city parks. Asps drop or are dislodged from foliage above and contact people below.

Pets: Dogs that walk under affected trees in fall sometimes contact asps that have fallen onto turf. Veterinary offices in our area see seasonal asp-related cases. Exam typically reveals paw or muzzle stings. Treatment parallels human first aid: spine removal with tape if possible, supportive care, antihistamines, monitoring for systemic symptoms.

The "caterpillar dropping from the live oak" scenario

The classic fall asp incident in our service area: A homeowner sits in an outdoor chair under a mature live oak in October. A caterpillar drops onto bare skin (forearm, neck, ankle). The person brushes it off reflexively, embedding spines deep into the skin. Pain develops within minutes. By the time they realize what happened, they have a dozen or more spines embedded and are in severe radiating pain.

This is the scenario behind a substantial fraction of fall ER visits for asp stings in San Antonio area hospitals.

Risk to humans and pets

Moderate to high. Individual sting can be severely painful and produces systemic symptoms in a meaningful fraction of victims. Children and people with allergies are at higher risk for severe reactions.

Asps are not aggressive — they cannot pursue a person, and they sting only on contact. The exposure pattern is therefore predictable: people who walk through known asp habitat, work on or under affected trees, or sit in outdoor furniture below host trees during peak season are at elevated risk.

The 3,484 stings reported to Texas Poison Centers between 2000 and 2016 averaged about 200 per year — and this is only the calls to Poison Control. Total annual asp envenomations in Texas likely run into the thousands.

Treatment approach

Asp control is genuinely challenging because:

• The caterpillars feed in the canopy of mature trees
• Treatment requires reaching elevated foliage
• Aerial application is impractical for residential properties
• The window of effective treatment is narrow (caterpillar stage only)
• Asp populations vary dramatically year-to-year
• Hard sprays risk killing beneficial pollinators

Our approach for properties with confirmed asp problems:

Identification and assessment:

• Visual inspection of host trees in known peak season
• Identify which trees on the property are actively infested
• Document caterpillar abundance for treatment decision
• Survey ground/turf areas for fallen caterpillars

Targeted treatment:

• For accessible foliage (lower branches, shrubs, vines), targeted application of labeled caterpillar-specific products such as Bacillus thuringiensis (Bt) — selective for caterpillars, low impact on pollinators
• For higher canopy, systemic insecticides applied via trunk injection or soil drench during the early larval period can be effective, though this is more invasive treatment
• Pyrethroid-based contact sprays for accessible foliage, with attention to avoiding bloom periods

Habitat and exposure reduction:

• Trim low branches that overhang outdoor seating, walkways, play areas
• Educate household members on identification and avoidance
• Clear ground litter under affected trees during peak season to reduce contact with fallen caterpillars
• Schedule outdoor furniture relocation away from worst-affected canopy during October–November

Education-first approach for low-density populations:

• Many properties have a few asps each year that don't justify chemical intervention
• Education on identification, school-age awareness, and reasonable avoidance is often sufficient
• We provide laminated identification cards for school-age kids in client households

What NOT to do:

• Do not crush or smash the caterpillars with bare hands (obvious but worth saying)
• Do not use a hose or pressure washer to dislodge asps from canopy (sends them flying onto people, pets, and surfaces below)
• Do not assume "I cleaned up the dead caterpillars" eliminates the hazard — 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)

• What is an asp caterpillar?
• I got stung by a fuzzy caterpillar — what do I do?
• Are puss caterpillars in Texas?
• How dangerous is an asp sting?
• When are asps active in Texas?
• What trees attract asp caterpillars?
• How do I get rid of asps in my yard?
• Can asps kill children or pets?

Sources consulted for this fact sheet include the Wikipedia account of Megalopyge opercularis, the Texas A&M AgriLife Extension Insects in the City fact sheet on asps and stinging caterpillars, the Merck Manual Professional Edition entry on puss moth caterpillar stings, the MSD Manual Consumer Version entry, Walker et al. 2023 PNAS paper "Horizontal gene transfer underlies the painful stings of asp caterpillars," the Forrester 2018 PubMed-published Texas Poison Center sting data study (3,484 stings 2000–2016), the Eagleman 2008 case report on envenomation by the asp caterpillar (Clinical Toxicology), Foot's foundational 1922 paper in the Journal of Experimental Medicine, and Bugs in the News first-aid coverage by entomologist Jerry Cates. Pain descriptions reflect quoted patient accounts in McMillan and Purcell 1964, Pinson and Morgan 1991, and Stipetic et al. 1999.