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The Terrifying Bug You Never Knew Existed

And How to Avoid It

…It was like holding my hand on an open flame…

… an hour later I was hyperventilating on the bedroom floor…

…I legit thought I might be having a heart attack…

…admitted to the hospital with severe pain, rash on torso, fever, and irregular heartbeat…

…the hospital hardly knew what to do with me…

…morphine did nothing…

…I felt dizzy for days…

These are a few of the comments on a recent Reddit thread left by people who’d been stung by a relatively unknown insect who may very well inhabit the trails you frequent.

We were surprised by the number of people who weren’t familiar with this bug, especially given the respect it demands.

So, we thought it was important to share some information about the puss caterpillar with our readers.

We’ll explain everything you need to know about the puss caterpillar – including its range, preferred habitat, and the way it can harm you.

We’ll also share some of the treatment options available for poor souls who cross paths with this innocuous-appearing insect. But unfortunately, some people find that no treatment really works.

The Puss Caterpillar: A Bug of Many Names


Like many other living creatures, the puss caterpillar goes by many names. The two most common alternatives include the asp caterpillar and the southern flannel moth caterpillar, but they’re also called fire caterpillars, woolly slugs, and tree asps.

These names all reference different characteristics of the animals:

  • Puss Caterpillar: This is thought to be a reference to the caterpillar’s hairy appearance, which looks somewhat like a pussycat. Some even describe them as looking like miniature Persian cats.
  • Asp Caterpillar: This moniker – which is more common in some geographic regions than others — is a reference to “asps,” which is the common name applied to a number of venomous snakes in Europe and the Nile Valley.
  • Southern Flannel Moth Caterpillar: This is a reference to the appearance of the adult moth, who’s clad in fluffy scales that somewhat resemble the napped surface of flannel fabrics.
  • Fire Caterpillar: This is a reference to the burning sensation caused by the caterpillar’s sting.
  • Woolly Slugs: Likely a reference to the fact that these insects are furry and slow-moving.

 Of course, to avoid confusion, biologists refer to it by its scientific name: Megalopyge opercularis.

The first part of the name (the insect’s genus) refers to the large abdomen of the adult moths (“megalo” is the Greek word for large, while “pygidium” means rump). Opercularis is a reference to the Latin word “operculum,” which means door. This was inspired by the cocoon of these insects, which features a conspicuous exit point for the pupating larva.

University of Florida

Note that the southern flannel moth has more than 40 close relatives, which are also classified in the genus Megalopyge. Most, if not all, of these are also capable of inflicting a venomous sting.

Few of these species have widely used common names; they’re often simply called asp caterpillars, puss caterpillars or flannel moth caterpillars.

The Puss Caterpillar Basics: Range, Natural History & More

We’ll detail the puss caterpillar’s sting and some of the treatments in a moment. But it’s important that we lay out some of the information regarding the lifestyle and habits of these insects first.

Geographic Range of the Puss Moth Caterpillar

The vast majority of puss cateripllars are native to South America, Central America, and Mexico.

However, a handful – including Megalopyge pixidifera, Megalopyge lapena, Megalopyge immaculata, Megalopyge crispate, Megalopyge lacyi, — are native to the United States.

Those living in the U.S. are primarily confined to a swath of land that stretches from Arizona, through the Gulf Coast States, and up the eastern seaboard as far as Virginia and New Jersey.  

Habitat of the Puss Moth Caterpillar

B. Team

The habitats of individual puss moth caterpillar species vary, but most are found in forests, agricultural areas and residential gardens.

The southern flannel moth caterpillar – perhaps the best-known species – is most often found on oaks, elms, hackberries, maples, and sycamores. They’re also known to feed on ornamental species, which is why they pose a particular risk to gardeners. Other flannel moth species consume leaves from plums, sycamores, and other species.

It’s worth noting that flannel moth caterpillars sometimes disperse into the area surrounding the tree(s) they been feeding on. They’re even found spinning cocoons on manmade structures at times.


Flannel moths start life as larvae (caterpillars), who immediately begin searching for food after hatching.

Over time, the growing larvae will periodically shed their exoskeletons. Each stage is called an instar, and flannel moth caterpillars pass through several (the exact number is unclear) during their development.

The early instars are pale yellow and clad in only a thin coat of hair-like setae. But later instars are clad in the long, earth-toned “fur” that gives them their characteristic appearance.

About six weeks days or so after hatching, the larva will begin constructing a cocoon.

After completing the cocoon and crawling inside, it’ll take about two weeks for the larva to pupate and another two weeks to emerge as an adult.

Once they emerge from their cocoons, southern flannel moths get down to the business of mating. 

Females typically mate on the first night of their adult lives and begin depositing eggs the following night.

About a week later, these eggs will hatch, and the process begins anew.

Adults often produce two broods per year, and some individuals living at the southern end of the range may produce three broods. The first occurs during the early summer, while the second takes place in the fall.  

Like some other moths and butterflies, puss caterpillars do not appear to feed during their adult (winged) life stage.

How Puss Caterpillars Sting

While the natural history of the puss caterpillar is interesting enough, it’s the painful sting that catches most people’s interest.

Puss caterpillars hatch bearing small hollow spines, with a venom gland located at the base of each.

The spines grow larger (as does the hair-like setae covering the larvae), as does the severity of their stings – the more advanced the larvae, the more serious the sting.

The stinging spines aren’t readily visible at advanced instars, as they are covered by the setae.  

The adult moths do not retain stinging spines.

Borrowed Bacteria

Researchers at the University of Queensland have discovered that puss caterpillar venom contains proteins “borrowed” from bacteria more than 400 million years ago in a process known as horizontal gene transfer.

Puss Caterpillar Sting First Aid & Treatment

Tampa Bay Times

Unfortunately, there doesn’t seem to be a magic bullet for treating puss caterpillar stings. Most authorities – including those of the medial and entomological bent – offer relatively boilerplate advice.

Most recommend trying to remove any spines still present with tape and then washing the area with soap and warm water. Some go on to recommend things like applying ice, calamine lotion and baking soda.  

But one thing is clear: If you suffer serious or systemic symptoms or the pain doesn’t resolve quickly, you should seek medical attention.

Sting victims report doctors trying a variety of things to alleviate the pain and treat systemic symptoms, but unfortunately, they seem to be a bit hit-or-miss. They work for some, but not for others.

As mentioned at the outset, some people don’t even enjoy pain relief from morphine.

So, the bottom line is that you want to avoid stings in the first place.

The Puss Caterpillar: Lasting Damage

The vast majority of people stung by these caterpillars seem to recover completely within a matter of hours, but some people seem to suffer the effects of a puss moth caterpillar sting for a long time. This is especially true of the characteristic sting pattern these bugs leave behind – this often remains for days or longer. Additionally, there are reports of people suffering the intense pain of the sting for days.

Avoiding Puss Caterpillar Stings: A Hiker’s Guide


OK, so there are these unassuming looking little fluffy caterpillars out there that can give you a sting you won’t soon forget.

So, uh, what do you do about it? How do you avoid puss caterpillar stings?

Well, there are no guarantees, but here are a few ways to reduce your chances of suffering a sting on the trail:

  • Limit your time under known host trees. You can’t exactly avoid walking under oaks and maples on the trail, but maybe don’t set up camp or enjoy a leisurely snack under oak groves when alternatives (such as pines, cedars, hemlocks or other evergreens) abound. Of course, this also means learning to identify common eastern trees.
  • Don’t put your hands on branches you can’t completely visualize. Anecdotes detailing people inadvertently grabbing a puss moss caterpillar abound – it’s certainly one of the most common ways by which people suffer stings. So, be careful when grabbing saplings, small trees and branches for support while you negotiate trail obstacles.  
  • Don’t shake trees when you walk under them. Many people are also stung when a puss moth caterpillar simply falls on them from a tree. This can certainly happen without you even touching the tree, but don’t tempt fate by bumping into trees or shaking them deliberately. This will only increase the chances of knocking a puss caterpillar (or any number of other stinging caterpillars) loose.
  • Inspect garments before wearing them. Just take a moment to look at shirts, coats, pants, or socks you’ve taken off on the trail. Make sure you don’t see any caterpillars clinging to the fabric before you rub it on your body. This is actually a good rule of thumb for avoiding a variety of different biting or stinging creatures.
  • Inspect blankets, tarps and ground covers before shaking them. It’s common practice to shake flat sheets and fabrics vigorously before packing them. And while this is a good idea in some cases, it may send a puss caterpillar flying in the general vicinity of your face, should one have crawled on your blanket.
  • Use extra caution anytime you encounter one puss moth caterpillar. Puss moth caterpillars are rarely abundant, but the larvae do sometimes feed communally. And that means you are sometimes more likely to see many whenever you see one.
  • Understand that you’re probably more likely to see them farther from farms and residential areas. Don’t misunderstand – people can and do find puss moth caterpillars in their backyards. But because residential and agricultural areas are more likely to be treated with pesticides and don’t offer any type of resource advantage over forests, you’re more likely to see these moths on isolated trails than trails at suburban parks.   


Look, puss moth caterpillars are definitely no joke – you don’t want to suffer a sting from one of these bad boys. But that doesn’t mean you need to panic or fear them irrationally. Chances are, you’ll never even see one on the trail, never mind suffer a sting from one.

So, just employ the tips we’ve shared, try to be careful, but don’t obsess over these little guys. After all, there are plenty of dangerous things on the trail – but getting out and enjoying the natural world is almost always worth it.

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