Arizona’s Mourning Cloak Caterpillars: Fuzzy, Cute…and Painful!

The desert bites. It’s also pokes, sticks, and stings. You’ve probably noticed that everything in the Sonoran Desert fights to defend itself with a pugilistic fierceness. Then they ask questions later.

You expect arachnids and serpents to defend themselves when they feel threatened, and they do when you venture too close to their territory. But there are many desert species beyond spiders and snakes that passively defend their territories. Jumping cholla springs to mind, literally attaching to your clothing and shoes, and unapologetically to your skin, when you get too close.

RELATED: Why You Should Be Careful Around Teddy Bear Cholla

I recently encountered another creature with a passive, yet potent, attack system, and I’m still reeling from the pain. Meet the Mourning Cloak Caterpillar. It’s late April in Cochise County, and right now these fuzzy, innocent-looking little wigglers are everywhere, and their velcro-stickiness makes them hard to avoid.

Warning: Do Not Pet the Fuzzy Caterpillar

It was after dark when I got home Saturday night. As I walked onto my porch, my shirt brushed up against one of the support posts. I went into my house to put my stuff down, then went back out to feed my horses. As I walked to the back pasture, my arm brushed against something attached to my shirt.

A fiery pain began to bloom in my forearm as I looked down to find a Mourning Cloak caterpillar clinging to my clothing. I knocked the insect off with a stick, but the pain in my arm kept growing.

I went back into the house and washed my arm, as the fire continued to grow across my skin. Welts were forming in a railroad-track pattern—a ghostly imprint of the caterpillar's outline.

I applied some topical lidocaine to the still-burning welt. That, and some triple antibiotic ointment, calmed the heat down to a dull throb. What was happening? Why did this sting so much?

READ MORE: This Creature is on the Move Now in Arizona: Know What to Do?

Lifestyles of the Mourning Cloak Caterpillar

The Mourning Cloak caterpillar (Nymphalis antiopa) is also called the spiny elm caterpillar. It’s easy to identify. Look closely at its black body, and you’ll see it’s covered in sharp spines and rows of red spots.

They’re found in Arizona, especially near our riparian corridors. These ‘teddy bear cute’ little monsters are deceptively painful.  They feed in groups before transforming into one of North America’s longest‑lived butterflies. I live near the San Pedro riparian area, and these caterpillars are finding a happy home all over my property right now.

Urticating Hairs and a Big Nope from Prey

The caterpillars hatch and feed together in tight clusters, chewing through young leaves before wandering off to pupate. Their group behavior and spiny appearance help deter predators. Most birds avoid them, and who blames them? They’re avoiding their stinging, urticating-like hairs.

These little fuzzy monsters have spiny black setae, similar to the urticating or stinging hairs on a tarantula.  While the caterpillar hairs can irritate skin on contact, they aren’t true urticating hairs. Their spines act more like tiny nettles. As I found out the hard way, they’re very unpleasant if you press into them with your skin.

Tarantulas, especially the species in Arizona, take things much further. They possess barbed, detachable urticating hairs on their abdomen that they can actively flick into the air, where they lodge in the skin, eyes, or airways. Compared to a tarantula’s airborne, barbed defense system, a Mourning Cloak caterpillar’s irritation is strictly contact‑based. Much more akin to the insect version of a cholla, in my opinion.

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Transformation from Fuzzy Demon to Beautiful Butterfly

Thankfully, the caterpillar explosion doesn’t last forever. According to What’s That Bug?, Mourning Cloak caterpillars usually only hang out as caterpillars for about 2–4 weeks before pupating. It all depends on temperature and food availability.

Once they form a chrysalis, they transform into the well‑known Mourning Cloak butterfly, a magical butterfly with one of the longest lifespans, up to 10–12 months—a long lifespan for butterflies.

Whatever you do, keep your eyes peeled during the stinging caterpillar phase, and keep pets and kids away. Take it from me, getting stung is the opposite of fun!