What You Need to Know About the Southern Flannel Moth—the Country's Most Poisonous Caterpillar

Watch out for this one!

Have you ever heard of the southern flannel moth (Megalopyge opercularis)? Maybe you've seen one, but didn't know what it was. It has a very distinctive look. So if you ever come across one, you definitely won't be able to miss it. But you should definitely avoid it.

The larval form of the southern flannel moth is unique looking as well, and it goes by many names: furry puss caterpillar, tree asp, and woolly slug. More important than its varied nomenclature is the fact that the larval, or caterpillar, form is also super venomous. In fact, it is one of the most dangerous caterpillars in the United States.

Southern Flannel Moth Caterpillar
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The Dangers of the Southern Flannel Moth and its Caterpillar

The inch-long caterpillars are covered in long, hair-like setae. These setae have the appearance of animal fur, which is why many say the caterpillars resemble Persian cats. Similar to cats, the colors of the caterpillars may vary from grayish white to a golden light brown with streaks of bright orange, to a dusty charcoal gray. During its early stages, the caterpillar's "fur" is sometimes extremely curly, making the larva look even fluffier.

Given their appearance and resemblance to soft, fluffy pets, it's easy to understand why people would want to touch the southern flannel moth caterpillar. Unfortunately, the "fur" is nothing like a cuddly, feline friend. It actually contains venomous spines. Coming into contact with these spines will cause extremely painful reactions in the skin. Caterpillars that are more advanced in the larval stage have the most potent defense.

As for the adult southern flannel moth, it's harmless on its own. The moth is covered in lengthy fuzz—not venomous spines—with colors ranging from a dull orange to a bright, lemon yellow, but its fuzzy feet are always black. Though the adult moths do not sting, if they are seen in an area, it is safe and best to assume that the more dangerous larvae are nearby or will soon be born.

The southern flannel moth caterpillar made the news because a five-year-old child in Texas was stung by one while playing outside at daycare. According to NBC Dallas-Fort Worth, the child was hospitalized after the caterpillar's spines stuck in her arm which led to pain and swelling. Her teachers removed the spines using tape, which was quick thinking, as it is best to treat a sting from the venomous spines within the first few hours. As reported by NBC Dallas-Fort Worth, according to the girl's mother, Lauren Chambers, "They said if that had not happened it could actually cause her whole body to go numb and start shutting down."

Getting Stung

The extreme pain-inducing spines are a form of defense meant to protect the caterpillars until they turn into moths and the venomous spines disappear. The slightest amount of contact, even merely brushing past them, will cause the spines to break off the caterpillar and inject venom into the skin. Sometimes, the spines become deeply embedded in the skin and need to be removed using tape, like the quick-thinking daycare teachers in Texas, or tweezers.

The pain from the southern flannel moth caterpillar sting has been described in several different, but all excruciating ways—a white-hot or knife-like sensation, a pain you can feel in your bones, or even a snake bite, hence why one of the alternative names for the caterpillar is "tree asp."

In the best-case scenario, reactions to the sting are localized to the affected area. However, what happens more often is that the pain radiates throughout the entire limb and eventually the entire body. It can cause burning, swelling, nausea, headache, abdominal pain, rashes, and bumps or marks on the skin. In more extreme cases, people experience difficulty breathing. Home remedies for mild cases include ice packs, antihistamine, baking soda paste, hydrocortisone cream, and calamine lotion. Emergency medical attention should be sought in cases of extreme reactions.

Where Southern Flannel Moths and Caterpillars Are Found

Because they are often found on trees and in plants and bushes, keep an eye out if you're near locations these caterpillars are known to frequent. They are commonly spotted in oak and elm trees. Caterpillars like to feed on the foliage as they grow into moths. Since these plants are frequently found in parks, gardens, and backyards, children are most likely to come in contact with the caterpillars. If you care for children, make sure you teach them about these caterpillars and warn them not to touch them.

Southern flannel moth caterpillars are widespread in the United States and can commonly be found on the East Coast in states stretching from New Jersey to Florida. They can also be found as far west as Texas and Arkansas, and also in the desert southwest. For more information, read this fact sheet from the University of Florida Department of Entomology and Nematology.

Have you ever seen one of these in person? If you ever do, make sure you stay very far away.

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