Stinging Caterpillars Pack an Unexpected Venomous Punch

Buck moth stinging caterpillar

You’ve protected against pests that can undermine your lawn, like rats and voles. You’ve kept the termites out and the ants in check. Even the wasps have been evicted.

But there’s one insect pest many people overlook. One that can pose a just as painful, even mortal threat: stinging caterpillars.

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The slow-moving, leaf-munching worms seem harmless enough beyond cutting holes in the leaves of your shrubs, but some of them pack a real punch.

It may just turn red. It may blister and sting, or it can leave you on the way to the emergency room.

The severity is comparable to a bee sting: It depends.

“What might be mild for one person might be life-threatening for another,” says entomologist Ric Bessin with the University of Kentucky Extension.

In studying these peculiar caterpillars, Bessin has been stung twice. All it takes is brushing up against them.

Ric Bessin
Dr. Ric Bessin, Entomology Extension Professor at the University of Kentucky, checking some pepper plants at the UK Horticulture Research South Farm in Lexington, Ky.

When stung, Bessin suffered mild irritation that built over time but wasn’t enough to really interrupt his day. But for the kids or pets playing in your yard it may be a different story.

So just in case, here’s how to spot dangerous caterpillars and keep your yard safe from them, too.

‘Venom-Filled Quills’

Do they really sting? Well, kind of. While they don’t pack stingers like bees and wasps, you don’t want to reach out and pick them up.

Across the country, certain caterpillars are covered in hairs that act like miniature syringes, called urticating hairs.

An article Bessin wrote for the University of Kentucky Extension explains that these quill-like hairs are connected to poison glands they use as defensive weapons. When the hairs break through the skin, the poison is released, and the unlucky person or animal on  the receiving end has been “stung.”

Bessin describes them as “hollow, venom-filled quills.”

Reactions to that sting vary, the article says. A caterpillar sting can leave you with a mild itch to severe pain and swelling, blistering, dermatitis and even intestinal problems.  Some of these urticating hairs Bessin has examined under a microscope have specific weak points to help them break off in the skin, slowly oozing out the poison.

Luckily, the caterpillars themselves are not aggressive, he says. They’re not out looking to come after you and the danger is in accidentally grabbing one or brushing up against it while pruning your redbuds.

Children are at particular risk though, says the Louisiana State University Extension. That’s because youngsters are attracted to the bright, unusual coloration which warns of a painful sting.

Types of Stinging Caterpillars

These spiny, venomous caterpillars become adult moths, many with wonderful colorations. They come in bright green and yellows, even reds, browns and blacks. They come covered in fur or in strange shapes. No matter their appearance, they have one thing in common: You don’t want to pick them up.

Stinging caterpillars thrive in a wide range of environments and vary widely in appearance across the country.

The University of Florida Institute of Food and Agricultural Sciences lists six species living in the Southeast alone. Around Kentucky and the Southeast there are probably a dozen total species, Bessin says.

His advice is don’t touch hairy caterpillars you can’t identify, especially if they’re brightly colored.

“Just don’t pick up one you don’t recognize,” he says.

Instead, if you find one on the willow tree next to the swingset, or in the front driveway, use a stick or other object to move it to a safer location.

But if you want to know exactly what you’re dealing with, here are some common stinging caterpillars you’re most likely to see in the back yard.

Saddleback Caterpillar (Acharia stimulea)

Saddleback caterpillar
The saddleback caterpillar sports a green saddle that makes it easy to spot. Credit: “Saddleback caterpillar,” Judy GallagherCC 2.0

The saddleback is the most well-known and easy to identify stinging caterpillar.

It’s easy to see why they got the name.  Small, brown and hairy, a green shape across the top of its back looks like a saddle blanket while a small brown circle directly in the middle of its back looks like a saddle.

Bessin says in Kentucky they’re called pack saddle caterpillars. It’s one people recognize more often.

According to the Florida institute, saddleback caterpillars range throughout the eastern half of the United States, from South Florida to Maine and from Virginia to west to Nebraska.

Fully grown, the caterpillar’s body is stocky, and a bit less than an inch long. Where not covered with the green “saddle blanket,” the caterpillar is brown.

Most of the stinging hairs are on horn-like formations on either end, with rows of more stinging hairs along either side. Some favorite foods include maples, hibiscus and crape myrtle.

If stung, burning, inflammation and red blanching may occur around the affected area, which can also become a rash or welts that swell and itch.

Puss Caterpillar (Megalopyge opercularis)

Puss caterpillar
Puss caterpillars

You could be forgiven for not recognizing a puss caterpillar as a caterpillar at all.

It looks more like a furry mouse than the typical green, worm-like picture most people bring to mind when they think caterpillar.

Completely covered with soft, brown, fur-like hairs, the puss caterpillar is stout-bodied, growing to about 1 inch long.

The hairs hide the caterpillar’s body, only identifying the rear end by tapering to a tail-like narrowness at the end.

They feed on lots of different plants, but favor oaks and elms.

Their spines are hollow with a toxin gland at the base. The larger the caterpillar is, the more toxic its sting.

Io Moth Caterpillar (Automeris io)

Now this one fits the classical picture of a caterpillar to the T, even if it’s named for the moth it becomes.

Io moth caterpillars are pale green with yellow-white and red stripes running down its sides. Its stinging organs are clustered on fleshy protrusions extending from the caterpillar’s back, usually yellow or green with black tips.

Much like the Saddleback, io moth caterpillars feed on a wide variety of plants, but prefer hibiscus, elms, maples, wisteria and willows.

If stung, a burning sensation begins after contact and may turn red and itchy.

Hag Moth Caterpillar (Phobetron pithecium)

Like the puss caterpillar, the hag caterpillar or hag moth caterpillar hardly looks like a caterpillar at all, but more like a small, browned leaf.

Also called the monkey slug, they’re light to dark brown. They have lateral spines that come in as many as nine pairs. Those spines, looking like disheveled hair, give the caterpillar its name, sharing it with its later form, the hag moth.

It feeds on woody plants including oak, dogwood and apple, the article says. If stung, symptoms can range from burning and stinging to itching, redness and inflammation.

Buck Moth Caterpillar (Hemileuca maia)

Buck moth caterpillars can be tricky to identify, with both a light form and a dark form.

The latter is more common, the Florida institute says, appearing black with tiny white dots on the body. The light variety shows white with a reddish head, but both types have dark, lateral rows of stinging spines on their backs.

You’ll find them munching on your oak and willow leaves, and can reach 2.5 inches in length.

And this is one caterpillar you definitely don’t want to be stung by. The spines break off into the skin, release the toxin and cause sudden stinging, redness and swelling that can last from one day to more than a week.

Flannel Moth Caterpillar (Norape ovina)

White flannel moth caterpillar
White flannel moth caterpillar

Now the Southern flannel moth caterpillar, or white flannel moth caterpillar, looks like what you’d expect a stinging caterpillar to look.

Growing as long as 1.25 inches, it has a yellow body marked with a wide black stripe down the back, bordered with red at each end.

This is a caterpillar whose poisonous warnings are certainly on display: along that black stripe are yellow bunches that contain non-stinging long hairs and shorter stinging hairs, says the Kentucky Extension.

Favorite foods for the flannel moth caterpillar are redbud, honey locust, hackberry, mimosa and beech.

Spiny Oak-Slug Caterpillar (Euclea delphinii)

The spiny oak slug is another species that uses its bold coloration to advertise its toxicity.

They come in a range of colors, but can be identified by their oval, stout bodies and abundance of spines protruding from them, UFIFAS says.

Rows of spine clusters also run down the caterpillars sides, with two to four large clusters of spines protruding from the rear.

Feeding favorites include oak, willow and cherry species.

And while they might be loud, but they’re small. Spiny oak-slug caterpillars grow to just under 1 inch long.

Luckily that small package doesn’t hide a powerful sting. Their spines are hollow with a toxin gland at the base and its sting is considered to be milder than others, with symptoms like a burning sensation, redness and inflammation.

First Aid for Caterpillar Stings

Unfortunately, there’s no easy effective home remedy to calm a caterpillar sting, says Bessin’s article for the Kentucky Extension.

But that doesn’t mean there’s nothing you can do.  Bessin says to stick a piece of tape where you’ve been stung and peel it off. Hopefully, this will pull out any quills still stuck in your skin.

Wash the area thoroughly with soap and water, which may help remove some of the venom. Quickly applying an ice pack or baking soda may help reduce pain and swelling.

Antihistamines, used mostly for bee and wasp stings, are apparently ineffective with caterpillar stings.

Very young, old or unhealthy people are more likely to suffer more severe reaction symptoms, and if those symptoms persist or if an allergic reaction occurs, seek medical attention.

Caterpillar Control

That’s a lot of little potential threats crawling among your oaks and redbuds. You may want to take action.

Fortunately, stinging caterpillars don’t pose much of a threat to your lawn or ornamental shrubbery, unless you get a large population on a single plant that can eat its way through too much foliage.

Every few years or so the populations wax and wane, Bessin says. So some years you may not see them at all, and several years later they may be all over the place.

Heavy infestation can become a possibility when populations are high.

Should you find yourself overrun with the hairy creatures, control is pretty straightforward. “If you’re seeing a lot, you may consider using a nice, friendly garden spray to get rid of them,” Bessin says.

The Kentucky Extension article says to use sprays with bacillus thuringiensis or carbaryl, spraying the entire infested shrub or bush.

There are even organic options for the earth-conscious homeowner, he says.

Sprays may be needed for buck moths in particular, says LSU, since they tend to emerge in large numbers.

But for other species, the LSU article says, control is “seldom needed except for knocking an occasional specimen to the ground and mashing it.”

Photo credit: Except where noted, photos courtesy of Dr. Ric Bessin.

Derek Lacey

Derek Lacey

Formerly the agriculture writer for the Hendersonville Times-News, Derek Lacey’s articles have appeared in U.S. News & World Report, The Charlotte Observer, News & Observer, and The State. He has won 15 awards from the North Carolina Press Association and GateHouse Media, for pieces ranging from news features and investigative reporting to photography and multimedia projects.