Woolly Bear Caterpillars and Weather Prediction

Do Woolly Worms Really Predict Winter Weather?

August 28, 2019

Based on the measurements of the distinctive woolly bear caterpillar, you can figure out your weather forecast!

University of Missouri

The woolly bear caterpillar—also called woolly worm or fuzzy worm—has the reputation of being able to forecast the coming winter weather. Whether this is fact or folklore, learn more about this legendary caterpillar and how to “read” the worm!

Here’s the legend: The Woolly Bear caterpillar has 13 distinct segments of either rusty brown or black. The wider the rusty brown sections (or the more brown segments there are), the milder the coming winter will be. The more black there is, the more severe the winter. 

How the Woolly Bear Caterpillar Became “Famous”

In the fall of 1948, Dr. C. H. Curran, curator of insects at the American Museum of Natural History in New York City, took his wife 40 miles north of the city to Bear Mountain State Park to look at woolly bear caterpillars.

Dr. Curran collected as many caterpillars as he could in a day, determined the average number of reddish-brown segments, and forecast the coming winter weather through a reporter friend at The New York Herald Tribune.

Dr. Curran’s experiment, which he continued over the next eight years, attempted to prove scientifically a weather rule of thumb that was as old as the hills around Bear Mountain. The resulting publicity made the woolly worm one of the most recognizable caterpillars in North America (alongside the monarch caterpillar and tomato hornworm).

Woolly Bear Caterpillar. Photo by SillyPuttyEnemies/Wikimedia Commons.
Woolly Bear Caterpillar. Photo by SillyPuttyEnemies/Wikimedia Commons.

What is a Woolly Bear Caterpillar?

The caterpillar Curran studied, the banded woolly bear, is the larval form of Pyrrharctia isabella, the Isabella tiger moth.

  • This medium-size moth, with yellowish-orange and cream-colored wings spotted with black, is common from northern Mexico throughout the United States and across the southern third of Canada.
  • As moths go, the Isabella isn’t much to look at compared with other species, but its immature larva, called the black-ended bear or the woolly bear (and, particularly in the South, woolly worm) is one of the few caterpillars most people can identify.
  • Woolly bears do not actually feel much like wool—they are covered with short, stiff bristles of hair.
  • In field guides, they’re found among the “bristled” species, which include the all-yellow salt marsh caterpillar and several species in the tiger moth family. Not all woolly caterpillars are true ‘woolly bears’ though!
    • If you find an all-black woolly caterpillar, don’t worry—this doesn’t mean that we’re in for a severe, endless winter! It’s just a caterpillar of a different species, and is not used for forecasting. The same is true for all-white woolly caterpillars. 
  • Woolly bears, like other caterpillars, hatch during warm weather from eggs laid by a female moth.
  • Mature woolly bears search for overwintering sites under bark or inside cavities of rocks or logs. (That’s why you see so many of them crossing roads and sidewalks in the fall.)
  • When spring arrives, woolly bears spin fuzzy cocoons and transform inside them into full-grown moths.
  • Typically, the bands at the ends of the caterpillar are black, and the one in the middle is brown or orange, giving the woolly bear its distinctive striped appearance.

Isabella Tiger Moth. Photo by Andy Reago & Chrissy McClarren/Wikimedia Commons.
Isabella Tiger Moth. Photo by Andy Reago & Chrissy McClarren/Wikimedia Commons.

Do Woolly Bear Caterpillars Forecast Winter Weather?

Between 1948 and 1956, Dr. Curran’s average brown-segment counts ranged from 5.3 to 5.6 out of the 13-segment total, meaning that the brown band took up more than a good third of the woolly bear’s body. The corresponding winters were milder than average, and Dr. Curran concluded that the folklore has some merit and might be true.

But Curran was under no scientific illusion: He knew that his data samples were small. Although the experiments legitimized folklore to some, they were simply an excuse for having fun. Curran, his wife, and their group of friends escaped the city to see the foliage each fall, calling themselves The Original Society of the Friends of the Woolly Bear.

Thirty years after the last meeting of Curran’s society, the woolly bear brown-segment counts and winter forecasts were resurrected by the nature museum at Bear Mountain State Park. The annual counts have continued, more or less tongue in cheek, since then.

For the past 10 years, Banner Elk, North Carolina, has held an annual “Woolly Worm Festival” each October, highlighted by a caterpillar race. Retired mayor Charles Von Canon inspects the champion woolly bear and announces his winter forecast. 

If the rusty band is wide, then it will be a mild winter. The more black there is, the more severe the winter. 

Woolly bear caterpillar in defensive posture.
Woolly bear caterpillar in its defensive posture.

Most scientists discount the folklore of woolly bear predictions as just that, folklore. Says Ferguson from his office in Washington, “I’ve never taken the notion very seriously. You’d have to look at an awful lot of caterpillars in one place over a great many years in order to say there’s something to it.”

Mike Peters, an entomologist at the University of Massachusetts, doesn’t disagree, but he says there could, in fact, be a link between winter severity and the brown band of a woolly bear caterpillar. “There’s evidence,” he says, “that the number of brown hairs has to do with the age of the caterpillar—in other words, how late it got going in the spring. The [band] does say something about a heavy winter or an early spring. The only thing is … it’s telling you about the previous year.”

Every year, the wooly worms do indeed look different—and it depends on their region. So, if you come across a local woolly worm, observe the colors of the bands and what they foretell about your winter weather.

What’s the real prediction for this winter? Read our official winter forecast here: 


The 1998 Old Farmer's Almanac

Reader Comments

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Tonight I went out into the garage & spotted something strange. I had my husband fetch the spotlight because I thought it might be a mouse. It turned out to be the largest & blackest woolybear I have ever seen! No orange at all. So I opened the door & gave it a little nudge outside with a broom.

To protect the wooley bear,

To protect the wooley bear, and others, I have stopped mowing the grass for the year. I don't want to run one over, I would feel really bad. Plus I think leaving the leaf litter will be more beneficial for bird watching.

Wooley Bear Worms

I'm not crazy or making this up. Yesterday I say a YELLOW one that had about 6 black spike like hairs sticking out of it. I've seen the ones described here, but NEVER a YELLOW one. The black spikes were two on each end and two in the middle and sticking out between the top and sides and were a little longer than than the rest of it's hair or fuzz or what ever you want to call it. I started to touch it but thought the black ones be a defense and poke or stick me with some kind of something than might hurt or make me really sick. So I just let it alone to do whatever IT wanted to do.

yellow caterpillar with black spikes

It sounds like you saw the larva of the American dagger moth (Acronicta americana); the larva eats leaves of certain trees, such as oaks and maples. Younger larvae are yellow fuzzy caterpillars with a few taller black tufts. As the larvae age, they become a paler yellow or white. You were right not to touch the caterpillar–the black tufts can cause irritation or a rash in some people with sensitive skin.

wooly worm

I just saw an all black wooly bear worm on my back patio. No orange stripes!. Does that mean a very severe winter is coming? plus it was very fat.

all black caterpillar

There are several fuzzy caterpillars that are essentially all black, such as the fully grown larval stage of the giant leopard moth. However, if the caterpillar you saw was indeed a banded woolly bear caterpillar, then yes, folklore warns that a severe winter is coming!

wooly bears

i saw an all black one last month, trying to come into my basement apt. scooped him up and put him by the garden. all black, so does that mean a horrible winter or what?

Woolly Catepillar

I saw my 2nd one and they both were pretty much black. You could barely see the orange stripes.

Wooley Bear Caterpillar-WA State

Just saw the first Wooley, all black!

It's when you first see them that counts

My experience has been that you can forecast a cold or mild winter based on when you see the woolly for the first time. This year I saw my first one on July 31st in central NJ. I've never seen one this early. So the forecast is for a cold winter. The earlier you see them the colder the winter. When you see the first one after the first day of fall(give or take a couple of days), that makes for a milder winter.

Woolly bear in PA

We were out splitting wood and came across not one but two all black woolly bears. We live in south central PA almost near the MD line. This is the first time in my 50+ years ever seeing a pure black one and I've lived in a number of different states.

Wooly Bear

I found two Wooly Bears while kayaking on the Clackamas River yesterday. We were near Estacada and the caterpillars' brown segment was 1/3.

wooly caterpiller

i am in the north east and all my caterpillar are all white what does this mean

White wooly bear

I don't think it's the wooly bear we're talking about here. I saw a white one up at my Dad's house in Pocono Summit, two years ago in late summer. I let it walk onto my hand and looked at it's face. What a face! It had sporadic black bristles in what looked like a weird angry expression. Reminded me of the Seinfeld episode where Uncle Leo loses his eyebrows and Elaine magic markers new ones on him, except everyone that looks at him thinks he's angry. Mother Nature sure has a sense of humor sometimes.

Wooley Bear Catapillar

I found a Wooley Bear Catapillar today in Patterson WA. Looks just like the above photo. Black on both ends and rusty color in the middle. Maybe half and half.

Woolly bear catapillars

I saw two woolly bear Catapillar's in central New Jersey today and both were totally rusty color

Yellow Wooly Bear

Pale Yellow Wooly Bear (no black) spotted today in 90 degree weather in Columbus, Ohio. Haven't seen many Wooly Bear's at all compared to 50 years ago. This one seemed slightly larger than usual.

yellow woolly bear

Had one on my deck railing today in South Jersey.

All orange woolly worm

Saw two all orange/rust woolly worms today here in Southern Maine. No black stripes or tips.

Spotted in MN

The dog and I found a wooly bear this morning in the Twin Cities! The middle band took up about 1/3 of it's body, so I'm hoping he's right and the meteorologists' prediction of a tough La Nina winter are incorrect!

No wooly bears at all!

I haven't seen any wooly bears at all this year. I've seen 3 of the all-black wooly's... all were dead.
Where have all the wooly's gone?
I live in Western Massachusetts and am outside much of the time. Surely I would've seen at least ONE by now!

Wooly Bear Caterpillar

I saw one today 9-19-2017 in DuBois, PA crawling across my driveway that was totally black no brown at all. Sounds like a bad winter. We will see as winter comes and goes.

White Wooly Bear

Yesterday my cat found a new friend in the house crawling through the living room, an all WHITE Wooly bear! Saved him from the cats claws! Hope it isn't true what they say...

White Wooly Bear

What DO they say about the white wooly worms?
We are in Southwest Ohio (Cincinnati area).

white woolly worms

i live here in IL. central part. Effingham. We are called Crossroads of I-70 and I-57.
We also have seen the while Woolly Worms. Did anybody ever answer you about
what it meant? Usually they say, the darker, colder and more precept but, never said
what white was.

Oregon sighting

Found one crawling on my deck tonight in Albany, OR. Less than one third was brown.

wooly bears

Yesterday l rescued (from my cat) what looked like a wooly bear but it was solid black. Could be a different species? Or should I head south for the winter?

Solid Black Woolly Bear

The Editors's picture

Yes, you may occasionally see the caterpillar of related species, like the Giant Leopard Moth, which are solid black in appearance! Fortunately, they don’t signal a harsh winter ahead. (Though we would still recommend heading south.)

Wooly Bear

I saw an insect that looked like a wooly bear but it's hairs were long. It kind of looked like a flannel moth caterpillar but same color pattern. As wooly bear.

Wooly Bear

I mean a caterpillar, not insect