Woolly Bear Caterpillars and Weather Prediction

Using Woolly Worms for a Winter Forecast


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 and 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 the most recognizable caterpillar in North America.

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

What is the 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, throughout 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 are ‘woolly bears!’
  • 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.

Then, again, you can always check the winter weather predictions in The Old Farmer’s Almanac!

Find out which other animals can predict the weather and learn how to tell the temperature from cricket chirps!


The 1999 Old Farmer's Almanac

Reader Comments

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Found an all black one. Paris

Found an all black one. Paris TN. Gonna be COLD!


Found an all black woolybear this afternoon in Sugarcreek, Ohio. First time I ever saw one that was all black!

all black i just saw it today

southpoint ohio

Wooly bear catipillar

Saw an all black one in knoxville tn. Well we need a good winter it's been a long time since we had a good snow. It would be nice before i check out of this world. old man

Wooly Bear

I live in central Maryland and just came across a solid black Wooly Bear in the back yard.

Gray wolly worms

Just seen a all gray wolly worm what does that mean

wooly bear

I'm in north central Ohio and the wooly bear I seen beginning of this week was mostly brown very little black on either end.


sorry, i forgot. Central New York state. Finger Lakes area.

Woolly worm

I just found one on my basement wall, about 2-1/2 inches long & mostly all black, i had to look hard to see any brown in the center. I'm tuning up my snowblower.

Wooly Bears

My daughter showed me a picture she took with her phone of an all black one she saw on our driveway this morning in central Massachusetts. :)

I'm in southwest michigan and

I'm in southwest michigan and am currently looking at an all black one

Saw one today, just a little

Saw one today, just a little rust in the middle and a lot of black on both ends! South Chicago suburbs!

willy worm

In Ohio saw willy worm l0-17-17 and it was all brown with a small black ring on both ends. Looks like your picture but almost all brown So hope this means a mild winter. Have also saw white willy worm too. and all brown one with a little black ring. All were in the same place on black top on our walk in park.

wooly bear

In Northeast PA I just saw one, and, the brown segment was larger than the black.. so perhaps our winter won't be as snowy this year! We had a pretty snowy one last year!

Woolly Bear

I'm in SW Ohio. I've seen 3 woolly bears in the last 2 days, all of them with wide brown bands. Hopefully the stories are true and we'll have a mild winter.

All-Rust Woolly Bear in NH

I saw my first Woolly Bear of the season today, and it was completely rust-colored. I live in south-western New Hampshire. It really has been a warm fall so far. Remains to be seen what the winter will be like!

Woolly Bear

We're on Pewaukee Lake Wisconsin, we see a lot of those cute little creatures, just took 2 out of the cottage and their brown bands were pretty narrow. Looking forward to a snowy winter!

Woolley worm

Saw one here in Northwest WA State near Canadian border. All dark rust color. No black. What up with that???

All Black Wooly Worm

Wednesday night (Oct. 4th, 2017) while out walking my dog, I spotted an all-black wooly worm. It was attempting to cross my neighborhood road. This, along with the fact that the Hickory trees in this area are fully loaded with nuts now supports my belief that winter in the Charlotte, North Carolina area may be severe.

All black wooly caterpillar

Driving out of Girl Scout camp on dirt road between brown corn stalks. Granddaughter saw a huge caterpillar almost across the road. She thought I ran it over. We backed up and got it. It is huge. Measured it, 2 inches long. It absoluteely lives lettuce.

Wooly caterpillar

Sorry forgot to mention we live in Nebraska.

Wooly bear

Found a wool
y bear on my husband's tire this afternoon. Didn't n
know about counting the different segments but the first black
segment was .50 inch, brown was atleast an inch if not a little more
and the last black segment was .25 inch

Woolly worm

This morning, as I left for work, I saw a solid black woolly worm. The first ever witnessed in my 50+ years.

All black wooly bear

Coming from the midwest I grew up with hearing the blacker the wooly the more severe the winter. However I have never seen an all black until today. Watch out Virginia I think we're in for a good one this year.


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!