Woolly Bear Caterpillars and Weather Prediction

Do Woolly Worms Really Predict Winter Weather?

September 15, 2021

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

University of Missouri

Woolly bear caterpillars—also called woolly worms—have a reputation for being able to forecast the coming winter weather. If their rusty band is wide, then it will be a mild winter. The more black there is, the more severe the winter. Just how true is this weather lore? Learn more about this legendary caterpillar and how to “read” the worm!

The Woolly Worm Legend

First of all, the “woolly worm” is not a worm at all! It’s a caterpillar; specifically, the larva of the Isabella tiger moth (Pyrrharctia isabella). Nonetheless, the name “worm” has stuck, at least in some parts of the United States. In others, such as New England and the Midwest, people tend to call them “woolly bears.” (Worm or not, at least we can all agree that they’re not bears!)

In terms of appearance, the caterpillar has 13 distinct segments of either rusty brown or black. Often, it is black on both ends with rust-colored segments in the middle, although it may sometimes be mostly black or mostly rust. (Note: All-black, all-white, or yellow woolly caterpillars are not woolly bears! They are simply different species and are not part of the woolly worm lore. So, if you spot an entirely black caterpillar, it isn’t forecasting an apocalyptic winter!)

According to legend:

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. 

Watch this short video

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

  • The Isabella is a beautiful winged creative with yellowish-orange and cream-colored wings spotted with black. It’s common from northern Mexico throughout the United States and across the southern third of Canada.
  • The tiger moth’s 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 Really 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 over forty years, Banner Elk, North Carolina, has held an annual Woolly Worm Festival in October, highlighted by a caterpillar race. Retired mayor Charles Von Canon inspects the champion woolly bear and announces his winter forecast. Similarly, there is a Woollybear Festival that takes place in Vermilion, Ohio, each October.

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.”

How to “Read” the Woolly Worm

Weather is local so you need to read your own woolly worm.

Look for these fuzzy wuzzies in the fall. According to woolly worm watchers, there are two generations of worms each year. The first appear in June and July, and the second in September. The second generation worms are the “weather prophets.”

To find a woolly bear, start looking under leaves and logs! Some are just crossing the road. Once you spot a woolly worm inching its way along the ground or a road, you’ll see them everywhere! The caterpillars are most active during the day (not at night). After filling up on food—including violets, lambs quarter, and clover—their goal is to find a place to hide for the winter. Interestingly, the woolly worm overwinters as larva. Their entire body will enter a “frozen” state until May when it will emerge as the Isabella moth.

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. Remember:

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

That’s it! Note that white, yellow, or other colors of fuzzy caterpillars are NOT the same type of woolly worm and are not used for weather forecasting. We’ll leave the weather-prognosticating “skills” to your own observation!

Speaking of Weather Predictions …

Did You Know: The 2021 Old Farmer’s Almanac is now available! The brand-new issue includes our famous 2021 Winter Weather Forecast.

Get your copy of the 2021 Almanac where books and magazines are sold or you can now order straight from Amazon to your home

Woolly Worm Video

In tribute to our fellow prognosticator, we made a woolly worm video … 

Whether the predictive powers of the woolly worm are fact or folklore, we always enjoy the fun!  Feel free to share your experience with the woolly worm in the comments below.


The 1998 Old Farmer's Almanac


Reader Comments

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Woolly Bear oddity in Upstate NY.

We recently saw several woolly bear caterpillars with black only on their head and none on the tail end. A few had just a black dot in the middle of the tail end and another we saw had a black stripe. Strange year.


I live in Vermilion Ohio and yesterday was the 49th annual Woolly bear festival mentioned in this article. I saw my first woolly bear a couple minutes ago. It had very little black on the head and tail and a lot of orange. Come on mild winter! BTW. We have a woolly bear song not sure if it is just Vermilion or not. "Old man winter is coming soon and we don't know what he gonna do. Will he cover us us up with white? Or is he gonna turn us blue? No one knows, but Woolly, me and youuuuuuu." LOL! Good luck this winter from Ohio.

Acorn Predictions

Our house was surrounded by huge oak trees and every year the acorns were a chore to bag and put to the curb. We noticed that on years where there were fewer acorns, the winter was milder. One year the 85 ft driveway was filled with them about 8 inches to a foot deep. It was a hella winter.

Wooley Worms in Northern Washington

I was out for a walk and had found a few of those fuzzy little ones. They had a longer black one the head, normal orange mid section and shorter black on the bottom end. Cool looking. (I try to rescue them from the middle of the road.)

Wooly Worm in eastern Utah

Wooly worm spotted in Vernal, Utah. Half was black and half (middle section) was brown. 50/50 split on our snowy vs. mild weeks of winter.

Wholly Bear

Thanks for all the information! I didn’t know if this was a bad pest in my garden or not I just saw my first one crawling across my driveway yesterday very cute just ambling along his way oblivious to anyone or anything just so intent on his destination. Very cool to observe.

Black Wooly Bear

Yesterday I found this gorgeous creature - at first I thought it was a misplaced craft item, it was jet black and fluffy like a pompon. I’m in northwest Georgia.

Wooly Bear 2021

So far I’ve only seen 1 wooly bear this year…it had a black head and an all brown body. I’ve never seen that in this area but the last few winters have been relatively benign for this area and the wooly bears in the corresponding falls have had wider brown bands than black so, if the legend is true, then I would expect a strong start to winter followed by relative warmth.

We are in Northern Vermont though…you really never know here.

Just crawling along ...

Came across a wooly bear yesterday when I was mowing the lawn here in Halifax, Nova Scotia. I let him/her crawl onto my glove so I could place him under the lilac bush, out of harm's way. Colours were about half and half brown and black. Such a beauty!

wooly bear sighting in my garage!

Hello, all. this is the second day of autumn in northern Georgia. I pulled into our garage three days ago and discovered a wooly bear following me into the garage! Well, I had no idea what it was, so I observed its very active motion before I collected it and placed it in the woods nearest the garage. Now that I read this article, I know it was a wooly bear worm! Hot dog! Oh, by the by, it was about three inches in length, and ALL ORANGE - no black at all! Let's see how winter turns out here in north Georgia. Happy fall, y'all!

Woolly Bears Article

I love this article! Love the humor all through it! I love learning bits of American folklore! Thank you for lifting-up my Friday afternoon!

wooley bear catapillers

We live in, I suppose what they are calling it now, is the lower hudson valley area, but I haven't seen any of these wooley bears at all these year. Why ?

Wooly worms

From childhood I have loved seeing the wooly worms. I would follow them, let them crawl on my hands, and just enjoy them. Even now as an adult these little critters make me pause for that moment of child like memory. Thank you to those wooly worms.

Wooly worms

Love the story!! My grandma and mom always shared the story about the woolly worm… it going to be a bad winter because…when I see a woolly worm I think of my grandma so I love the story true or not…

Predicting winter

I was always taught - and it seems to hold true-that the more nuts and berries available indicate the winter. Our black walnut, chestnut and butternut tress are heavy Kaiden with fruit. Our pear tree produced the sweetest and most abundant ever! We’ll see I guess how true it is here in western pennsylvania.

All brown wooly bear

I saw an all brown Wooly Bear on the edge of the road on a walk recently. I’ve never seen one like it before. Could it have been a Willy Bear? Or was it a different caterpillar?

woolly worm

The Editors's picture

The wooly bear aka wooly worm is the type of caterpillar that’s a black/brown and rust colors and fuzzy with 13 bands. IWhite, yellow, or other colors of fuzzy caterpillars are not the same type of woolly worm. And if it’s all brown and doesn’t have ANY bands, it’s probably not the wooly bear.

article on wooly bear caterpillar

So I just realized that the content and reason for reading this article was not worth reading as there is no scientific basis for folk lore. Why did you write the article...if it was meant for fun, you should have been upfront about that....

Weekly worms

Very interesting. Want to tell Grandchildren and great grandchildren about them works!!! Thanks

Wholly bear caterpillar

I live in Barrie Ontario north of Toronto. Today, on March 2nd, I found a wholly bear caterpillar in my backyard, very much alive crawling in an area where the snow had melted. Is this unusual for this time of year? Is there any significance in this?

Updated Data Collection Project

Hi all! I am deeply interested in these wondrous little bears and their divination. I am a naturalist by hobby, but a very amateur one. Does anyone know of any recent data collection on the woollies and their band size? Or has anyone considered using something like iNaturalist for data collection? I can't stop thinking about how we could use technology to get more scientific answers than what Curran was able to do pre-internet. Any info you have on more reading or other enthused individuals would be wonderful!

Wooly Caterpillar; Conn.

I found a wooly caterpillar today; January 23rd; in Conn.; I was tempted to build a terrarium; instead I drove her to a local garden and put her in some nice tall green winter plants; I hope she makes it !!!

Woolly caterpillar

Saw one outside on patio concrete 12-21-20 Winter solstice

Wooly Worm

I'm writing a novel based upon the Daphne Greek myth. Set in Eastern PA, it involves an apple tree in a field near a stream, Daphne. I'm now working on her first winter though the novel already has 50,000 words. She's not watching TV to predict the weather, being in a field. My understanding is the '72 was a warmer and dryer winter. Other than the wooly worm, how is she going to know its going to be warm and dry? How is she going to know, other than her leaves turning up, that its going to be cool/warm/hot, the next day, or rain the next day. She can hear, see, smell, taste, and feel touch.

Woolly Caterpillar

Caterpillar just seen in Ohio. Black in color. Walking on my front porch. Awe sooo cute

woolly bear in my bedroom

i just woke up from an afternoon nap, after a long morning and early afternoon catching up on errands. The weather has been in the low 30's, so had stayed inside instead of going out unless necessary. (I am 74 yrs old and don't do well at those temps. Anyway, when i got up and looked at my hardwood floor, there was a woolly bear! At least 1.75 inches long, brown middle, black ends, very much awake and active. i picked it up gently a put it on a small dish; it curled quickly, but not for long. Stretched out again he was on the move.
wasn't sure where he wanted to be. More to the question: how he got here. i live in a small Little House in the woods (truly), but at least 6 feet off ground level. there is a fieldstone terrace and stone stairs and now closed/locked heavy glass sliders in the living room and kitchen. before starting this, i put it on a clean towel, opened the kitchen slider and let it be. just now checked: he is not in the kitchen and nowhere in sight on the fieldstone steps or terrace. he must have been Moving! could he have been in the house since summer and become a "worm" recently? or come in with the wretched red squirrels when they bring in the odd black walnuts still on the ground to eat in some dry nest they must have made in some crawl space we have yet to discover?
we are in the process of redoing the exterior of the old building, so expect to find an assortment of critters. but i did not know woolly bears were naturally indoor types. this one was really magnificent, quite furry, but obviously curious about his surroundings and eager to get out. it was t treat to meet (him).

stay well, please, everyone. Priscilla Van de Workeen, Dudley, MA 10 Dec. 2020 +/- 4:45pm

Will it survive?

So I live in Vermont.. I had woolly bear caterpillar that I finally after months of being in my room. I just let it go. It is 45° outside. Did I do right thing?? I feel so bad.

woolly bear release

The Editors's picture

You did exactly the right thing, under the circumstances. A woolly bear caterpillar needs the cold temperatures to get ready for hibernation; it combats cold temperatures by producing a sort of antifreeze as temperatures cool. The caterpillar likely would have died if it were kept inside a warm home over winter. Now, it might still have a chance to find a sheltered, warm place, build up that antifreeze, and then hibernate (such as under leaves) before winter sets in. In spring, it will wake up from hibernation, cocoon, and turn into a happy moth.

Wolly Bear

A picture that was taken today (11/18/20) shows the caterpillar having a very long band of brown with only a thin black stripe on the ends. Curious to see what winter has in store for us.

Woolyworm color

Live in northern ky and work in cincinnati.
Have seen 20 or so this fall in cincinnati and ky ....all black.
Grandma was a country gal and could accurately predict how our garden would fare based on the previously winter. She loved seeing the all black ones saying, "the more harsh the winter was the more insects, ( that would be harmful to the garden) would be killed off"!
We always had a great garden when we had black wollyworms the prior winter.