Woolly Bear Caterpillars and Weather Prediction

Do Woolly Worms Really Predict Winter Weather?

September 21, 2020
woolly-bear-caterpillar-winter

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.

Source: 

The 1998 Old Farmer's Almanac

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Reader Comments

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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.

Woolly Caterpillar

Found one today 11/10/20...We live in Northville, Michigan, between Detroit and Ann Arbor.

It was small black on the ends and a rust band in the center about the length of the black ends put together. So a quarter of the length on the ends was black and the band in the center was twice either one of them.

Wooly Worm

I found a wooly worm in my kitchen yesterday. It was all black with a thin bit of orange. I am in Missouri and we haven't had a bad winter in awhile. I sure hope this year is not the year it decides to play "catch up!"

Wooly worm picture i took 2020

The wooly was all tan? Anybody else see these in ohio?

totally black woolly bear

I live in Mountain, WI and I saw my first black woolly bear. should I be worried

all black fuzzy caterpillar

The Editors's picture

According to folklore, an all-black woolly bear caterpillar would indicate a severe winter to come. But more than likely, the caterpillar that you saw was of a different species, such as the giant leopard moth, which can be found in Wisconsin. This caterpillar shows red bands on its skin. These aren’t very obvious, though, unless the caterpillar curls up, moving the black bristles out of the way. The adult is a beautiful creamy white moth with black leopard spots.

Woolly Bear sighting

Today I found a completely black Woolly caterpillar in my garage. Maybe this is a good sign we won't have any snow this year?? I live in CT.

Woolly Bear Caterpillar sighting

today I saw a woolly Bear Caterpillar with skinny rust bands,thick black bands, it was striped not two color sections. what does this mean? found in Maryland. of course went to get camera and it was gone

Wooly Bear

Saw one this evening, touch of black on front and back, thick brown middle!!!!!!! Bring on winter, I love seeing the beautiful snow! Could do without extreme cold though. We shall see!!!!

wooly bear

i live in western new york by the shore of lake erie. on oct 10th of this year i saw a wooly bear with just a tiny bit of black. like just his nose was black. will see in the spring if he was right with his forecast

Willy bear caterpillar

Found Them to be mosty black this year. Boy this should be some severe winter.

Wooley Bear

Front was bushy black thick brown color over half of it body last back small black area

Sept 25 2020 Wooly

Saw a Wolly today in Upstate NY Traveling North. Head Segment longer than Middle or Tail Segment

Wooly

I forgot to put that I’m in Quebec, Canada

Thin rusty band colour

I took a picture of one today, it has just a little rusty band in middle and black is wide. Wish I could send a picture. Early in the month there was another one with rusty band abit wider but black was also more dominant.

Woolly Bears

I love this!
I read about these cute little insects years ago. I have to say, each Fall when they are crawling all over the roads and grassy spots, I look at their bands of colour to predict our Winter forecast, and the result is true. Every year now for the past 6 years. Even if it is folklore it’s still fun

WOOLLY WORM

When I was about 10, My Grandfather taught me about the WOOLLY WORM.
Here is what he said, First off the WOOLLY WORM has 13 segments and Winter has 13 weeks, each segment represents one week of winter. and yes you have to find your own
in your immediate area to be your winter forecast. the front being Dec. 20, 2020 and the back being Mar. 21, 2020. So you can tell what week the milder weather will begin and end.
Also when not to plan your travels ( in the Black weeks ) . Like the one in your story on 9/22/2020, The head is very small ( that means a slow start x-mas week ) then a little rougher the next week, Then 2 bad weeks before the mild weather begins. the first 3 weeks of rust color has 3 dark spot in them ( means there will be one bigger snow in each of those weeks) , The next 3 weeks are mild and best to make plans for winter travel. The last 2 segments mean that the 2nd and 3rd week of marh will be very wintery. Use this information how you see fit.

not woollys but tons of acorns !

no woollys yet ...but tons of acorns! Someone told me that if trees shed tons of acorns
the winter will be awful ! Is this true ? Please do an article on that.

Woolly Bear

..saw solid orange Woolly Bear on my driveway today.TN

Oh boy

I've seen two so far this year. One was huge, the other was really small. They were black. I'm in PA. Good thing I replaced the heater a few weeks ago.

Black Woolly

Saw my first Woolly yesterday in St. Louis, sat., Sept. 19, 2020, he was ALL black. Should I worry, I'm a "summer baby"?

WOOLLY BEAR in NEW HAMPSHIRE

Found a large all-black Woolly in my yard yesterday! I don't remember seeing an all-black one before. I love shoveling snow - bring it on. Maybe it will relieve our drought. Wondering where Barbara Lamb (9/6/20) lives?

Woolly caterpillars predicting winter

I gather a minimum of 3 caterpillars each fall to see what they predict for the coming winter. My grandmother taught me as a child how to do this and at 68 yrs old I enjoy memories of her by continuing this tradition!
Good information in this article! Thank you!

Wooly Bear Caterpillar

I just now (9-6-2020) found an all black Wooly Bear caterpillar on my porch. It's larger than any other caterpillar I've ever seen: 2" long and 3/4" wide and solid black, no hint of brown/orange anywhere. I'll have to keep track of the weather this year and see if there's any truth to the folklore saying that it'll be a severe weather.

forecasting winter

Here in the redwood forest of northern California along the coast, we have wooley bears in the Fall. Love them. They appear everywhere around here. / Another predictor of winter weather is the holly tree - the more red berries on the bush, the colder the winter weather.

Long red section

I never have catapillars in my back yard. But mine are red in the middle and black ends. Larger red

Wooly Bear Caterpillar May 2020 CLE, OHIO

All black wooly bear caterpillar found on our patio. All black with orange banded body, no brown/orange spikes/wooly bands across.

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