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 or all-white 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. 

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. 

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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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.

Springtime woolly worm

I just seen a wooly worm on my deck and it’s the end of March. Is that normal and what does it mean?

NONE

I have found at least one woolly worm every year, at least for 40 years! This year, I have found none. What does this mean?

Solid Black Wooly Bear Catapillar

Well, I've heard the predictions all my life and told that the blacker the catapillar, the more severe the winter. Well, I have an all black one here in Virginia which should mean a severe winter, however the weatherman is predicting an extremely mild winter. Go figure.

Wooly Caterpillar

Today is January 2, 2020 and I found one of these crossing the road in Exeter, NH. (Black ends Brown middle section) I'm wondering if it is unusual for one to be out this far into winter.

Wooly Bears

I was getting the yard ready for winter, stumbled upon several of these lil guys. All curled up. I thought they might have died. I put them in a container and brought them in to show my mother. To my surprise they all opened up and started moving. I did some research on them and was amazed by what I found out. What a wonderful creator we have. This lil guy has some type of serum that freezes to protect them from the bitter cold. In the spring they come out and go through their metamorphosis. Fascinating.

Wooly Bears

I was getting the yard ready for winter, stumbled upon several of these lil guys. All curled up. I thought they might have died. I put them in a container and brought them in to show my mother. To my surprise they all opened up and started moving. I did some research on them and was amazed by what I found out. What a wonderful creator we have. This lil guy has some type of serum that freezes to protect them from the bitter cold. In the spring they come out and go through their metamorphosis. Fascinating.

Wooly Bears

I was getting the yard ready for winter, stumbled upon several of these lil guys. All curled up. I thought they might have died. I put them in a container and brought them in to show my mother. To my surprise they all opened up and started moving. I did some research on them and was amazed by what I found out. What a wonderful creator we have. This lil guy has some type of serum that freezes to protect them from the bitter cold. In the spring they come out and go through their metamorphosis. Fascinating.

Wooly bears (catapilar)

I just found one on my doormat and it's 11/18/19 but we have had many inches of snow, but it has all melted. We have also have had below zero temperatures. So I brought him in fearing that he would die. I put him on a paper plate with a piece of iceberg lettuce, a raspberry, a grape and a blueberry. What should I do with him??? Please help.....

Wooly bear

Just found one in our basement

All black wooly bear caterpillar

Just found an all black one outside our back door

woolly worm

i found a all black one in my house tonight .

all black wooly bear caterpillar

Found an all black one in my carport today. I scooped him up and moved him to the back yard.

woolly worms

I have seen all black woolly worms where I live here in Jackson, Kentucky. I have heard that this could be a bad winter, however I also read that solid black woolly worms could not determine the severity of the winters. Only the red centered ones.

wooly bear caterpillars

I have seen more of these little caterpillars this year than any other time. Most of them have been relatively "standard", but there have been a few that have a very wide brown patch, and even a few that had the black head and an all brown body. They are looking for somewhere warm-like my garage most of the time.

Wooly Bear Caterpillar

The one we had found had only one end of it black, rest of it was rusty brown.

Woolly bear caterpillar

Siting: SE Michigan
Barely any brown

wooly bear sighting

here in Southern Coastal Or...I just saw one crossing my back yard..we have a FEW of them..lol anyway..huge middle RED segment ..10/17/2019 ..we shall see...

Wooly bear caterpillar

In East Troy Wisconsin yesterday a lone caperpillar was entirely black.

Wooly Worms

I live in Western North Carolina and I saw a yellow wooly worm for the first time ever. What does that mean for our weather prediction?

Woolly worms

I saw 2 diff ones today out in my barn, both were entirely black and very woolly! My mom swears by this, my Grandfather was a farmer and they paid attention to the woolly worms, so I guess we will see:)

WOOLLY BEAR CATERPILLAR

Found on October 10th 2019 in Valley Washington.
Mostly black about 1/3 brown.

Auburn Hills, MI

In years past, I would only see 1 or 2 of these little guys crawling around town...but over the past few days, I have seen about 2 dozen of them! All different sizes, and all different 'band widths'...They keep trying to come in the vestibule at my work, and I keep relocating them to a nicer, outdoor 'no-squish' zone. I wonder why there are so many of them this year...anyone know?

Black wooly bear

For the first time in my live(64), I seen a all black wooly bear. That's in Minnesota

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