Woolly Bear Caterpillars and Weather Prediction

Do Woolly Worms Really Predict Winter Weather?

August 28, 2019
woolly-bear-caterpillar-winter

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: 

Source: 

The 1998 Old Farmer's Almanac

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Reader Comments

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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

White wolly worms

I live on the West Virginia/Ohio border and for the first time ever, I have seen all white wolly worms. I heard the ones with horns can be poisonous but what is the story with any of the white worms? I've seen with horns and without.

white wooly worms

I live in Winchester, TN, and saw a white wooly worm 2 days ago. I'd like to know if there's any significance to their being white?

Found one in Gurnee, IL.

These caterpillars are a beauty. They’re very agile and don’t mind being in the captured environment at all. Its Gorgeous and loves eating our wild growing dandelions

Wooly worm sighting

I saw one for the first time this week and it was entirely black! I live in NE Kansas...

wooly bears and winters

The ones I am seeing have small black ends and a large orange midsection = easy winter?

Wooly Bears

The same here in Central New York State

Wholly worm

All the wholly worms I have seen this year are solid black. Think its gonna be bad here in east Tennessee.

Wooly worm

I found one today in Blount County. It had black ends and a large brown middle.

Obviously you didn't read the

Obviously you didn't read the article completely, it stated the all black are a different species

Woolly bear caterpillars

I live in Oakville, Ontario, Canada, and have never seen one before -this morning, my cat found 4 in the grass! He thought they were toys, and kept patting them to make them move quicker - no harm done! I phoned a friend who knows all about these things, and she told me they were woolly bears - not harmful. The brown band in the middle is about equal amount to the black at each end.

Woolly Caterpillar

Had one on my patio in Flint Michigan, had more rust than black tried putting it in a tree but the woolly bear wouldn't stay . Hoping for a mild winter!

found a wooly caterpillar all orange

what does that mean

Woolly bear

Just saw our first woolly bear. We found it in our hydrangeas. It is about equal parts brown and black. So cute! I live in NE Ohio, not far from the home of the Woolly Bear Festival.

Woolly bear sightings in Maine

4 woolly bears sighted in various locations in foothills of western Maine, all rusty colored, no black.

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