Woolly Bear Caterpillars and Weather Prediction

Using Woolly Worms for a Winter Forecast

August 30, 2018

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.

What’s the real winter weather prediction for 2018The 2019 Old Farmer’s Almanac was just released! Look inside for our official forecast!



The 1998 Old Farmer's Almanac

2020 Almanac Calendar Club

Reader Comments

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woolly Bear

Here is an all beige Woolly Bear on our house in Central Indiana. Oct 10, 2018

Black Wolly

Here in Eastern PA, I've been seeing a lot of black/all black woolly's. With the amount of rain we have received this summer and fall, I would not be surprised if the trend keeps up. Bought a snowblower over the summer just incase!

Wooley worm

I saw a all black wooley worm today .looks like a bad winter in sterling illnois!!!! Yeaaaa

Woolly caterpillar

I read that a young girls was hospitalized because they're poisonous. I keep crawling to the front of my door guessing they're trying to come into my house. Also finding them in my yard. I just found 4 of them. Two by my front door and two in my yard.

Winter is coming

Saw orange in middle with black on both ends today, looking like. Mild winter but I predict rainy!

Woolly caterpillar

I saw one today as well, from Bowman, Quebec Canada, the orange was smaller than the black, so its going to be a harsh winter.

Rule of thumb?

In the early 1900’s it was legal for men to beat their wives, as long as they used a stick no bigger than their thumb... Now you know, and knowing is half of the battle... (Yes part of my comment is from the movie, Boondock Saints). I couldn’t resist.

Woolly bears

What does "rule of thumb," which goes back far more than the early 1900s, have to do with wooly bears? BTW, by the dawn of the 2oth Century and even earlier, such barbaric laws were being phased out. Now, if it was a rule of wooly bears . . .

Wooly caterpillar

Saw an all black wooly on my deck this morning...he was very fat and very black. Plus a lot of acorns this year. Looks like a bad winter here in Pennsylvania.

Orange Wooly

First wooly I've seen this season was on my porch this morning and he's all orange. It's a mild winter for Theresa, NY. Oh darn! I like 20 below & snow.

Black Wooly Caterpillar

I found one tonight crawling on my deck here in Worcester MA Sept 26th. He is completely Black! About 2.5 inches and fat! Does not look good for us this winter according to the almanac!

Orange brown

Came in from garden this morning and had an orange brownish with black head caterpillar crawling up my ankle. Mild winter??

Wooly Bear Caterpillar

it was actually an indicator from the previous winter. Up here in Mass I found one today and it had more orange/rusty color and black and we had a pretty mild winter last year. But this year I do believe is the La Nina season which is due to bring us a pretty nasty winter. I will be more interested in what wooly worm says next year.

Wooly worm

Seen one Today and it was black but very thin didn’t have much hair on it they say if it’s black and bushy were going to have a bad winter but hopefully here in southern Missouri it won’t be bad

Orange Fussy

2018, Sunday 23
I was outside looking for flowers to pick for my mom and I found this Fuzzy orange fur with white hairs. I named it sugar. Also I have some questions, 1# why does it stop moving when you put it inside your house? 2# what does a Fuzzy Orange caterpillar eat? 3# do toads eat orange Fuzzy caterpillars?


I don't know what Sugar might prefer to eat, but I love the name and your story.

Big Black Fuzzy

I saw a 1.5 inch long big huge fuzzy one! Looked like a small puppy lol
... says bad winter :(

Wooly Worms

We saw a black and reddish colored wooly bear worm a little while ago, there were the two black bands on each end and a reddish colored middle band. We need some cold wet weather in KC so I hope I see more black ones!

Woolly brown caterpillar

Saw a solid colored reddish-brown woolly caterpillar today Sept. 18, 2018 in Anthony (south-central) KS!

Solid tan wooly worms

We are seeing solid tan wooly worms in southern Indiana. Does that mean no winter ;)

Gray Woolly Worms

I have several gray Woolly Worms and I've never seen in that color. Do you know why this is?

Wooly worms.

Just saw a good sized wooly worm. It was black with SEVERAL narrow rusty colored stripes. Not just one.

Woolley worm

I saw a two inch Wooley, all black with orange bands in between that were not as noticible. But he was certainly noticible and truckin along at a good pace.

Woolly bears

I have been seeing a lot of totally black woolly bear caterpillars this year. What does that mean. You are predicting a wet, warm winter for my area (northeast IN) but the woolly are saying it's going to be a bad one. I don't know which one I should listen to. lol

Black or White Woolly Bears

The Editors's picture

Caterpillars that are completely black or completely white are different species than the ones traditionally used to predict winter weather. The weather-predicting woolly bears referenced in this article will have at least some brownish-red banding on them.

Wooly Worms

my children have collected quite a few today. We have three pitch black ones, some dark rusty colored ones and two completely white woolies. They are a lot of fun. We will be curious how our winter turns out.

wooly worm

I have seen a white one What does that mean?

white worm

Seen a white wool worm here in (south) St Louis.

First time ever seeing one.

Wolly Bear

In Indiana husband and I have seen two white Caterpillars this year!!

Yesterday I saw a caterpillar

Yesterday I saw a caterpillar on my porch that is very unusual for this time of year. It looked very close to the yellow & black tent caterpillars we usually see in the spring. But this one was more grey black & white. I've never seen one like that, especially this time of year. I have not seen any wooly worms yet either. But seeing that one catarpillar this time of year-- what could that mean for us here in the foothills in northern NC?