Woolly Bear Caterpillars and Weather Prediction

Do Woolly Worms Really Predict Winter Weather?

August 28, 2019

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: 


The 1998 Old Farmer's Almanac


Reader Comments

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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?

Wooly worm

This last month of Aug. I found several all black wooly worms........eeew! Looks like we here in Kentucky will be having an all wet or snowy colllddd Winter!!!

wooly bear catapillers

Never seen one all black till today in my driveway in northeastern NY state.

Woollies & winter weather

I have limited experience with woolly bear caterpillar forecasting, but two years I found totally black woollies. Those winters, following those finds, the snow seemed endless. It was up to my hips! Usually in our valley, there is not much snow & it melts quickly. I usually do not see woolly bear caterpillars.

Wooly Worm

Saw an all black wooly worm this week - in Jacksonville, Florida!

wooly worms

I live in Norton, Virginia.

This year I have not seen any wooly worms at all. I'm finding that a bit odd as every previous year I have seen a few.

Hi I live in New Brunswick

Hi I live in New Brunswick Canada and my Willy bears last year and this year is all black lol and last winter was really bad . Had frost even clear up into June . This year suppose to be bad to. You even predicted its going to be a bad winter . My dad and his dad went by the cows and other farm animals .

Wooley worm more accurate than TV weathermen!

I've followed the wooly worm since growing up in Minnesota, life in South Dakota, Michigan, Massachsetts and now here in Tennessee on the Cumberland Plateau and the wooley worm seems to get it right. This year I've been seeing increasing numbers of wooley worms. Some are nearly all rust but most have a few rings head and tail. Yes, they say mild winter here in Zone 6 Tn. In fact, The Old Farmers Almanac says the same thing with more wet than usual...so there you have it. BTW old timers are right about a lot of things!

Woolly bear

About 10 yrs ago, I came across a Albino wooly bear! I live on the south shore of Massachusetts. What does that say about the weather? Also around the same time, we were visited by a albino skunk. We named him, El Blanco.

white woolly bear?

The Editors's picture

We’re wondering if that might have been a hickory tussock caterpillar…

white woolly Caterpillars with black dorsal markings

I'm seeing them too in south-central New Hampshire. I've seen four so far as of 9/26/18 but not a single one of the brown-orange variety and all within the past week. It's as though the brown-orange Isabela Tiger moth variety went extinct since last year.

White hickory tussock caterpiller

This fall I am seeing lots of wooly bears but yesterday for the first time, a white furry one with a linear mid-back stripe lying on the grass close to shagbark hickory trees, etc. Don't touch it! it can cause a rash, etc like poison ivy as it secretes venom.

wooly worms

The wooly worms are brown in S W Ohio this year.Last winter was extremely cold so I'm hoping for a mild winter this year.

brown woolly bears

The Editors's picture

According to the lore, if the rusty/brown band is wide, then it will be a mild winter. The more black there is, the more severe the winter. So if it’s brown you’re seeing, it seems to be a match for the mild winter that we’re forecasting. Coincidence? :-)

Black and yellow woolybear catopilers

I seen so many black and yellow woolybear catopilers this last month . What does the black and yellow mean
I’m in Alaska
Thank you

Long brown band

If I’ve seen the proper wooly caterpillar in our yard, its sides were rust colored, but the black/brown band was running straight down it’s back, from head to tail end. It wasn’t segmented like I observed last year. Is that possible for this species? Bucyrus, KS

I'm in missouri and have been

I'm in missouri and have been seeing the same, mostly black with rust on the undersides.... wondering what this might mean for us since the weather predicted was not really a prediction for us.

brown wooly bear

Reading PA - 9/2/2018 - first wooly bear is all brown . . .

all-brown woolly bear

The Editors's picture

If the rusty/brown band is wide, or all encompassing for all intents and purposes, then it will be a mild winter. The more black there is, the more severe the winter. So, it sounds like a mild winter—as predicted. We’ll all just have to wait and see.

Aug 31 2018 Wooly Bear Caterpillar St Louis-Illinois boarder

All Black this year, but if you look at them in the sun you can tell there is a reddish tinge along w the all black. No stripes of any kind. All the same in various areas on both sides of the river. Cold harsh winter??

Aug 2018

I have ran over 100's of solid black wooly worms in Central Illinois in the last week. Wonder about the upcoming winter.

wooly bear caterpillar

Saw black wooly bear caterpillars all day crossing the road everywhere i went in Northeast Arkansas.

All black wooly bear caterpillars

Saw an all black wooly bear caterpillar today in Minnesota! Very sad to see this predicts a long hard winter ~ hope this isn’t the case!

Wooly worms & winter

Midwest Indiana I saw only black ones crossing the road yesterday

All black woolly bear

In central Illinois, week of 8-20-2018, seeing numerous solid black woolly worms. Are we in for very long hard winter?

All-Black Woolly Bear

The Editors's picture

All-black woolly bears are actually a different species of caterpillar and are not used for traditional weather prediction. True woolly bears will have at least a few rust-colored segments in the middle. As for winter predictions, read our 2019 Winter Weather Forecast to find out!