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


Reader Comments

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All Black Caterpiller

I saw an All Black Caterpiller yesterday crossing my front porch, Va Beach, Va.

Thawing today wooly bear on doorstep

Was on doorstep during a thaw day today. Central brown area about 7/12ths with ends black the remaining 5/12ths.
It was soaked with saltwater so I dried it with tissue and placed on a plant inside to rest and recuperate. Has been some very cold spells this winter, but only moderate snow. Near Ottawa, Ontario, Canada.

Wooly Bear Caterpillar

Found one late fall put him or her in my garage in a caterpillar container and was amazed to find its alive and doing well .

Woolly worm

I seen one just like that in the picture in the fall of 2017 and it’s been a moderate winter here in Nebraska.

Mostly brown wooly worm

I just found a dead one on my living room carpet in Philadelphia, PA. Mostly brown. Must have come in under doorjamb.

I saw an all black Woolly

I saw an all black Woolly Bear worm in my garage. I am in Bryant, AR

Woolly bear Catapillar

Woolly bear Catapillar〰I found one in front of my house Nov.13,2017 ALL BLACK Burr!!! Bundle up Maryland!

First woolly worm in MN

First one I have ever seen in Minnesota. Mostly black, probably going to be a really cold winter.

Saw one yesterday all black-

Saw one yesterday all black- southern oh- here o i guess a bad winter for us

Wooly Worm

Saw a smaller all black wooly worm on my porch today Nov 15, 2017!!!

I saw an all black one

I saw an all black one yesterday!

Wolly "BLACK"

Found an all black wolly walking into my garage November 7. Next day I found him curled into a ball on the cement floor. I put him in a tissue box and brought him inside.

Wooly Bear

Just saw an all black wooly bear in Orangeville Pa 11-12-2017 at 1pm
It is a nice sunny day in the lowers 40

just found a wooly bear worm

just found a wooly bear worm almost all Brown, I guess we will have a mild winter in cincinnati. Yeh !

Wooly worm

Saw a solid black wooly worm today!!!

Black woolly worm

Just walked out my front door and there was a solid black woolly bear on my brick wall. That is the first one I have seen this year. Powell Ohio

Woolly Bear

Do woolly bear caterpillars make a cocoon, ??

Found one today 11/3/17

On my front porch.. small orange bands.. mostly black! I hope the legends are true!! Azle, Texas

Wooly (brown) Bear

Just found an all rust wooly bear! All the ones I found early fall at least had a black band, though smaller than I've seen years past.

black woolly bear

I seen several the last few months while walking and all were rust color in the middle with balck on each end. Yesterday 11-02-17 I seen a all black one.Went out the front door this morning there was a black one at the door husband almost stepped on it until I noticed .Look like woolly was was wanting to get in the house.It was 52 degrees this morning.Punxsutawney Pa



Woolly bear caterpillar

Just saw an all black one. Nov. 2, 2017. Hopkinsville, Ky.


Found all black one in NewAlbany, In, today!

Woolly Bear

Just spotted an all-black one as well...Upstate South Carolina better hunker down this Winter!

Wooly Bears in NY State

I just found a small one (average size) crawling on my deck at Albany NY. The head was almost a half inch black and the rest of it was rust brown.

Found an all black one. Paris

Found an all black one. Paris TN. Gonna be COLD!


Found an all black woolybear this afternoon in Sugarcreek, Ohio. First time I ever saw one that was all black!

all black i just saw it today

southpoint ohio

Wooly bear catipillar

Saw an all black one in knoxville tn. Well we need a good winter it's been a long time since we had a good snow. It would be nice before i check out of this world. old man

Wooly Bear

I live in central Maryland and just came across a solid black Wooly Bear in the back yard.