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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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

WOLLEY

SAW ONE TODAY NOVEMBER 2 2017 IN EUFAULA,ALA.ALL BLACK.

Woolly bear caterpillar

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

WoollyWorm

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!

Woolybear

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.

Gray wolly worms

Just seen a all gray wolly worm what does that mean

wooly bear

I'm in north central Ohio and the wooly bear I seen beginning of this week was mostly brown very little black on either end.

forgot

sorry, i forgot. Central New York state. Finger Lakes area.

Woolly worm

I just found one on my basement wall, about 2-1/2 inches long & mostly all black, i had to look hard to see any brown in the center. I'm tuning up my snowblower.

Wooly Bears

My daughter showed me a picture she took with her phone of an all black one she saw on our driveway this morning in central Massachusetts. :)

I'm in southwest michigan and

I'm in southwest michigan and am currently looking at an all black one

Saw one today, just a little

Saw one today, just a little rust in the middle and a lot of black on both ends! South Chicago suburbs!

willy worm

In Ohio saw willy worm l0-17-17 and it was all brown with a small black ring on both ends. Looks like your picture but almost all brown So hope this means a mild winter. Have also saw white willy worm too. and all brown one with a little black ring. All were in the same place on black top on our walk in park.

wooly bear

In Northeast PA I just saw one, and, the brown segment was larger than the black.. so perhaps our winter won't be as snowy this year! We had a pretty snowy one last year!

Woolly Bear

I'm in SW Ohio. I've seen 3 woolly bears in the last 2 days, all of them with wide brown bands. Hopefully the stories are true and we'll have a mild winter.

All-Rust Woolly Bear in NH

I saw my first Woolly Bear of the season today, and it was completely rust-colored. I live in south-western New Hampshire. It really has been a warm fall so far. Remains to be seen what the winter will be like!

Woolly Bear

We're on Pewaukee Lake Wisconsin, we see a lot of those cute little creatures, just took 2 out of the cottage and their brown bands were pretty narrow. Looking forward to a snowy winter!

Woolley worm

Saw one here in Northwest WA State near Canadian border. All dark rust color. No black. What up with that???

All Black Wooly Worm

Wednesday night (Oct. 4th, 2017) while out walking my dog, I spotted an all-black wooly worm. It was attempting to cross my neighborhood road. This, along with the fact that the Hickory trees in this area are fully loaded with nuts now supports my belief that winter in the Charlotte, North Carolina area may be severe.

All black wooly caterpillar

Driving out of Girl Scout camp on dirt road between brown corn stalks. Granddaughter saw a huge caterpillar almost across the road. She thought I ran it over. We backed up and got it. It is huge. Measured it, 2 inches long. It absoluteely lives lettuce.

Wooly caterpillar

Sorry forgot to mention we live in Nebraska.

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