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

Reader Comments

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I live in southwestern

I live in southwestern Pennsylvania I just seen a all red or light in color woolly bear but it looked young so don't know if it will get black on the ends before the summer is over, does anyone know?

Found a white wooley in ohio

Found a white wooley in ohio today

What did yours look like?

What did yours look like? Just seen our first one today! Ours was COMPLETELY orange. In past several yrs it's been thick black bands w little orange, remember last winter how bad it was here, WW said awful winter. We are eastern central Ohio.

Found a big fat thick wolly

Found a big fat thick wolly bear worm on my porch today. What does that gonna mean for Kentucky's winter?

Found a woolly bear

Found a woolly bear caterpillar on brick of house tody
It was all a rust color with no black at all.
Ontario Canada, Very mild winter coming if coloring & Almanac is correct.

Ah now I see what these

Ah now I see what these things are .

They're crawling all over the place here at the horse ranch .

Recently moved out to the corn fields from a major city and never seen so much wildlife all at once , all right in front of ya .

This past fall was the wooly invasion .

Didn't know exactly what they were so I basically left them alone .

I do know there will be plenty of them moths this coming spring and summer .

That is alright , the reason why I moved out here is to get away from the corruption , greed , drugs and crime and that is just in the police department .

I found one on my old

I found one on my old christmas tree I was like "yayay" and then I got it and put it in a box so yaaaaa......... anyways it was black and in the middle it was redish orangeish brownish all mixed together can anyone tell me what kind of caterpillar it is and if its poisonious

Seen an all black wooly bear

Seen an all black wooly bear caterpillar 3 inches long an really bushy seen another by my house again another black one this is in Allentown, PA.

when did you find a all black

when did you find a all black wooly bear

I just found a black and

I just found a black and brown wooly bear catapillar We are having that strong winter storm on v day here in southern maryland. Should i bring this guy insince we r havingbelow zero. tonight and the next week. Wont he freeze? What do i do?????

I FOUND AN ALL BLACK ONE ON

I FOUND AN ALL BLACK ONE ON CHINCOTEAGUE ISLAND, VIRGINIA. BLACK SEEMS TO BE THE NEW TWO TONE FOR THIS WINTER SEASON!

We are in St. Louis, MO &

We are in St. Louis, MO & just found one in the house that is about 1/3 brown in the middle. We'll probably have a harsh winter but not as bad as the worst. Hope everyone has a safe season!!

We found one north of

We found one north of Toronto, Ontario with a tiny patch of brown! Looks like it's going to be a cold winter!

I found an all black one

I found an all black one crawling on my pillow about to crawl on my arm ewww

And this was in Southeast

And this was in Southeast Texas

Well folks...just found one

Well folks...just found one in Orlando, FL. Almost all black. Not looking forward to a cold winter!!

Found one in New Jersey, also

Found one in New Jersey, also almost all black! Bundle up!!

Wooley

funny...I've noticed them over the years and never paid attention to their colors. Just assumed they had different markings. This year in North Jersey I noticed they seem to be all black...usually they have some orange at either end. Well...we got away with a warm winter in 2015 guess we are due! lol

Here in Beebeetown, IA (by

Here in Beebeetown, IA (by Missouri Valley) the single brown stripe takes up over half of all our Wooly's bodies. I hope they don't freeze because it is 11degrees out now at 8am Nov. 12, 2014. Expected high for today is low 20's.

found a wooly bear

found a wooly bear caterpillar at the wyndotte county lake and i have to admit theyre pretty but i have read some articles and i think they can sting you ande cause a painful burn, so be careful with them.

I am in the Baltimore suburbs

I am in the Baltimore suburbs and have a solid black woolly that decided to hang out in our garage for the last 4 days. Today, It finally moved about 8 feet along the wall, right towards our door-to come inside!

Just found an all black wooly

Just found an all black wooly bear about 1.5 inches in suburbs of Pittsburgh PA!

Just saw the largest wooley

Just saw the largest wooley bear Ive ever seen moving across my driveway around 3" long and completely black, first time Ive ever seen that. Central NJ. Seems fitting since this is about the coldest November I can remember.

Are all these fully black

Are all these fully black woollies? Or another species like the one with light red "stripes" on the body below the black hairs. I've never seen an all black but will keep on alert. Woollies are one of my most favorite childhood memories as I raised them often. When I see them they make my day!

I have not seen as many

I have not seen as many woolly worns as usual..we live in a rual area in Tennessee...however the three that I have seen have been ALL BLACK. Not looking forward to the winter. Good luck to all.

Oh no we're screwed this

Oh no we're screwed this winter - I too saw an all black Woolly worm this past weekend in SE Pennsylvania. In fact that's how I ended up on this site.

Last fall I found a couple with narrow orange and we had a pretty significant winter. We're in for a treat this winter!

I found an all black one

I found an all black one today in N.E Indiana.

Found a very large all black

Found a very large all black one in Nashville TN. Absolutely no reddish brown coloring. Heating bill will to be just delightful this year

I just found a black wooly

I just found a black wooly worm in my laundry room where the cold garage is, it was all ball up. Virginia Beach

Found a fat, all black one in

Found a fat, all black one in the basement. About 2 inches long and 3/4 inch wide. Never seen one so big.

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