Woolly Bear Caterpillars: Winter Weather Predictors | Facts & Folklore | The Old Farmer's Almanac

Woolly Bear Caterpillars and Weather Prediction

Primary Image
a woolly bear or a woolly worm caterpillar, brown and black, on pavement. use a woolly bear to predict winter

Based on the measurements of the distinctive woolly bear caterpillar, you can figure out your weather forecast!

Photo Credit
University of Missouri

Do Woolly Worms Really Predict Winter Weather?

Print Friendly and PDF
No content available.

Woolly bear caterpillars—also called woolly worms—have a reputation for being able to forecast the coming winter weather. If their rusty band is wide, it will be a mild winter. The more black there is, the more severe the winter. Just how true is this weather lore? Learn more about this legendary caterpillar and how to “read” the worm!

The Woolly Worm Legend

First of all, the “woolly worm” is not a worm at all! It’s a caterpillar, specifically, the larva of the Isabella tiger moth (Pyrrharctia isabella). Nonetheless, the name “worm” has stuck in some parts of the United States. In others, such as New England and the Midwest, people call them “woolly bears.” (Worm or not, at least we can all agree that they’re not bears!)winter weather folklore, using a woolly bear caterpillar to predict winter. According to legend:  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.

In terms of appearance, the caterpillar has 13 distinct segments, either rusty brown or black. It is often black on both ends with rust-colored segments in the middle, although it may sometimes be mostly black or rusty. (Note: All-black, all-white, or yellow woolly caterpillars are not woolly bears! They are different species and not part of the woolly worm lore. So, if you spot an entirely black caterpillar, it isn’t forecasting an apocalyptic winter!)

According to legend:

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. 

Love weather lore? Read about more signs of a hard 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 that Dr. Curran studied, the banded woolly bear, is the larval form of Pyrrharctia isabella, the Isabella tiger moth.

  • The Isabella is a beautiful winged creative with yellowish-orange and cream-colored wings spotted with black. It’s common from northern Mexico throughout the United States and across the southern third of Canada.
  • The tiger moth’s immature larva, called the black-ended bear or the woolly bear (particularly in the South, woolly worm), is one of the few caterpillars most people can identify.
  • Woolly bears do not feel much like wool—they are covered with short, stiff hair bristles.
  • 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 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 many of them crossroads 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 Really 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 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. Since then, the annual counts have continued, more or less tongue in cheek.

For over forty years, Banner Elk, North Carolina, has held an annual Woolly Worm Festival in October, highlighted by a caterpillar race. Retired mayor Charles Von Canon inspects the champion woolly bear and announces his winter forecast. Similarly, there is a Woollybear Festival that takes place in Vermilion, Ohio, each October.

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.”

How to “Read” the Woolly Worm

Weather is local, so you need to read your own woolly worm.

Look for these fuzzy wuzzies in the fall. According to woolly worm watchers, there are two generations of worms each year. The first appear in June and July, and the second in September. The second-generation worms are the “weather prophets.”

To find a woolly bear, start looking under leaves and logs! Some are just crossing the road. Once you spot a woolly worm inching its way along the ground or a road, you’ll see them everywhere! The caterpillars are most active during the day (not at night). After filling up on food—including violets, lambs’ quarters, and clover—their goal is to find a place to hide for the winter. Interestingly, the woolly worm overwinters as a larva. Their entire body will enter a “frozen” state until May when it will emerge as the Isabella moth.

Every year, the wooly worms 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. Remember:

If the rusty band is wide, it will be a mild winter. The more black there is, the more severe the winter.

That’s it! Note that white, yellow, or other colors of fuzzy caterpillars are NOT the same type of woolly worm and are not used for weather forecasting. We’ll leave the weather-prognosticating “skills” to your observation!

Speaking of Weather Predictions …

Did You Know: The 2024 Old Farmer’s Almanac is now available! The brand-new issue includes our famous 2024 Winter Weather Forecast.

Get your copy of the 2024 Almanac, where books and magazines are sold, or you can now order straight from Amazon to your home

Woolly Worm Video

In tribute to our fellow prognosticator, we made a woolly worm video … 

Whether the predictive powers of the woolly worm are fact or folklore, we always enjoy the fun!  Feel free to share your experience with the woolly worm in the comments below.

About The Author

Catherine Boeckmann

Catherine Boeckmann loves nature, stargazing, and gardening so it’s not surprising that she and The Old Farmer’s Almanac found each other. She leads digital content for the Almanac website, and is also a certified master gardener in the state of Indiana. Read More from Catherine Boeckmann