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.
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.
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 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.
Then, again, you can always check the winter weather predictions in The Old Farmer’s Almanac!