Woolly Bear Caterpillars and Weather Prediction (2024)

Woolly bear caterpillars—also calledwoolly worms—have a reputation for being able to forecast the coming winter weather.If theirrusty 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 caterpillarand how to “read” theworm!

The Woolly WormLegend

First of all, the “woolly worm” is not a worm at all! It’s a caterpillar, specifically, the larva of the Isabella tiger moth(Pyrrharctiaisabella). 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, atleast we can all agree that they’re notbears!)

In terms of appearance, the caterpillar has 13 distinct segments of either rusty brown or black. Often, it is black on both ends withrust-colored segments in the middle,although it may sometimes be mostly black or mostly rust. (Note: All-black, all-white, or yellow woollycaterpillars 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 forecastingan apocalypticwinter!)


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According tolegend:

The wider the rusty brown sections (or, themore brown segments there are), the milder the coming winter will be.The more black there is, the more severe thewinter.

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 bearcaterpillars.

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 Caterpillars and Weather Prediction (1)

What Is a Woolly BearCaterpillar?

The caterpillar that Dr. Curran studied, the banded woolly bear, is the larval form of Pyrrharctia isabella, the Isabella tigermoth.

  • The Isabella is a beautiful winged creative with yellowish-orange and cream-colored wings spotted with black. It’scommon from northern Mexico throughout the United States and across the southern third ofCanada.
  • The tiger moth’simmature 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 canidentify.
  • Woolly bears do not actually feel much like wool—they are covered with short, stiff bristles ofhair.
  • 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 woollycaterpillars.
  • Woolly bears, like other caterpillars, hatch during warm weather from eggs laid by a femalemoth.
  • 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 thefall.)
  • When spring arrives, woolly bears spin fuzzy cocoons and transform inside them into full-grownmoths.
  • 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 stripedappearance.
Woolly Bear Caterpillars and Weather Prediction (2)

Do Woolly Bear Caterpillars Really Forecast WinterWeather?

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’sbody.

The corresponding winters were milder than average, and Dr. Curran concluded that the folklore has some merit and might betrue.

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 WoollyBear.

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, sincethen.

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

Woolly Bear Caterpillars and Weather Prediction (3)

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 toit.”

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 WoollyWorm

Weather is local so you need to read your own woollyworm.

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 “weatherprophets.”

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 upon food—including violets, lambs quarter, and clover—their goal is to find a place to hide for the winter. Interestingly, the woolly worm overwinters as larva.Their entire body will enter a “frozen” state until May when it will emerge as the Isabellamoth.

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.Remember:

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

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 ownobservation!

Speaking of Weather Predictions…

Did You Know: The 2024 Old Farmer’s Almanac is now available! The brand-new issueincludes our famous2024 Winter Weather Forecast.

Get your copy of the 2024 Almanac where books and magazines are soldor you can noworder straight from Amazon to your home.

Woolly WormVideo

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

Whether the predictive powers of the woolly wormarefact or folklore, we always enjoy the fun! Feel free to share your experience with the woolly wormin the commentsbelow.

Woolly Bear Caterpillars and Weather Prediction (2024)
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