Groundhogs Are Old News. In This Tiny Town, Caterpillars Predict the Weather

Last weekend, thousands of people flooded into a tiny town in North Carolina to watch caterpillars race. The creepy-crawly contest is at the center of the annual Woolly Worm Festival, a fiercely popular local event that’s been going on for nearly half a century. The tradition runs deep, and the stakes are high: The winning human gets a $1,000 prize, and the winning woolly worm caterpillar (Pyrrharctia isabella) gets the honor of predicting the weather for the winter to come.

Folk traditions are common in this part of the country. August fog means a snowy winter, and low beehives portend a blizzard. But the caterpillar myth has been one of the most persistent, and it’s certainly the only one with serious competition behind it.

But is there any truth to it?

How to read a worm

Ask Tommy Burleson if the caterpillars’ predictions have merit, and his answer is an emphatic yes. That’s not surprising; he’s been the festival’s official “worm reader” for the past 30 years. The method, he explains, is very specific.

“In reading the worm, you break it down into 13 segments. Those are the 13 weeks of winter,” Burleson says. “If the segment is black, that’s going to be below-average temperatures with snow. If it’s what we call ‘amber,’ which is a dark brown, temperatures are going to be right around average. Here in Avery County, that’s about 27 degrees Fahrenheit. If it’s a light brown, that’s above-average temperatures. And then if you have ‘fleck,’ which is a band that has a combination of brown and black: That’s going to be below-average temperatures with frost or maybe a little bit of ice in the morning.”

To read a worm, you start near the head and move backward from there. The foremost segment corresponds with the first week of winter; the tail-end segment corresponds with the last.

The approach was handed down to Burleson from former worm readers, who, in turn, learned it from generations before them. No one seems to know exactly where the forecasting technique started. But Burleson says it’s stunningly accurate; in the time he’s been worm-reading, his predictions have been correct “87 to 90 percent of the time,” he asserts. “This past winter was absolutely spot on.”

Can woolly worms really predict the weather?

Scientific studies on worm forecasting are few and far between. The most often cited is a small trial that American Museum of Natural History entomologist Howard Curran conducted in 1948.

“I really love the story of Curran,” says Joe Boggs, an entomologist at Ohio State University Extension who has studied woolly worms. According to the legend, Curran heard about the woolly worm caterpillar myth and decided to test it for himself. So, each fall for eight years, he and a fellow entomologist and their wives traveled to Bear Mountain State Park along New York’s Hudson River to collect woolly worms. He found that if the worms he collected had brown markings on more than a third of their body, winters tended to be milder.

“It’s a wonderful story, but I do think it was a tongue-in-cheek endeavor,” Boggs says. “Because Curran was a real scientist. He had a bunch of papers published. But he never published this one—probably because he knew it wouldn’t stand up to peer review.”

Plus, Boggs adds, if the caterpillars got their markings from environmental conditions, one would expect them to all look the same. In reality, though, there’s wide variation.

“It’s possible that the light-colored band in their middle changes size as they stretch and grow,” says Brent Sinclair, a zoologist at the University of Western Ontario who studies how arthropods survive at low temperatures. “So, the size of the band might be telling you about what the summer was like, in terms of how far along they are in their development.”

That could in turn say something about how summer weather affected local plants, their food sources. Still, that would simply mean that the caterpillars’ coloring was a record of past weather—not a forecasting tool for the weather to come.

“So, sadly, you can’t use them for winter forecasting. For that, you have to go back to the groundhog, which is really the gold standard,” Sinclair jokes.

Boggs guesses that the coloration is more likely a camouflage technique. If an individual caterpillar (and its predecessors) thrived on grasses, they might end up with thinner stripes for better camouflage, for example. Or, if a lineage of caterpillars was predated on by wasps adapted to quickly spot a certain pattern, future generations might develop a different pattern.

“I think you just have to look at the basic facts, and the fact is that there’s no evolutionary reason for caterpillars to predict the weather,” Boggs says. However, he says the persistence of the myth doesn’t bother him a bit.

“I love the idea that people are coming together, enjoying the fall weather, and having fun. And they’re coming together for an insect—what could be better?”

The real occult powers of the woolly worm

Just because woolly worms’ forecasting abilities are suspect doesn’t mean they don’t have other incredible powers. The critters can curl into a ball tight enough to fend off attacks from wasps. They’re relatively fast, traveling up to four feet per minute, and they’ve adapted to survive temperatures as low as minus 40 degrees Fahrenheit.

That’s pretty astonishing given their big, juicy size, says Sinclair.

“At large volumes, it becomes harder to keep yourself unfrozen,” he explains. So, woolly worm caterpillars don’t fight it: Instead, they try to freeze.

“Insect cells and tissues are surrounded by hemolymph, which is basically like their blood,” Sinclair says. “If you cut me in half, you’d see solid organs with a bunch of blood vessels containing blood. If you cut an insect in half, you find one big cavity, or hemocoel, with all the insect’s tissues sort of suspended in it.”

It’s in the insect’s best interest to ensure that ice forms within the hemolymph—not within their more delicate tissues. Some insects have developed a means of intentionally kick-starting ice formation just to make sure that happens. This is smart for another reason, too: When water freezes, it gets drawn out of the blood. The remaining blood technically goes into a dehydrated state. Because molecules move from areas of high concentration to low concentration, this dehydrated blood pulls water out of the insect’s other tissues via simple osmosis. As a result, these delicate tissues dry out—making them even more freeze-resistant.

“It’s like making ice wine,” Sinclair explains. “When you freeze grapes, all the water molecules get packed up in the ice. You’re then left with this really concentrated mixture of sugars. It’s the same thing with an insect. It’s nothing fancy—just a bit of chemistry and a bit of physics. What the insect is doing is using that chemistry and that physics to make sure that it survives.”

That’s the theory, anyway. Testing has been limited, Sinclair says, but so far, evidence seems to support it.

It’s also known that woolly worm caterpillars accumulate large quantities of glycerol, a substance similar to antifreeze.

“Glycerol is very easy to make biochemically, so it’s a pretty standard thing to have kicking around inside an insect,” Sinclair says. “But the concentrations these caterpillars can achieve are really high—higher than there are salts in seawater.” In the studies he’s done with emerald ash borers, the bugs’ glycerol-laced hemolymph has been so thick that he hasn’t even been able to squeeze it through a pipette. Instead, he’s had to cut out little blocks of hemolymph, like Jell-O. Glycerol essentially works as a filler, fluffing up the insect’s cells so that they don’t shrink too much during dehydration and get damaged.

Sinclair suspects there are advantages to spending most of the winter frozen. That’s because caterpillars are ectotherms; when they get warm, their metabolism revs up, and they have to expend energy. But when they’re frozen, they exist in an extremely efficient state. That’s important because as soon as they come out of hibernation in the spring, they have almost no time to eat; they have to start making a pupa right away.

“The energy they have left at the end of the winter is what they use to make a moth,” Sinclair says.

The woolly worm population in North Carolina is smaller today than it was five or ten years ago. Winters are getting warmer, which could mean that the caterpillars are spending more time tossing in their sleep and less time preserving their energy. In turn, that could mean that fewer of them have the energy stores they need to metamorphose come spring.

Of course, “fewer” is a relative term; this year, North Carolinians still prepared for the annual festival by collecting worms by the bucketful.

Woolly Worm Central, USA

Banner Elk is a one-stoplight town folded into a valley between Appalachian ridgelines. This time of year, the oaks and maples are cast in rust-colored hues, so warm and glowing that the place seems to exist within a perpetual sunset. Avery County feels trapped in time in other ways, too. It’s quaint and quiet, and life moves at a gentle pace—with the exception of every third weekend of October.

“Can I call you right back?” asks Mary Jo Brubaker, Woolly Worm Festival chairperson, when I call her the week before the event. “The stage has just arrived, and I’ve got to make sure they set it up facing the sun so the worms stay warm.”

Brubaker has been racing worms for 20 years. She got involved through the Kiwanis Club, the local charity organization that now runs the festival in partnership with the Banner Elk Chamber of Commerce.

The festival was first conceived of in 1978 by Avery County local Jim Morton. A few Banner Elk business owners had gathered to brainstorm ideas for an annual community event when Morton spoke up. He suggested a fall festival centered around the popular woolly worm caterpillar.

Morton had long been fascinated by the folklore surrounding the woolly worm, but he felt it seemed wrong to select just any old caterpillar for such a monumental task as forecasting the weather. If Morton was going to trust his winter plans to a bug, he wanted it to be the fittest bug around. The race was born.

Today, the Kiwanis Club uses the proceeds from the event for children’s programming in Avery County. The festival is its biggest fundraiser of the year, Brubaker says. So big, in fact, that all the lodging in town books up as early as February.

On festival weekend, as many as 21,000 people flood into Banner Elk (population 1,035). The air smells of frying batter, and the tents are bright with crafts and costumes. Local artists sing gospel and bluegrass. Elvis wanders around shaking hands and taking selfies. But the highlight of the weekend is always the worm race.

How to race a worm

“The races are a really big deal,” Brubaker says. Some families rally together behind a single family worm. Some make team T-shirts.

The caterpillars are usually plucked out of trees in the days before the race and given names, like racehorses. Only these names are a little less highbrow: Chonk, Mr. Fluffy, Cotton Ball. Last year’s winner had been christened Porta Potty.

The worms are inspected by the official worm doctor. (This year, it’s the local dentist.) Once given a clean bill of health, they’re off.

The races take place on a series of dangling parallel strings made of two-millimeter to four-millimeter nylon, because that’s what Burleson says the worms race best on. Instead of furlongs, the strings are measured in “wor-longs.” Each string is carefully cut to 8 wor-longs, or 32 inches.

The caterpillars each race on their own string, 25 to a heat, with 60 to 70 heats happening throughout the day on Saturday. (A much smaller corporate race happens on Sunday. Unlike the open race, there is no cash prize for the corporate race—only a small trophy and bragging rights.) Top finishers in the open heats go on to a semi-final, and from there to a final race, where thousands gather to cheer themselves hoarse. Just because the race is family-friendly doesn’t mean it isn’t also extremely competitive.

Nancy Owen, who’s been racing for 20 years and won the 2021 corporate race, says some people take their worm-rearing very seriously.

“When my son, Hampton, was younger, my husband would tie several strings to our curtain rod at home, and Hampton would go out and find worms and bring them inside and train them,” she says. “He would feed them apples and lettuce. It’s also important to keep your worm in your pocket and to blow on them. That keeps them warm. They like to be warm.”

This year’s winner was a woolly worm named Jeffery, raced by Brook and Max Wright from Newland, North Carolina, a 20-minute drive southwest of Banner Elk. This is the first time in years that the winner has been a local worm, Brubaker says.

It was a fast race, thanks to sunny, warm weather all weekend. Ectotherms like heat. But Jeffery ultimately pulled ahead, to roars of approval from the crowd.

Burleson took Jeffery aside after the race to do the official reading. Then, he stepped up to the microphone on the competition stage to announce the results. Burleson admits he sometimes gets nervous reading out his predictions, afraid to be the bearer of bad news to local ski areas or to kids hoping for snow days. But this year, the residents of Avery County should be pretty happy:

Weeks 1-4 – Below-average temperatures with snow

Week 5 – Below-average temperatures with light snow or frost

Weeks 6-7 – Average temperatures

Weeks 8-10 – Above-average temperatures

Weeks 11-12 – Below-average temperatures with light snow or frost

Week 13 – Below-average temperatures with snow

Corey Buhay | READ MORE