A banded wooly bear caterpillar, Pyrrharctia Isabella larva. (Photo provided — Whitney Cranshaw, Colorado State University, Bugwood.org)

It’s been a remarkably mild fall. In fact, at the time of this writing (Oct. 27), I still haven’t had a frost at my home, near the Canadian border. But winter is coming. And while winter can be a very picturesque time of the year and getting outdoors in winter can be a lot of fun, harsh winter weather can stop most of us … well … cold.

Someone recently asked me, “What do you think the winter will be like this year?” I simply replied, “I don’t know.”

Even meteorologists, using state-of-the-art models can’t predict weather with 100% accuracy. In fact, it seems to me that meteorology is a notoriously inexact science, with accuracy dropping especially quickly as you look more than a week or so into the future.

The question took me back to a time several years ago, when a friend’s intrepid little granddaughter overheard a conversation that he and I were having about this and jumped right in, informing us that the way to know what the winter will be like is to look for a woolly bear. She wasn’t talking about Winnie the Pooh or a massive, omnivorous mammal with coarse, heavy fur and sharp claws. She was referring to banded woolly bear caterpillars (WBC); also called woolly worms, weather worms, fuzzy bears, and fuzzy-wuzzies. They’re the bristly caterpillars with black bands at either end of their bodies and a coppery, reddish-brown band around their middle. They characteristically curl up when you touch them or pick them up.

The WBC is actually the larva of the Isabella tiger moth, Pyrrharctia Isabella, which overwinters as a caterpillar, often in or near its final stage of larval development. We most often take notice of them at that time, when they’re roughly 1.5 to 2 inches long and seeking overwintering shelter in plant debris and leaf litter, amongst roots, in wood piles, beneath loose tree bark, or in other protected places.

WBCs don’t hibernate, at least not in the usual sense, by resting or sleeping or slowing their breathing and lowering their heart rates to become inactive. When winter sets in, WBCs actually freeze solid. They produce a cryoprotectant, glycerol; a type of sugar that acts like antifreeze in their circulatory fluids, inhibiting damage to cell tissue and allowing them to survive until spring, at which time they feed primarily on wild herbaceous plants, rarely attacking desirable ones, before pupating within cocoons made from their setae (hairs) and eventually emerging as adult Isabella tiger moths in the spring.

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Predicting winter weather

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Can banded WBCs really be used to predict the severity of the coming winter weather? Popular folklore says yes. One widely accepted perception is that the fatter and fuzzier WBCs are, the worse the coming winter will be. Another alleges that if they’re seen moving south, the winter ahead will be a harsh one. If they’re headed north, you can expect the winter to be milder. (I guess easterly or westerly movement indicates uncertainty.) The most widely maintained belief, however, has to do with the length of the caterpillar’s coppery reddish-brown banding; the shorter the center band, the harsher the winter ahead will be.

There’s little, if any, scientific evidence to support the accuracy of any of this, although from 1948 through 1956, Dr. C. H. Curran, then curator of insects at the American Museum of Natural History in New York City, along with a group of friends and colleagues, calling themselves the Original Society of the Friends of the Woolly Bear, conducted some imaginatively offbeat research on the subject.

Year after year, they went bear hunting (woolly bear hunting that is) in Bear Mountain State Park, near Peekskill, catching as many specimens as they could find and counting the number of reddish-brown segments on their bodies, before releasing them. Isabella tiger moth larvae have 13 distinct body segments which, according to legend, represent the 13 weeks of winter.

Dr. Curran publicly reported his findings and yearly forecasts through the New York Herald Tribune. And they proved accurate every time. After eight years of monitoring, he concluded that there was a correlation, however negligible, between the number of reddish-brown segments and the severity of the winter that followed.

Most entomologists will tell you, however, that the pigmentation of WBC setae reveals more about the age of the caterpillar than it does about future winter weather. Each time a WBC molts (sheds its exoskeleton to grow larger, replacing it with a new one), at least one black segment is replaced by a reddish-brown one, so a greater number of reddish-brown segments indicates that the caterpillar is an older or later-instar larva. Evidently, more black setae are produced during periods of wet weather, too.

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Caterpillar racing

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Since 1978, the town of Banner Elk, North Carolina, has held an annual Woolly Worm Festival, in which as many as 1,000 wooly bear caterpillars compete in wooly worm races. Aside from the distinction of predicting the upcoming winter weather, the winning larvae receive top honors. And the person or persons that entered the winning worms take home grand prizes of $1,000 on Saturday and $500 on Sunday.

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Conclusion

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The bottom line: Using insects to forecast winter weather probably isn’t very accurate. But it’s fun!