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Woolly bear caterpillars are easily recognized by their black and orange bands. Photos by Chris Young.

Woolly bears predict mild winter ... or not

November 07, 2013 at 12:43 PM

The State Journal-Register.

If this coming winter seems especially harsh, or if it is unseasonably mild, the credit or blame could fall to the woolly bear caterpillar.

Or it might not.

Woolly bear caterpillars, with their bands of black and orange, are familiar sights each fall when they cross roads and footpaths. The three bands make them easy to identify.

Folklore has it that winter’s severity can be predicted by the width of the woolly bear’s middle band. If the band is wide, a mild winter is predicted.

But don’t count on it.

“We just don’t have magical caterpillars around,” said Tim Cashatt, chair of zoology at the Illinois State Museum.

How wide the bands appear is more accurately tied to the caterpillar’s age.

According to the Peterson Field Guide to Eastern Moths: “Colors change as caterpillars molt to successive instars (growth stages), becoming less black and more reddish as they age.”

Isabella tiger moth.

The woolly bear is the larva of the Isabella tiger moth (Pyrrharctia Isabella).

While the caterpillar’s supposed ability to predict the weather gets the most attention, it is it’s ability to survive the cold of winter in the caterpillar stage that has piqued the interest of scientists.

“Some insects have built-in antifreeze mechanisms that allow them to survive cold temperatures until it warms and their metabolism speeds up again and they undergo metamorphosis,” Cashatt said.

The insect’s body chemistry keeps water in bodily fluids from forming ice crystals.

Researchers Jack Layne, Christine Edgar and Rebecca Medwith studied woolly bear caterpillars in Pennsylvania, and tried to understand how they survived winter temperatures by hiding in leaf litter.

They found that caterpillars could withstand several days of sub-freezing temperatures at a time and live. Their study was published in 1999 in the American Midland Naturalist.

Many butterflies and moths spend the winter in a chrysalis or cocoon, but the woolly bear waits until spring to make its cocoon.

When you see a woolly bear on the move, rest assured it is not just out to see the world, but looking for a place to spend the winter.

“They are looking for something to crawl underneath, looking for a warm spot,” Cashatt said. “Don’t ask how they know where that is. They have their own mechanism for finding those places they hole up in.”

In the spring, the caterpillars feed for a short time before spinning their cocoon.

But can they really predict weather?

As for the story about their weather predicting abilities, here is the account from The Old Farmer’s Almanac.

The story goes that Charles Curran of the American Museum of Natural History in New York City counted woolly bear caterpillars during a fall getaway in the late 1940s and used his measurements to forecast the winter.

Over the next several years, he repeated his observations to try to prove the weather rule scientifically.

He never did prove his point, but he did manage to make the woolly bear a widely recognized caterpillar.

Now, for those who still believe in the caterpillar’s prognosticating power, there’s one caveat.

A mild winter still can include a cold snap or spring blizzard.

So, no matter the prediction, it won’t do any good to blame the foot of snow in your driveway this January on the woolly bear caterpillar.

Chris Young can be reached at 341-8487 or .(JavaScript must be enabled to view this email address). Follow him at

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