Friends of Woolly Bear Caterpillars and Winter Weather Predicting

moth and caterpillar

I have written before about the “signs of winter” that are part weather folklore and part science. Those posts have touched on the a caterpillar known as the woolly bear (the larval form of Pyrrharctia isabella, the Isabella tiger moth) being seen as a winter weather indicator.

I did some further digging on a recent cool summer night that felt more like autumn to see how much science might be behind this weather lore.

The science part seems to go back to 1948, when Dr. Curran, curator of insects at the American Museum of Natural History in New York City, started collecting the caterpillars in Bear Mountain State Park in NY. He decided to research the weather lore and determined the average number of reddish-brown segments.

He had a reporter friend at The New York Herald Tribune and they published his winter weather predictions and the publicity made the woolly bear a celebrity caterpillar in North America.

The Isabella tiger moth is common from northern Mexico throughout the United States and across the southern third of Canada. The immature larva is called the black-ended bear, woolly bear or woolly worm. They don’t feel wooly or fuzzy. I held some at a nature center once and they were more bristly.

This wooly bear walking across the patio is saying the winter will be mild.
This wooly bear walking across the patio is saying the winter will be mild.

Woolly bears, like other caterpillars, hatch during warm weather and search for overwintering sites under bark or inside cavities of rocks or logs, so you might see them crossing roads and sidewalks in the fall.

The lore is that the wider that middle brown section is (i.e., the more brown segments there are), the milder the coming winter will be. Conversely, a narrow brown band is said to predict a harsh winter.

Between 1948 and 1956, Curran’s average brown-segment counts ranged from 5.3 to 5.6 out of the 13-segment total. That means the brown band took up more than a third of the body and should have predicted mild winters. And the winters in NY were milder than average.

But even Curran didn’t take his small and unreplicated research too seriously.  he and some friends would go off to look at fall foliage each fall, collect caterpillars and they called themselves The Original Society of the Friends of the Woolly Bear.

Most scientists consider the folklore of woolly bear predictions to be folklore. Some entomologists have made a tentative connection between wooly bears and winter. The number of brown hairs seems to be related to the age of the caterpillar. Since that age  is based on how late it emerged in spring, the band does tell us about a heavy winter or an early spring. Unfortunately, that is a backward look at the previous year rather than a look ahead at the upcoming season and year.

Still, being a Friend of the Wooly Bear is an excellent excuse to go out into the autumn woods, look at the foliage and count those brown segments if you come across our bristly friend.

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Ken Ronkowitz

Random by design. Predictably irrational. It's turtles all the way down. Dolce far niente. A lifelong educator.

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