The Legend of the Woolly Bear

Woolly Bear Caterpillar (Pyrrharctia isabella). Photo by Bob Jenkins
Woolly Bear Caterpillar (Pyrrharctia isabella). Photo by Bob Jenkins

By Christine Harris

Chances are that while enjoying the outdoors on a cool fall day you’ve run across a woolly bear caterpillar.  Topping out at speeds of four feet per minute, these brown and black fuzzy critters are most active in the fall months throughout the contiguous United States as they seek out a cozy, subterranean space to spend the winter.  Woolly bears need to grow and molt six times before they can develop into Isabella tiger moths, which can take several years in colder climates with shorter growing seasons.  Fortunately woolly bears have a unique adaptation to allow them to survive cold winters.  With tissues that contain antifreeze-like compounds called cryoprotectants, woolly bears can survive freezing and thawing throughout the cold winter months.

If you know anything about woolly bears, you’ve likely heard that the width of the brown band on their bodies can serve as a predictor of the severity of the coming winter, with wider bands indicating milder winters.  This well-known piece of folklore was first popularized in 1948 by Dr. C. H. Curran, Curator of Insects at the American Museum of Natural History in New York City.  In that year, Curran and his wife visited Bear Mountain State Park, forty miles north of the city, where they collected a number of woolly bears and measured the length of their brown bands. He found that the brown bands were wide in comparison to the black bands and predicted that the coming winter would be mild.

Curran’s prediction was published in the New York Herald by a personal friend who wrote for the newspaper, and proved to be correct.  Over the next seven years, Curran and his wife traveled annually to Bear Mountain State Park to measure woolly bears.  Through their measurements over those years they found that the bands averaged more than a third of the length of the caterpillars bodies, and the corresponding winters proved to be mild.

 As any well-practiced field biologist knows, the small sample size and limited time period of Curran’s study could not serve as conclusive proof of the woolly bear/winter weather connection, and Curran was under no illusions that it did.  His visits to Bear Mountain with his wife were a much anticipated autumn escape and in later years when his friends joined them, they called themselves The Original Society of the Friends of the Woolly Bear, a group that enjoyed the foliage and caterpillars, but did not take themselves too seriously as scientific researchers.

Isabella Tiger Moth (Pyrrharctia isabella). Photo by Jurvetson (flickr)
Isabella Tiger Moth (Pyrrharctia isabella). Photo by Jurvetson (flickr)

Thirty years after Curran’s last survey, the nature center at Bear Mountain State Park resumed the sampling of woolly bears and continues to do so today.  Though it will take many years and many caterpillars to draw any kind of reasonable conclusions, the park continues the project to maintain an intriguing part of its history and may one day have enough data to determine whether or not a correlation between these caterpillars’ appearance and winter weather actually exists.

 According to Mike Peters, an entomologist at the University of Massachusetts, the brown band on a woolly bear may in fact correlate to the severity of the previous winter rather than the severity of the winter still to come.  There is evidence to suggest that the width of the brown band has to do with the age of the caterpillar.  The earlier in the spring the caterpillars emerge, the sooner they pupate into moths and the sooner the moths lay eggs which will become the next generation of woolly bears.  The earlier the eggs are laid in the spring, the earlier the woolly bears emerge and the more time they have to grow and develop wider bands before they head into hibernation in the fall.  Therefore, instead of predicting a mild winter ahead, a wide, brown band most likely means that the previous winter was mild.

Are they two inch insects, or nature’s oracles? Whether or not they have predictive powers, these ubiquitous, fuzzy little caterpillars, will likely continue to fascinate us for generations to come.

2 thoughts on “The Legend of the Woolly Bear

  1. Christine, you know lots of stuff that I don’t. Thanks for sharing the article. Any day that you lear something new, is a good day!

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