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Fearless winter weather prognosticators

According to legend, the woollybear caterpillar can predict the severity of winter.

Woollybear caterpillarWhat kind of winter will we face this year?

Don’t ask the National Weather Service. They have a hard time getting their predictions correct for next week, let alone next season.

Ask your local woollybear caterpillar (Pyrrharctia isabella), the larvae of Isabella tiger moth. You can find them as they crawl around looking for a place to spend the winter.

According to legend, the wider the copper band on the caterpillar, the milder the winter we can expect.

The black bands on either end can be informative. If the head end is very dark, the beginning of winter will be very cold. Likewise if the back end of the caterpillar is very dark, the end of the winter will be very cold.

A woollybear caterpillar has 13 segments and there are officially 13 weeks of winter. Is this a coincidence? Perhaps not.

This legend has been around since colonial times. Colonists would see the caterpillars crawling out of pastures and across dirt roads this time of year. The severity of winter was a life-or-death matter to colonists and they were desperate to find any way of predicting the weather.

The legend grew in popularity after Dr. Howard Curran, Curator of Entomology from the American Museum of Natural History, did a study in the 1940s and 50s. He went out to Bear Mountain in New York every fall and measured the copper bands on the caterpillars. He then made a prediction for the winter. His predictions were published in newspapers across the nation. 

The stories surrounding the woollybear have grown over the years. If the coat of hair is very thick, we will have a cold winter. If the caterpillar is crawling south, it will be a cold winter because the woollybear is heading for warmth. 

Festivals, parades and caterpillar races are held in many towns every autumn to celebrate the woollybear, much like we celebrate Punxsutawney Phil and Groundhog Day. Woollybear Day is the biggest one-day festival in Ohio.

Unfortunately, a woollybear cannot predict the severity of winter. Its coloring is primarily based on its age. A woollybear caterpillar molts and sheds its skin six times. Its copper band becomes wider each time it molts. Complicating matters is there are hundreds of related tiger moth caterpillars, each with slightly different color patterns.

Nevertheless, this is a fun legend and woollybear caterpillars are amazing survivors. Once they find a place to overwinter, they produce an anti-freeze in their body, glycerol, which allows them to survive frigid temperatures. They have been reported to survive temperatures as low as –90°F and can survive an entire winter inside an ice cube! That’s cool, excuse the pun.


National Weather Service. 2013. Woolly bear caterpillar: A winter weather predictor or not? www.crh.noaa.gov/arx/?n=woollybear. Accessed 9/14/2014.

Raupp. M. 2012. Wooly bear caterpillars - purported peerless prognosticators. Extension news. Pennsylvania State University: State College.

Written by Tom Kalb, Extension Horticulturist. North Dakota State University. Published in NDSU Yard & Garden Report, September 15, 2014. Photo was made available under a Creative Commons license specified by the photographer: Dave Govonia.

Filed under: Insect, Tom Kalb
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