Can woolly bear caterpillars predict how harsh winters will be?

Each year, people living in the mountains of North Carolina and central Pennsylvania turn to an unconventional source to get an idea of whether the upcoming winter will be mild or harsh.

Unlike your typical meteorologist, however, this “forecaster” crawls on the ground, has six legs and sports fuzzy brown and black bands.

The fuzzy prognosticator is a tiger moth caterpillar known as the woolly worm or woolly bear.

According to The Old Farmer’s Almanac, the woolly bear legend is based on the caterpillar’s 13 distinct segments of either rusty brown or black. The wider the brown sections, the milder the coming winter will be. The more black there is, the more severe the winter. 

Since 1978, residents of Elk Creek, North Carolina, have celebrated the coming of the snow season with the Woolly Worm Festival. They set aside the third weekend in October to determine which one worm will have the honor of predicting the severity of the coming winter. That worm earns the honor by winning heat after heat of hard-fought races – up a three-foot length of string.

In addition to winning the privilege of predicting the weather, the winning worm also receives a $1,000 prize.

Adam Binder, co-organizer of the Woolly Worm Festival, said that the woolly worm has an 87 percent accuracy rate in its predictions. The Woolly Worm Festival is a prime attraction for the one-stoplight town of Banner Elk, drawing over 22,000 visitors annually.

While newer to the scene and somewhat smaller in scale than the Banner Elk event, the Woolly Worm Festival in Lewisburg, Pennsylvania, has nonetheless carved its own quirky niche in the woolly bear folklore.

Similar to the Banner Elk festival, the “competitors” in Lewisburg’s Woolly Worm Festival crawl up strings, with the fastest among them given the honor of predicting the upcoming winter weather.

“We’re correct 108 percent of the time,” said Mike Glazer, who founded the unapologetically wacky festival in 1997.

While woolly worm festivals may be fun for everyone involved, is there any scientific merit to the legend?

In the fall of 1948, according to The Old Farmer’s Almanac, C. H. Curran, curator of insects at the American Museum of Natural History in New York City, decided to put that claim to the test.

Between 1948 and 1956, he conducted experiments on woolly bears in which he found that the brown bands took up more than a third of the woolly bear’s body. The corresponding winters were milder than average, and Curran concluded that the folklore has some merit.

However, according to the Almanac, he realized that his samples were small and saw his experiments as an excuse for having fun.

According to Michael J. Raupp, “the bug guy” and an entomology professor at the University of Maryland, there is some loose logic behind the concept of predicting the winter weather by analyzing the width and color of the woolly bear’s bands.

When the caterpillars hatch from their eggs, he said, their bands are largely black but gradually turn reddish-brown as they mature. As a cold-blooded creature, the woolly bear’s rate of development is proportional to its surrounding temperatures.

“The warmer it is, the faster they develop,” Raupp said.

If the woolly bear grows faster during warmer summer weather, its brown bands will be quite broad. Since a hotter-than-average summer can be a presage of a warmer winter, Raupp said, the width and color of a woolly bear’s bands can possibly indicate what type of winter to expect.

According to AccuWeather Senior Meteorologist Alex Sosnowski, there is about a 50-50 chance that there will be a correlation between a region’s summer and winter temperatures.

Since an individual’s perception of what constitutes a cold versus mild winter is highly subjective, he added, meteorologists prefer to make seasonal forecasts in terms of normal temperatures—the average of a 30-year period.

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