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Chicago Urban Gardening

The day to day experiences of a University of Illinois Extension Urban Horticulture Educator in Chicago, Illinois

Wooly Worms and the Weather

Posted by Ron Wolford -

Despite the scientific evidence that discredits any super weather-predicting power for the wooly worm, people are very passionate about them, said a University of Illinois Extension horticulture educator.

"Several towns in the United States have their own wooly worm festivals," said Jennifer Schultz Nelson. "Banner Elk, North Carolina, hosts an annual festival complete with a wooly worm race. The winner is declared the 'official' predictor of winter weather."

Each fall, it's hard not to notice little wooly worms crawling across roads, sidewalks, and paths everywhere. Whereever they are going, they seem very determined. Many people wonder what they actually are. Do they stay as they are or morph into something entirely different in time? And just how do they forecast the winter weather?

"Different species of wooly worms are found all over the world," Nelson said. "The ancient Romans used the Latin term catta pilosa, literally "hairy cat," to describe them. The term caterpillar traces back to these early descriptions.

"The creatures we call wooly worms are, in fact, caterpillars, so their time spent as a wooly worm is limited. The United States is home to at least eight species of the hairy caterpillars that are commonly called wooly worms."

In central Illinois, the most common is the banded wooly bear or worm, Pyrrharctia Isabella. Following the pupal stage, the banded wooly bear emerges in its adult form, the Isabella tiger moth.

"Experts estimate around 260 species of tiger moths in North America," she said. "Their caterpillars share some degree of hairiness--from a few hairs to being completely covered in hairs, like the wooly worms.

"Tiger moths get their name from their bright coloration. Their colors are typically some spotted or striped combination of gold and black. Some species also have red, white, and gray colors interspersed as well. This Isabella tiger moth is a dull yellow-orange with some black spotting, and its first pair of legs has bright red-orange segments."

Each year brings two generations of banded wooly bears, or Isabella tiger moths. One is typically hatched in May, the other in August.

"Most people don't notice wooly worms in the spring and early summer, but they're around, feeding on various plants and growing," she said. "One reason we probably don't notice them is that essentially they're hermits, preferring solitude over socializing."

Another tiger moth larva, fall webworm, is far friendlier, and tends to congregate in large populations on their host plants.

"It is only in the autumn that most people notice wooly worms, and the worms are usually in a big hurry to get somewhere," said Nelson. "What's the rush? Each wooly worm is scrambling to find a warm nook or cranny in which to overwinter.

"Unlike other butterflies or moths, the wooly worm spends the winter as a caterpillar, not a pupa or chrysalis. The hairs covering their bodies are thought to offer some degree of insulation from cold winter temperatures. When the weather warms in the spring, the banded wooly worm becomes active again. They feed for a short time, then pupate, and emerge a few weeks later as adult Isabella tiger moths."

A lot of folklore surrounds the banded wooly worm, particularly related to its supposed ability to predict upcoming winter weather each fall.

"The typical banded wooly worm has sections of black hairs at each end, and a section of orange-brown hairs in the center," she explained. "Legend says that the more black on a banded wooly worm, the more severe the winter will be.

"Some folks have taken this to an extreme, and noting that there are 13 segments in a typical banded wooly worm, they argue that each segment represents one week of winter. Orange segments predict mild weeks, and black ones foretell bad winter weather."

Some people also insist that the thickness of the hairs is the predictor--thick hair equals a bad winter, sparse hair a mild one. Another legend says that the direction a wooly worm is found traveling is a hint about the coming winter. If the wooly worm is traveling north, count on a mild winter. If he's headed south, get ready for a long, cold winter.

"Research has shown repeatedly that the colors or hairs of wooly worms have no bearing on weather the following winter," she said. "Larvae hatched from the same clutch of eggs reared under one set of environmental conditions will show a range of hair thickness and colorations, from fully orange to fully black.

"This variation in a constant environment is a strong argument against wooly worms being able to predict the weather. There is genetic segregation for color and hair thickness present in the wooly worm population. This segregation would make it nearly impossible to visually sort out any environmental influence if it did exist."

Still, the wooly worms have their defenders.

"People come from miles around to participate in the Banner Elk wooly worm festival races," Nelson noted. "Some bring wooly worms they collected; others prefer to buy a wooly worm from one of the 'breeders' who bring their best racing stock to the festival. I guess you can sell just about anything!

"Illinois had its first official wooly worm festival in Camargo in 2006. Will wooly worm breeders be far behind? Time will tell."

Source: Jennifer Schultz Nelson, Unit Educator, Horticulture,

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