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Wooly Worms & the Weather

Photo of Jennifer Schultz Nelson

Jennifer Schultz Nelson
Extension Educator, Horticulture

Each fall, it's hard not to notice them–little wooly worms crawling across roads, sidewalks, and paths everywhere. Where ever they're going, they seem very determined. More than one person in recent weeks has asked me what they actually are. Do they stay as they are, or morph into something entirely different in time? And how does a wooly worm forecast the winter weather?

Different species of wooly worms are found all over the world. 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 U.S. is home to at least eight species of hairy caterpillars that are commonly called wooly worms. In central Illinois we are most familiar with 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. 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. The Isabella tiger moth is a dull yellow-orange with some black spotting, and its first pair of legs have bright red-orange segments.

There are two generations of banded wooly bears, or Isabella tiger moths, each year. 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. 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 more friendly, and tends to congregate in large populations on their host plants.

It is only in the early autumn that most people notice wooly worms, and they're usually in a big hurry to get somewhere. 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.

There is a lot of folklore surrounding the banded wooly worm, particularly related to its supposed power 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. 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 thirteen 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 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. 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.

Despite the scientific evidence that discredits any super weather predicting power of the wooly worm, people are very passionate about their wooly worms. Several towns in the U.S. have their own wooly worm festivals. Banner Elk, North Carolina hosts an annual festival complete with a wooly worm race. The winner is declared the "official" predictor of winter weather. People come from miles around to participate. Some bring wooly worms they collected themselves, others prefer to buy a wooly worm from one of the "breeders" that brings 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, Illinois last Saturday, October 7th. Will wooly worm breeders be far behind? Time will tell.

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