Why did the woolly worm cross the road? Maybe it was to set up a joke about a road-traversing chicken. Wooly bear caterpillars (a.k.a. woolly worms) are commonly seen crossing roads and sidewalks this time of year. Lore about woolly bear caterpillars predicting the upcoming winter has existed for a long time, with little evidence of accuracy.
John Obermeyer, Purdue entomologist and integrated pest management specialist, recently wrote about woolly bear caterpillars. In answer to the above question, Obermeyer stated, “The purpose, observers for many generations tell us, is to find the perfect over-wintering site, under leaf litter, fallen tree branches, rock crevices, etc.” He added that within the last couple hundred years, they have also used our structures such as barns, outhouses, etc.
If these insects overwinter as caterpillars, how do they keep from freezing and avoid being eaten?
“Amazingly, because they are able to chemically alter their bodily fluids to much like anti-freeze, the shelter is probably more to protect them from predators, e.g. raccoons and birds, during this winter stasis,” he said.
And now, to the subject of their supposed ability to be winter weather prognosticators.
“Folklore tells us that the woolly bear is able to predict how cold/mild the winter will be,” said Obermeyer. He offered a tongue-in-cheek comment about its ability and added that the Internet will give you all kinds of “answers” about this phenomenon. Supposedly, the wider the width of the brown band, as opposed to the black, predicts a milder winter.
With his tongue still firmly implanted in his cheek, he adds, “If you are going to do your own local observations and measurements for a precise forecast, you must use the correct species of woolly bear, Pyrrharctia isabella, more commonly known as the banded woolly bear, or woolly worm. Using another species of caterpillar will throw the ‘science’ off!”
According to the Old Farmer’s Almanac, woolly bear caterpillar lore began in 1948, when Dr. C. H. Curran, curator of insects at the American Museum of Natural History in New York City, began studying the caterpillars. Over the course of eight years, he captured, measured, and documented the relative colors of woolly bear caterpillars and tried to associate upcoming winter conditions with the color patterns. He would forecast the coming winter weather through a reporter friend of his at The New York Herald Tribune. Woolly bear caterpillars became North America’s most recognizable caterpillar.
Winter survivors pupate in the spring and emerge as moths with yellow-tan wings and sparse black dots. The moths are named either wooly bear caterpillar moth, banded woollybear moth, or Isabella tiger moth.
There is one forecast you can count on, however: when you see woolly worms in the fall, winter will soon be upon us. Additionally, it will probably be cold at some point, and we may even have some snow!
Find Obermeyer’s original article at https://extension.entm.purdue.edu/newsletters/pestandcrop/.