Woolly Weather

A blustery fall wind swept away any warmth from the pale sun. Brown and yellow leaves skittered across the pavement. As I bent down to dip my hands in the lake, my arm (wrapped in fuzzy fleece) brushed the seed head of a burr marigold—so named because the cheery yellow flower matures into a cluster of pokey stick-tight seeds. Yet another sign of the season…

Distracted, I meandered back toward the car, picking little two-pronged seeds out of my armpit as I went. Then...whoa!  Something caught the corner of my eye, and I pulled my foot back from its next step. There, in the middle of the parking lot, was a woolly bear caterpillar. Not the most exciting find, but it would have been unpleasant for both of us if my shoe had continued on its original path.

I’ve always enjoyed seeing these fuzzy, black and brown-striped critters. As a kid I tortured them—poking one to watch it curl into a ball…patiently waiting until it uncurled...and the poking it again. You’ve probably had your own encounters with these cute little critters, no matter what age you happened to be. A few years ago, while I was teaching in Maine, my class found a woolly bear’s hairy cocoon in the woods. The students just shook their head at my enthusiasm for the discovery.

Seeing this woolly bear didn’t strike me as anything spectacular, but I took some photos anyway, because a friend had asked me about the caterpillar’s famed weather predicting skills. My research turned up a great story about that, and so much more.

Back in 1948, Dr. C. H. Curran, curator of insects at the American Museum of Natural History in New York City, wanted an excuse to get out of the city and enjoy the fall colors. He decided that the question of woolly bears’ weather prediction skills needed some field research (sounds like a great excuse to play hooky from work to me!), and took his wife a nearby state park. They gathered as many woolly bears as they could find, and measured the sizes of the black and brown stripes. This was so fun that they invited friends the next year, and began calling themselves The Original Society of the Friends of the Woolly Bear.

For eight years they “gathered data” in the beautiful fall woods. At first, it seemed like they were on to something. Several years in a row, the brown stripes took up more than a third of the caterpillar’s thirteen body segments, on average. Folklore says that wider brown stripes forecast milder winters, and that did indeed play out for Curran. Then came the year that two groups of caterpillars in neighboring habitats gave opposing forecasts. That year, Curran gave up.

As it turns out, some scientists now think that the width of the brown band IS related to the weather—of the previous spring. Each time the caterpillar gets too big for its skin and molts, one black segment changes to a brown segment. So, a w-bear who starts eating early (they are generalist feeders who eat a variety of plants), during a mild spring, will have a wider brown stripe by the following fall. A caterpillar who gets a late start, perhaps due to snow in April (not that we know anything about that!), will likely have fewer brown segments before the growing season ends again.

So, with one riddle solved, we’re ready for another one. Why do you see so many woolly bears crossing the road this time of year?  To get to an overwintering site, of course! Like many northern critters (bears, for example), woolly bears are short-distance migrants who need to travel a little ways to find a nice place to spend the winter. And they can get there fast (for a caterpillar) traveling at 0.05 miles an hour, or about a mile a day.

Beneath a rock, under a log, in a bark crevice—almost any protected place will do for an overwintering woolly bear. Warmth is not a major factor, since these little guys will freeze solid, and thaw, and freeze again, many times throughout the winter. On a warm day, they may even get out and crawl around.

If a tomato spent the winter like a woolly bear, it would soon be mush. But woolly bears use chemicals known as cryoprotectants to safeguard living tissue against damage from freezing and thawing. Woolly bears who live in the arctic (the species we know as well as some relatives) may take 14 years to complete their life cycles. They freeze solid every winter, and grow just a little bit during each brief summer.

Our Wisconsin (or Minnesota) woolly bear only needs two years to complete its life. In spring, it will thaw and resume eating. Once large enough, the caterpillar spins itself a cocoon using silk and its own hairs. In two weeks, it metamorphoses into a pale yellow Isabella tiger moth. In another two weeks, the moth will mate, lay eggs, and die.

Tiger moths, in the family Arctiidae, are amazing, colorful creatures in their own right. Some tiger moth caterpillars eat toxic plants, just like monarch caterpillars, in order to protect themselves against predators. The toxin persists in the adult moths, who use bright warning coloration to tell potential predators that they taste bad. Since one of their main predators—bats—can’t see colors in the dark, the moths emit ultrasonic sounds to warn bats of their unpalatability.

All the moths are surely dead by now, and their offspring, the caterpillars, are racing toward their overwintering rocks. I hope the one I almost stepped on found a cozy place in the leaf litter. With snow in the forecast, I’m glad I overwinter in a house. I wonder how mild or harsh this winter will be?  I wish a woolly bear could tell me!