Can these endearing roaming caterpillars predict the weather?

Did you happen to observe more than the usual number of woolly bear caterpillars motoring across your lawn this fall? There seemed to be a definite uptick in their populations this past year, and I remember seeing them everywhere! Woolly bears, or Isabella Tiger Moths as they will become, are one of the few caterpillars that overwinter in their larval stage in Maine. Lately my family has found many of them in our woodpile, looking quite dead in their wintertime state of torpor. But in just a matter of months, they will pupate, transition through metamorphosis, and fly off to mate and lay their own eggs.

Of course, what everyone really wants to know is – can they predict the weather, as folklore suggests? As it turns out, there may be a kernel of truth to this idea. An American entomologist collected woolly bears north of New York City, over a nine-year period beginning in 1948. He observed that in years when the brown band was longer, winters tended to be milder than normal. Other sources suggest that the black segments of the woolly bear’s coat lengthen as the animals age, making older caterpillars more likely to “predict” harsh winters. It will be interesting to see if, with climate change, we see a change in the percentage of woolly bears with longer black segments.

So how does this insect survive the cold? Why doesn’t its cells freeze and burst as the water in them expands? In the fall, as temperatures cool, the caterpillar finds a pile of wood or leaves in which to overwinter. According to scientists, the woolly bear acclimates to sub-freezing temperatures in response to dehydration and cold temperature cues. These cause the caterpillar to begin producing glycerol, a sugar, that reduces the freezing point of the caterpillar’s body so it can undergo low temperatures without damage to its cells. As the weather turns colder, the caterpillar slowly freezes, but the glycerol prevents its inner cells from freezing, allowing it to survive even the most extreme winter weather conditions.

And why did the woolly bear cross the road, anyway? When spring arrives, woolly bears start to wander about again. Scientists are not sure why they travel so much. They are not picky eaters and can eat a variety of common plants including dandelion, asters, goldenrod, plantain, clovers, and maple leaves. Once their wanderlust has been satisfied, eventually they spin cocoons of silk and the caterpillars’ own setae – the hair-like strands of proteins that cover their bodies. In about two weeks, Isabella Tiger moths will appear.

If you want to provide safe places in your yard for caterpillars to overwinter, make sure you “leave the leaves.” There are many little creatures that will appreciate the protection your leaf litter offers!