Spineless Wonders

Why Did the Woolly Bear Cross the Road?


John Obermeyer, Purdue University (Woolly Bear Caterpillar on road)

Dear Dr. Tim:

Have you noticed the hairy caterpillars crossing paved roads recently? What are they, where did they come from and why are they crossing the roads? Isn’t that a dumb thing to do? Don’t they know they could be run over?

Concerned Driver.


Dear Concerned Driver:

If you are driving right now, I hope you are listening to my podcast rather than reading my reply. Your focus needs to be on the road. And, while it is fine to observe the caterpillars while driving, please don’t swerve to avoid hitting them. That can be very dangerous, and your safety is most important.

To answer your question, yes, I have noticed this phenomenon. The fuzzy caterpillars scurrying across the road at this time of year are woolly bear caterpillars, also known as woolly worms. Until your letter I have not given much thought about their reasoning skills, but you are right: We have to wonder what they are thinking.

Woolly bear caterpillars are about 2 inches in length, and their color varies from light tan to orange, or even dark brown or black. The two things that all woolly bear caterpillars have in common are long fuzzy coats and the fact that they are prone to play in traffic at this time of year.

One of the most common species sports a characteristic fuzzy coat — a combination of orange and black bands.

During the early or midsummer months, they go unnoticed even though they complete two full generations, from egg to adult. The adult moth, commonly called a tiger moth, is white, yellow or orange in color with black specks on its wings. If noticed at all, it is because it flies around porch lights at nighttime.

Eggs are laid on the undersides of leaves on trees, shrubs or weeds, wherever the fuzzy caterpillars feed. While most caterpillars overwinter in protective silken cocoons, the woolly bear passes the winter as a caterpillar. It curls up in a winter sanctuary under a warm pile of leaves, a rock, or a fallen log, then relies on its fuzzy covering as well as a natural antifreeze inside its body to keep it warm enough to survive until spring.

Folklore has it that the orange and black-banded woolly bears we are seeing on the roads right now can actually predict the severity of the approaching winter. The legend says the coming winter will be cold IF the woolly bear has a narrow, orange middle band, and the winter will be warm if the band is wide. Certainly one would have to know what is normal, and what is not, to compare against but apparently there are actual studies that have measured the average differences in band width from year to year. The studies concluded that there are in fact differences in average band widths from year to year but — sorry, folks — the studies failed to demonstrate a significant correlation with the coming weather.

I know that is a letdown, but before you get too disappointed, let’s think about this for a few minutes from the caterpillars’ perspective. Nobody asked them if they were meteorologists; they never applied for the job; they never actually said they could or wanted to predict the winter. I can’t even speculate as to why these little caterpillars would need to know the severity of the coming winter, considering that the ones that do not get run over will be bundled up all nice and snug in their little sleeping quarters by winter. They won’t really care about what we humans call a cold or a warm winter. They sleep through it either way.

Entomologists at the University of Massachusetts don’t disagree. They have also discredited the prediction theory, but they say hang on a minute: There could, in fact, be a link between winter severity and the orange band width of a woolly bear caterpillar. They point to their own evidence that the number of orange versus black hairs on a woolly bear has to do with the age of the caterpillar. In other words, the longer the caterpillar lives, the more orange hairs it will have.  The length of time a caterpillar spends as a caterpillar depends upon how soon it began feeding in the spring. The more orange, the longer the caterpillar has been feeding. So their conclusion is this: Band width does say something about a late cold winter or an early warm spring. However, it’s telling you about the previous year, not the coming year. Wide orange bands simply mean the caterpillars went through a warmer winter than those with narrow orange bands.

All of this is interesting but seems to lack value as a weather predictor. It is like looking at a wet rock outside and deducing that it going to rain very soon, or a white rock and deducing that we can expect snow. Most people have already figured out that a winter was especially cold or not long before the woolly bear caterpillar is found and the width of its orange band is measured.

As to your second question about reasoning skills, like you I have I have stopped to watch the behavior of woolly bear caterpillars during late summer and early fall. My personal observations have confirmed to me that woolly bears aren’t very smart. They can’t be very smart, really. I watch them travel back and forth across a busy road as fast as they can. How intelligent is that? I see one desperately humping it across the road, as if it really has a destination, a place to go and a short time to get there — as though a caterpillar starting gun triggered a frantic race.

I have also observed woolly bears going one way and some the opposite, often passing each other in the middle of the road. It is apparent that neither one knows which side of the road the finish line is on, but neither one wants to slow down to ask directions. (Maybe they are all males?) Perhaps a little voice in their brain is saying, “I have absolutely no idea where I am going but I am going to be the first one to get there!”  How smart is that?

I doubt there is much active brainwork or reasoning going on. I am developing similar doubts about people who actually believe that woolly bears can predict the severity of the coming winter. Woolly bears are not meteorologists; never were, never professed to be. People, not caterpillars, started this legend about woolly bear caterpillars predicting the winter.

As for me, I rely on woolly bears to predict one thing, and I am pretty confident in this. Mark your calendar. If you see woolly bears crossing the road in autumn, the coming season is always going to be colder. So, I guess, in a sense they can be used to predict an oncoming winter.

One more prediction that I can make is if the little humpers don’t keep moving, they are going to become road splat when they get run over by my car. Then they will not have to worry about winter. That is my prediction!

Woolly bear caterpillars can’t predict the severity of a coming winter but they do provide bored motorists with something to contemplate: Why DID the woolly bear cross the road?