The Woolly Bear Crosses the Road

Recently, I watched a documentary that depicted the Serengeti’s famous wildebeest migration as the animals forded a river in Tanzania. The wildebeests are reluctant to cross the river because of the huge crocodiles waiting to catch and eat them. While the majority safely make the crossing, a significant number are killed. We, too, have a perilous yearly migration here in coastal Humboldt (though thankfully without the blood and guts of the Serengeti).

First, replace the crocodile-rich rivers with our roads, and substitute the ravenous crocodiles with our car tires. Then, in place of the wildebeests, picture fuzzy black-and-russet woolly bears. And there you have it, a wildebeest migration in miniature. Well, to me, anyway, it’s very similar.

Why do woolly bears cross roads and streets every fall? This annual insect migration is undertaken by moth larvae (aka caterpillars) in the family Erebidae, which includes tiger moths. Locally, the banded woolly bear, which is the caterpillar of the Isabella tiger moth, is quite common. While the banded woolly bear is active all summer, grazing on many different herbaceous plants, it becomes most visible in the fall, when the large colorful caterpillars start looking for somewhere safe and dry to spend the winter. This is when we start to notice them hurrying across roads and sidewalks, bustling here and there. What sort of winter haven are they searching for? Stacked firewood is a favorite, as well as winter tarps and brush piles, all of which make great hostels in which to spend the winter.

Most moth caterpillars spend the winter as pupae inside protective cocoons, but woolly bears overwinter as caterpillars, which is why they need to find winter housing every year. Then, in the spring, they break their hibernation, gorge on plant leaves, and finally get around to pupating. The adult Isabella tiger moths emerge from their silk cocoons in the summer and after the moths mate, the females lay their eggs on vegetation. Then the cycle is repeated and a whole new batch of woolly bears prepares to cross Humboldt’s roads.

Evolution has provided woolly bears with a thick coat of setae (hair) to protect them from being eaten by birds and other insectivorous predators, but it is useless against our vehicles. Some autumns, I Street near the Arcata Marsh is spattered with small piles of orange and black mush.

Nevertheless, the woolly bears persevere. Their great migration story occurs right in our neighborhoods, parks and yards. How many of us see our homes as places to learn about nature? Look closely. What are the big black beetles in your backyard, what does a western swallowtail caterpillar look like and how bright is the glowworm’s glow? Searching for answers (stay tuned for some of them) has truly made my life richer. I hope it can do the same for you.

Pete Haggard (he/him) is the coauthor of Insects of the Pacific Northwest. He writes with editorial assistance from Jane Monroe (she/her), who is determined to help all the woolly bears cross the street. Pete and Jane are the coauthors of the upcoming book ReWilding: How to Create a Pacific Northwest Native Garden.

This article appears in Holiday Gift Guide 2024.