They’re on the March

We often think of migration as long-distance treks by birds, mammals, and fish traveling to wintering or nesting grounds. Animal migrations, however, can be short. They can even happen right in your own yard or neighborhood. Right now, Banded Woolly Bear caterpillars (Pyrrharctia isabella) are on the move to find places to overwinter, such as under a log or in leaf litter. You can see them crawling around in many different habitats. Taking a walk around the neighborhood recently, I spotted quite a few and helped them cross the road.

The Woolly Bear caterpillar is the larva of the Isabella Tiger Moth. They are found throughout most of the United States, Canada, and Mexico. These two-inch (5 cm) caterpillars are usually black on the ends and brown in the middle. If you have never touched a Woolly Bear, they don’t actually feel like wool. They are covered with 13 rows of soft bristles called setae.

There is some weather folklore associated with Woolly Bears. You might have heard that if the caterpillars have a wide brown middle band, it will be a mild winter. Or if the black bands at the end are wide and the middle brown band is narrow, expect cold and snow and a more severe winter. In reality, differing band length is partly due to genetics and how mature the caterpillar is, as the brown bands tend to widen with age. Research suggests wetter weather can also widen the black bands.

Other lore has to do with direction. If the caterpillar is traveling south, expect a harsh winter. If it is going north, winter will be mild. I’m not sure about east or west though!

Woolly Bear larvae have been found as far north as the Arctic, where temperatures can reach minus 90°F (–68°C)! These larvae are amazingly adapted to get through harsh winters—their bodies produce a cryoprotectant, an anti-freeze with glycerol, that keeps cells from freezing. The caterpillar’s setae also protect it from fluctuating winter temperatures. When spring temperatures rise to 50°F (10°C), the larvae will spin cocoons and a few weeks later pupate into adults.

In Connecticut, there are two broods of Woolly Bears. The first brood ecloses, or hatches, from cocoons and emerges as adult Isabella Tiger Moths in mid-summer. The second brood overwinters as larvae to pupate in the spring. Female moths put out a scent to attract males and after mating will lay batches of 100 or more eggs. The eggs hatch four to five days later. Woolly Bear larvae are generalist feeders and eat a variety of herbaceous plants, shrubs, and trees.

Next time you see a Woolly Bear caterpillar, you can thank yourself for leaving leaves and logs in your yard to help them to get through the winter.