Banded Woolly Bear (Pyrrharctia Isabella)

A sure sign that summer is waning and winter not far behind: a banded woolly bear caterpillar creeping across a road or path. Where do these ubiquitous caterpillars come from? And where are they going?

Life Cycle. The banded woolly bear is the larval form of the Isabella tiger moth, Pyrrharctia isabella, a creamy to pale orange moth you may see flitting around outdoor lights at night in summer. Adult female Isabella tiger moths lay clusters of eggs on the surface of plants. After a few days, a tiny caterpillar emerges from each egg. The caterpillar goes through four to six growth stages, shedding its skin each time, then spins a silk cocoon and forms a pupa inside the cocoon. In one to three weeks, the moth emerges, finds a mate, lays eggs, and begins the cycle again, sometimes two life cycles in a summer. Woolly bears at the end of summer overwinter underneath leaf litter and pupate in spring. A chemical called glycerol inside its body acts as an antifreeze, preventing the frigid temperatures from killing the caterpillar.

Appearance. A woolly bear caterpillar is about 2 inches from end to end. Long, stiff fibers called setae radiate from its body, giving it a furry look. The setae on two ends are dark brown to black, and those toward the middle of the body are reddish-brown. The woolly bear often rolls into a ball when disturbed.

Range and Habitat. Woolly bear caterpillars live throughout most of Minnesota. They prefer damp places with lots of vegetation.

What They Eat. In the caterpillar stage, Pyrrharctia isabella eats pretty much any kind of plant material. The adult moths sip nectar from flowers.

What Eats Them. Some birds, such as cuckoos, catbirds, and spotted towhees, can eat hairy caterpillars such as the woolly bear. However, other birds and mice are deterred by the caterpillars’ long, prickly hairs and bad taste.

Climate Connection. Scientists have found that freezing and thawing multiple times can make it harder for woolly bear caterpillars to survive the winter. As climate change makes periodic thaws more frequent, woolly bears may be less likely to survive long enough to make more woolly bears.

About Those Bands. You might have heard that the width of a woolly bear’s brown and black bands can predict how severe a winter might be. In reality, the width of the bands has to do with the caterpillar’s age. The older the caterpillar, the wider the reddish-brown band around its midsection.