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Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

October 1999

A woolly bear bundled up against the cold. © Don Blegen

A slow crossing to winter

The woolly bear's stripes don't predict cold weather, but we know what makes its coat change color.

Anita Carpenter


A woolly bear bundled up against the cold.

© Don Blegen

Each September and October, legions of woolly bear caterpillars (Acrea sp.) decide that the grass is greener on the other side of the highway. Humping across the road at a fairly respectable four foot per minute pace, the furry caterpillars, (black on both ends, reddish-brown in the middle) are blithely unaware of the consternation they cause drivers who try to avoid them.

Why do these multitudes cross the highways? It's more than getting to the other side. The traveling woolly bears have finished feeding for the year and are moving about searching for the perfect spot to curl up and spend winter under bark, a rock or a fallen log, although I often wonder if the warm road somehow appeals to them.

Since woolly bears are so noticeable in autumn, they have become legendary predictors of the coming winter's severity. Folklore states that the wider the black bands, the colder the winter. Also, some believe the hairier the critter, the harsher the winter.

We take great interest in the woolly's winter coat, but don't give a second thought to what the caterpillar will become. Each fuzzy, 1½ inch caterpillar becomes an Isabella moth.

After wintering in its chosen spot, the caterpillar awakens on a warm spring day and continues to feed.

An Isabella moth, formerly a woolly bear caterpillar.

© S. Borkin
An Isabella moth, formerly a woolly bear caterpillar. © S. Borkin

Soon it forms a cocoon and pupates. In about two weeks, an orange-yellow moth with 1 - to 2-inch wingspan emerges. The wings lack distinctive markings but the abdomen is spotted with three longitudinal rows of small black dots. The moths are active at night throughout summer.

Fertilized female Isabella moths lay eggs in small clusters on a variety of plants including birches, elms, maples, asters, and sunflowers. The eggs hatch in four to five days and the tiny caterpillars begin feeding on their host plants. The caterpillars shed their skins or molt six times before reaching adult size. With each molt, their colors change, becoming less black and more reddish. Thus differing colors among the caterpillars merely reflect age differences, not an indication of impending harsh weather. Two generations of caterpillars are produced each year, so it's the second generation that overwinters.

If you try to pick up a woolly bear, it curls defensively into a tight ball with thick -inch hairs sticking all over to dissuade would-be predators. With luck, the woolly bear survives its cross-country journey, finds a snug place for the winter, re-emerges in spring, and transforms into quite a different creature from the furry little beast that crossed the road last fall.

Anita Carpenter follows the seasons from her Oshkosh home.