Article by Anita Carpenter  in the Wisconsin Department of Natural Resources magazine

From the October 1999 issue:
 

                                   A slow crossing to Winter

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

                                    by:  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.  Soon it forms a cocoon and pupates. In about two weeks, an
                orange-yellow moth with 1 ½- to 2-inch wingspan emerges. The wings lack distictive
                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 ¼-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.

All credit for this article goes to Anita Carpenter as published in the Wisconsin Department of Natural Resources magazines.
All photos are credited to the contributor as seen in the article at: http://www.wnrmag.com/stories/1999/oct99/wbear.htm