The Woolly Bear (Pyrrharctia isabella) is probably the most widely-recognized of caterpillars. Even people with no interest in butterflies and moths can put a name to the black and rust fuzzy creepers. Woolly Bears are especially conspicuous in the autumn, around the time of the first frost. They are often seen crossing driveways or roads in their search for, presumably, an appropriate spot to pass the upcoming winter. Why they should wander is not really well understood, as their diet of greens, including grass and dandelions, is generally readily satisfied. Woolly Bears are neither woolly nor bears. Although they look fuzzy, they are actually covered with short, stiff bristles. Perhaps the “bear” part of their name comes from the way the caterpillars hibernate, similar to bears.
Woolly Bears are so commonplace, they are even the subject of folklore, according to which the width of the orange band can be used as a predictor of the severity of the upcoming winter: the wider the orange band, the milder the winter will be. Really devoted folklorists go even further, suggesting that each of the caterpillar’s 13 segments represents a week of winter. In fact, the condition of the caterpillar is more likely to represent the bounty of the season past, as the caterpillar passes through up to 6 stages or instars on its way to maturity. At each molt, a section of the black is replaced by orange, and thus the orange band is broadest in the last instar. There are a couple of generations over the summer and it is the last generation that overwinters as a caterpillar. It will hide away under bark, leaf litter or a log, and can survive temperatures as low as -90 F.
Surprisingly, while many people can identify a Woolly Bear, few can tell you what the caterpillar will become in its next stage of life. The answer is an Isabella Tiger Moth. In the spring, the overwintering caterpillars warm back up and begin to feed. They then form a cocoon and pupate. When they emerge as moths, fertilized females lay eggs on trees and grasses. When the caterpillars hatch, the cycle begins again. Male and female Isabella moths may be differentiated by the pinkish colour of the underwings of the female.
While Woolly Bears are the most easily-spotted caterpillars out and about, they are not alone. I found another interesting caterpillar on a leaf of a Manitoba maple tree. It’s a Banded Tussock caterpillar (Halysidota tessellaris). Their preferred foodplants include a variety of trees and woody shrubs.
These caterpillars, while not as obvious as Woolly Bears, tend to be conspicuous, resting on the upper surface of leaves in full view. They make little effort to hide themselves, suggesting that they aren’t very tasty morsels. The caterpillar will overwinter as a pupa in a cocoon and will emerge in the spring as a Banded Tussock moth, pictured below. Thank you to Birdgirl for the outstanding moth photographs. You can visit her moth site at North American Moths.