Woolly Bear Caterpillar

Pyrrharctia isabella
Wednesday, July 21, 2010

This specimen does not immediately resemble the “normal” woolly bear caterpillars, which are black on both ends. It is, however, the same species; these caterpillars also occur in morphs of plain brown, rusty, tan or blonde. They are distinguished from the similar larvae of other tiger moths by the uniform length of all of their bristles, rather than having some longer than others.

The “banded” woolly bear caterpillar (distinct from the yellow woolly bear—a different species) is the larval form of the Isabella Tiger Moth. The moths have a wingspan of 4-5 cm, and are somewhat unremarkable-looking. Their wings are yellow-brown with a series of small, black dots; their hind wings are slightly paler and pinkish with gray spots. The abdomen has three black dots on the rear edge of each segment.

The caterpillars, however, are distinctive and quite well-known. They are generally no larger than 55 mm, and the typical color pattern is a rusty red body with black anterior and posterior ends. However, as mentioned above, they have a variety of different color morphs. In the black-ended caterpillars, however, the black bands shrink while the red part grows as they age. Thus, an older caterpillar would have smaller black bands. The caterpillars are generally very docile and are popular with humans. Unlike other similarly barbed caterpillars, they cannot sting or inject venom with their bristles.

The caterpillars feed on almost any kind of vegetation; they explore many human areas. They are inquisitive, but some will curl up and play dead if threatened. There are two generations of caterpillars each year; one in May and one in August. Caterpillars that have overwintered emerge in the spring, grow, pupate, become moths, mate, and lay eggs that will hatch in time for the August generation. These caterpillars develop into larvae that overwinter in their caterpillar form. The caterpillars can produce an “antifreeze” in their tissues that enables them to survive the winter. Upon the arrival of warm weather, they promptly eat as much as they can, before pupating to become the May generation.

There is some evidence that Woolly Bear caterpillars eat alkaloid-laden leaves to help combat a parasite in their abdomen. This is, among insects, an extremely unusual example of self-medication.

This caterpillar was found by Kim at the golf course.

Article by Hazel Galloway

See images of this organism here, courtesy of Bugguide.net.