Woolly Bear Caterpillar - Pyrrharctia isabella
(Photo: Jo Smith on 11/14/09)
Folklore says: The longer the black ends on a Woolly Bear caterpillar, the longer and more severe the winter will be.
Entomologists say: The amount of black varies with the age of the caterpillar and the moisture levels in the area where it developed. Also, the length of the black ends can vary on caterpillars grown out together from the same group of eggs.
BugGuide makes a half-hearted defense of the Woolly Bear's powers of weather prediction by saying: The variability of the bands depends on many factors. As larvae mature, the reddish bands lengthen. Wetter weather lengthens the black bands. So while not a reliable measure, it makes some sense that onset of an early and thus longer winter will force younger and less red caterpillars into hibernation.
Woolly Bear caterpillars are the larval stage of Isabella Tiger Moths (Pyrrharctia isabella). They are common throughout almost all of North America. Larvae eat many plants and trees including grasses, asters, birches, clover, corn, elms, maples and sunflowers. There are usually two broods of P. isabella each summer. The first of two broods pupates in summer. The second brood overwinters as a caterpillar and pupates in spring. A photo of an adult Isabella Tiger Moth is here.
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