Isabella Tiger Moths (we call their larvae Woolly Bears) are one of the few moths or butterflies that overwinter as caterpillars. In the fall they produce a chemical which acts like anti-freeze and protects them against damage from freezing and thawing. The caterpillars remain curled up in a protected spot, such as in leaf litter or under loose bark, nearly frozen solid all winter.
When spring arrives and the temperature reaches the high 40’s and 50’s they become active again, feed for a few days, and then pupate inside a cocoon made with their own bristles. Adult Isabella Tiger Moths emerge in about a month, anywhere between April and June, mate, and lay eggs. Within two weeks the eggs hatch. In New England a second generation of woolly bears will be produced and these are the larvae that overwinter.
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The Isabella Tiger Moth typically has two broods during the summer. The caterpillars (Woolly Bears) in the first brood pupate and emerge as adult moths mid-summer. The second brood overwinters as caterpillars and pupate in the spring. The Woolly Bears we see crossing roads at this time of year are second-brood caterpillars in search of protective hibernation sites (hibernacula).
Old-timers predicted the severity of the coming winter by the relative lengths of the black and brown bands of the caterpillars when they became easy to observe in the fall – the longer the black sections and narrower the brown section, the harder a winter they were in for. In fact, this may have had some validity, as brown hairs (setae) are added to the middle band every time the caterpillar molts. Therefore, the older the caterpillar, the wider the brown band. If winter comes early, the caterpillar’s brown band would be relatively narrow due to the fact it didn’t have time to mature fully and develop a wider brown section before hibernating.
The adult stage of the Isabella Tiger Moth is often overlooked, due to the appeal of the larval stage. This tan moth, with a wingspan of 1 ½ – 2 inches, has tiny black markings on its wings. Male and female are sexually dimorphic and can be distinguished by the color of their hind wings. Males have yellow-pale orange hind wings while the hind wings of females are rosy. (Photo: Woolly Bear; photo inset: female Isabella Tiger Moth)
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