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Woolly Bears Seeking Hibernacula

10-10-18 isabella tiger moth 119

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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6 responses

  1. Brooke Beaird


    Are black squirrels uncommon up here? Never seen on in 40 years until this morning.

    Thanks for your great site,

    Brooke Beaird

    Sent from Brooke’s iPad


    October 10, 2018 at 8:59 am

  2. Alice Pratt

    I’ve seen many Wooly Bears this fall.

    October 10, 2018 at 10:22 am

  3. Kathryn

    If you see some fool driving erratically down the road – that’s me, trying to avoid the woolly bears!

    October 10, 2018 at 10:39 am

  4. Thanks for clarifying the “myth” about the bands predicting the severity of the winter. I always thought the myth was that the wider the brown band in the middle, the longer/harsher the winter. I’ve seen some this year with BIG brown bands and was getting worried 🙂

    October 10, 2018 at 10:46 am

  5. Char Delabar


    October 10, 2018 at 6:20 pm

  6. Jean Harrison

    Beautiful photo. I love the sheen you show on the head and hairs.

    October 11, 2018 at 11:47 pm

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