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Can Woolly Bear Caterpillars Predict the Weather?

Updated on October 24, 2017
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Caren White is a Master Gardener and longtime volunteer at Rutgers Gardens as well as an instructor at Home Gardeners School.

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I’ve been seeing a lot of woolly bear caterpillars this fall. They triggered a memory from my childhood of predicting the winter by examining their stripes but I couldn’t remember how to do it. Time for a little research.

What are woolly bear caterpillars?

Woolly bear caterpillars are the larval stage of the Isabella Tiger Moth. You’ve probably never heard of it because they are active at night and therefore rarely seen. Isabella Tiger moths are orange-yellow in color with a wingspan of 1 ½- to 2-inches. Their wings have black dots in a random arrangement. The abdomens are more distinctive with three parallel rows of black dots. They lay their eggs on weeds such as grasses, dandelions, nettles and plantains.

The resulting caterpillars are “furry” with black hair on the front and rear of their bodies and rusty brown in the middle. They are most often seen in the fall when they are hunting for a place to hibernate. They have been known to travel up to a mile a day in search of the perfect spot to spend the winter. Their favorite spots are under rocks, logs or inside bark.

The hair on their bodies is not to keep them warm. Instead, it aids them in freezing in a controlled fashion. They don’t hibernate by slowing down their metabolisms and spending the winter in a stupor like bears. Woolly caterpillars actually freeze almost solid. The interior of their cells is the only part that doesn’t freeze because the caterpillars produce glycerol, a natural organic anti-freeze. They can survive temperatures as low as -90⁰F (-68⁰C).

As the weather warms in the spring, the caterpillars thaw out and start feeding again. Two to four weeks later they pupate, that is create a cocoon, and after another two weeks, emerge as the Isabella Tiger moth.

Isabella Tiger Moth
Isabella Tiger Moth | Source

Why don't I see them in my garden?

Woolly bear caterpillars are assets in your yard because they eat weeds rather than vegetables and ornamental plants so don’t kill them. Just enjoy watching them scurry around looking for a hiding place. But don’t touch them. Their defense is to curl up in a ball and play dead until they feel safe enough to continue on their way.

Playing Dead
Playing Dead | Source

How do their stripes predict the weather?

What I learned as a child is that if the middle brown stripe is wider than the black stripes on the ends, then the upcoming winter will be a mild one. But if the two black strips are wider than the brown stripe, beware! The winter will be severe. The caterpillars have 13 segments, supposedly representing the 13 weeks of winter so you can “predict” when the severe weather will occur. If the front black stripe is wider than the back black stripe, there will be severe weather at the beginning of winter. If the rear black stripe is wider than the front one, the weather will be severe at the end of winter.

So can woolly bear caterpillars really predict the winter weather?

Sadly, no. That is just a delightful myth. The truth is that width of the stripes depend on how old the caterpillar is and how well it has been eating. Older caterpillars will have larger black stripes because they have been feeding and growing longer. Also, if the growing season was good and the caterpillar had plenty to eat, then it will have gotten larger and the black stripes will be longer. If the growing season wasn’t good, with too much or too little rain, then the caterpillars don’t grow as much and the black stripes are smaller.

So the size of their stripes tells you how the weather was during the previous summer rather than what it will be during the upcoming winter.

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    • Kristen Howe profile image

      Kristen Howe 4 weeks ago from Northeast Ohio

      Caren, this was an interesting hub about those wooly bear caterpillars. I haven't seen one in ages. Good wonderful insight on them.