Can Caterpillars Forecast the Weather?

by Bram de Veer

This week, we have a guest blogger! Bram de Veer looks at signs we can (or can’t?) use to tell if we will have a long, snowy winter.

Growing up in New England, I learned that a big crop of acorns in the fall (and a sudden proliferation of squirrels) were a sure sign that Mother Nature was preparing for a long, cold, and snowy winter. I wasn’t sure if this was an ancient legend or folklore, but it made sense.

My schoolmates and I also learned that the “Woolly Bear” caterpillar with its distinctive black and brown stripes was another harbinger of the winter ahead. These adorable pandas of the insect world (also known as the woolly worm) are the larvae of the Isabella tiger moth (Pyrrharctia isabella) and it’s commonly believed that the severity of winter can be foretold by the width of its brown middle stripe. The thicker the stripe, the milder the winter.

I never even thought to question it until this fall, when I spotted 2 woolly worms while out for a walk. The first one was almost entirely brown (uh oh, I thought – another example of global warming). But a minute later I found one that was equally brown and black. Here they are:

   Was this simply an example of genetic variability? Is there any truth to the myth? Is there a correlation between Woolly Bear caterpillars and the intensity of winter?

As it turns out, in the 1940s and 1950s, Dr. C. H. Curran, curator of insects at the American Museum of Natural History in New York City, studied the Woolly Bear caterpillar in an effort to make that determination. Over a period of eight years, Dr. Curran carefully sampled and measured caterpillars and their stripes and recorded the weather, and in the end he found that there was virtually no correlation.

More recently, other hypotheses have suggested that the brown stripe is actually an indication of the age of the caterpillar. The more brown, the older the caterpillar. In fact, seasonal variability can influence when they hatch. So, the width of the stripe may be dependent on weather – not the other way around.

Natural selection may also play a role. Woolly Bears typically overwinter under leaves before pupating in the spring, and their survival (as well as the traits passed to their offspring) depends on their ability to camouflage in a suitable spot. Over time, it’s possible that the predominant color may trend towards brown or black depending on the weather and their environment.

In addition to acorns and Woolly Bear caterpillars, The Farmers’ Almanac cites other omens such as “Thicker-Than-Normal Corn Husks,” “Woodpeckers Sharing a Tree,” “The Early Arrival of the Snowy Owl,” “Thick Hair on the Nape of a Cow’s Neck,” and “Spiders Spinning Larger-Than-Usual Webs and Entering the House in Great Numbers” among the Signs of a Hard Winter Ahead.

Interestingly, Woolly Bear caterpillars are also known for “enrollment”, a protective reflex where they curl up instinctively to protect their vulnerable underside. Many other animals and insects exhibit this behavior; ancient trilobites can be found enrolled in the fossil record, and their living relatives are sometimes called roly polys (they are also known as potato bugs, pill bugs, or doodle bugs). These tiny terrestrial crustaceans can usually be found under a rotting log and play a critical role in the decomposition of wood and cellulose, and in forest regeneration.

All insects have a close relationship with the weather. For example, many species of cicadas famously stay dormant underground – some for up to 17 years before they emerge. Cicadas are also known as “nature’s thermometers” because they have been known to start “singing” in late summer, 6 weeks before the first frost in temperate climates.

Some insects even impact the weather. Locusts are a type of grasshopper that have been known to swarm in their own clouds consisting of millions of insects (also known as a “grasshopper plague”). These swarms can be carried for thousands of miles by the wind, while destroying crops and vegetation as they devour every green thing in their path.

Getting back to acorns. It seems logical – almost like second nature – that Oak trees would produce an abundance of acorns before the arrival of severe weather. Why? Investing their energy in producing lots of seeds increases the chances of viability and propagation. Maple trees do the same thing when they produce thousands of “helicopters”; these winged seeds (also known as “samaras” and sometimes called “whirligigs”) travel with the whim of the wind like the seeds of a dandelion. And of course, most seeds are entirely dependent upon temperature and moisture in order to germinate.

So, plants and animals might not predict the weather, but all migratory species – from whales and sea turtles, to geese and monarch butterflies are somehow programmed to travel thousands of miles to other climates in anticipation of the changing seasons. It’s pretty impressive. And I’d venture to say that not one of them has a degree in meteorology.

I have noticed a lot of spiders in my house lately. I guess it’s time to stock up on firewood.

5 thoughts on “Can Caterpillars Forecast the Weather?

  1. James Morris Post author

    This is a great blog post – thanks so much. It reminds me of a science fair project that my sister Pam did in high school. She looked at the leaves of Rhododendrons, which curl up when it’s cold. In fact, the amount of curling is correlated with the temperature, so you can use the leaves as a thermometer. Another example of nature’s thermometer!

  2. Emily Koch

    Perfect timing! I have been seeing a bazillion wooly caterpillars recently and was wondering about that old myth!

  3. Paul S Morris

    What a fascinating blog. Dot Andrews, everyone’s favorite science teacher from The Bromfield School in Harvard MA would be so pleased! I love hearing about new ways to predict events and trends like the weather, but it’s a tricky business.

    1. James Morris Post author

      Yes, Dot Andrews was a fantastic teacher, and encouraged all of us to “stand and stare,” which Bram does so well in his blog.


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