Can Wooly Worms Predict The Weather?


Ron Wilson

Wooly worm

Friend and colleague and garden expert Ron Wilson shares another interesting post:

Oct 29, 2020

Bristly “woolly bear” caterpillars commence their annual crawl-abouts in search of sheltered winter quarters in the fall. You may see noticeable numbers crossing roads with some unfortunates becoming laminated onto tires. Their crawl-abouts may start as early as late September and continue until early November in Ohio. It depends on the weather.

Woolly bears (woolly worms in the south) are the caterpillar stage of medium-sized moths known as tiger moths (family Erebidae; subfamily Arctiinae). The caterpillars are so-named because of their short, stiff bristles. The sharp-pointed bristles serve to defend the caterpillars. However, they are not stinging hairs; they do not inject venom. Still, some people suffer severe localized reactions if the hairs penetrate their skin. 

Woolly bears will roll themselves into a tight ball when disturbed to bring to bear their defensive bristles. Their resemblance to hedgehogs is referenced by the alternate common name “hedgehog caterpillars.” You may see this defense posture in response to various wasps such as yellowjackets (family Vespidae) that attack and feed on caterpillars.

Four of the most common woolly bear species found in Ohio are the banded woolly bear which develops into the Isabella tiger moth (Pyrrharctia isabella); the yellow woolly bear which develops into the Virginia tiger moth (Spilosoma virginica); the salt marsh caterpillar which develops into the salt marsh (tiger) moth (Estigmene acrea); and the giant leopard moth caterpillar (Hypercompe scribonia (syn. Ecpantheria scribonia)).

These woolly bear caterpillars may be found feeding on a wide range of plants including some field crops. In fact, crop harvests commonly produce an early flush of caterpillars crawling across nearby roads.

The four species of woolly bear moths have two generations per season in Ohio with the largest number of caterpillars occurring in the second generation. This is one reason we typically see more caterpillars in the fall. Of course, the other reason is that their mass fall crawl-abouts in search of protected winter quarters commonly bring them out onto hiking trails, sidewalks, roads, and up onto the sides of homes and other structures. 

Research conducted by Jack Layne, Department of Biology, Slippery Rock University, revealed that the woolly bear caterpillars of the Isabella tiger moth and the giant leopard moth produce antifreeze-like chemicals, collectively known as “cryoprotectants,” to prepare themselves for winter. The cryoprotectants prevent sharp-pointed ice crystals from forming which would puncture cell membranes.

Do Banded Woolly Bears Predict Winter Weather?

The banded woolly bear (P. isabella) is the species most often referenced as the “official” predictor of winter weather for one obvious reason; it’s banded.  Giant leopard moth caterpillars are completely black which may provide a good excuse to spend the winter in Florida.

According to folklore, the greater the amount of black on a banded woolly bear, the more severe the winter weather. Also, the position of the widest dark bands predicts which part of the winter will be the coldest. If the dark band is widest at the head end, the beginning of winter will be severe. If the dark band is widest at the tail end of the caterpillar, winter will go out like a lion. The predictive ability of the caterpillars is further fine-tuned by “reading” the 13 segments of the caterpillar’s body which are said to correspond to the 13 weeks of winter.

This weather folklore dates back to the American colonial days. However, we can thank Dr. Charles Howard Curran for giving credibility to this myth; perhaps inadvertently.

Curran was a noted entomologist who served as Curator of Insects and Spiders for the American Museum of Natural History (AMNH) in New York City (NYC) until his retirement in 1960. On October 27, 1948, Curran and fellow AMNH entomologist Mont Cazier, along with their wives, traveled to (Woolly?) Bear Mountain State Park about 40 miles north of NYC. They collected 15 banded woolly bear caterpillars and dutifully measured the lengths of the black end bands and rusty brown middle bands.

Curran was a respected scientist who published widely in scientific journals, most often on Diptera. There remains much debate as to whether or not this expedition was a serious attempt to test the theory wrapped in folklore that the caterpillar bands predict winter weather.

Instead of producing a scientific paper to be perused and parsed by his entomology colleagues, Curran’s “survey results” predicting the winter weather for 1948 were announced by news reporter John O’Reilly on the front page of the October 28, 1948, issue of the New York Herald Tribune. Curran’s caterpillars predicted a mild winter … which turned out to be correct.

Of course, this produced a demand by the Tribune readers for an annual winter weather prediction by Curran’s caterpillars which continued for seven more years. I couldn’t find information on how often the caterpillar predictions were correct. However, I found several reports that Curran recognized his sample size was always too small to be of any scientific value. One may assume he simply used the caterpillars for their entertainment value.

More rigorous research subsequently debunked the winter weather prediction value of banded woolly bears. The caterpillars commonly show high variability in their coloration based on their age, food sources, and moisture levels in the area where they develop.

You can see the variability in the image below. These banded woolly bears were collected on the same date from around a building in southwest Ohio that is surrounded by landscape flower beds as well as nearby crop fields or fallow ground with a wide range of native and non-native vegetation. It was not a scientific study, but no color form was excluded during the collection. I’m not making a winter weather prediction, but I like the prognostication of the soothsayer caterpillar pictured at the top which I named Optimystic.

Of course, caterpillar coloration also varies between woolly worm species. If weather prognosticators accidentally use the yellow color form of the yellow woolly bear, they would assume there will be no winter. Conversely, the mostly black color form could create mass panic causing folklorists to flee to the south for the winter!

Regardless of the folklore, I believe Curran was onto something by recognizing the pure entertainment value of woolly bear caterpillars. After all, there aren’t too many insects that have engendered annual festivals in their honor. The annual Woollybear Festival has been held every fall since 1973 in beautiful downtown Vermilion, OH, on the shores of Lake Erie. The Woolly Worm Festival of Banner Elk, NC, has been held each fall since 1978 in the beautiful North Carolina High Country. Sadly, both were canceled this year because of COVID-19. But just as woolly bears will surely reappear in 2021, so will their festivals (we hope). Me and Optimystic are planning some road trips!

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