Tuesday, March 23, 2021

As the temperature heats up, so too does the activity of woolly bears

Woolly BearAs the old saying goes, “March comes in like a lion and goes out like a lamb.” During this transition, overwintering insects begin to reanimate. One insect that will soon regain mobility is the woolly bear, Pyrrharctia isabella Smith (Lepidoptera: Erebidae). The life cycle of this insect is complex, but if it is properly understood, then lepidopterists will have a much better chance of seeing one in the wild. 

The woolly bear overwinters as a larva. As the temperature gets cooler, the woolly bear larva will bask in the sun, using its dark coloration to gather heat. When the autumnal temperatures drop too low for basking to be sufficient, woolly bears ensconce themselves in leafy detritus. Snowfall serves to further insulate the moth from biting winter winds. Woolly bears are further protected by the chemical glycerol, which is produced by their bodies to protect them from extreme cold. This chemical is found in some antifreeze brands, and can be used in cars. Through strategic selection of overwintering sites and the use of glycerol, the woolly bear can survive temperatures as low as -60 oF.

Their survival can be put at risk if they are brought out of dormancy by unseasonal warmth, because they stop producing the chemicals necessary to protect themselves. Therefore, if a woolly bear is encountered in the wintertime, it is recommended that nature enthusiasts leave it alone.

Supposing that the insect survives the winter, it breaks dormancy in the spring. The larvae briefly feed on violets, clovers, nettles, lambsquarters, dandelions, burdock, maple, elm, and birch. After breaking their fast, they seek a location to pupate. To do so, woolly bears form webbed cocoons on the sides of buildings and trees. Despite looking like a clump of hair that was removed from a clogged sink, the woolly bear cocoon is capable of concealing the insect as it turns into an adult. The cocoon’s silk will hold it in place, provide camouflage, and protect it from the elements. The insect uses saliva, hair-like strands called setae, and silk to construct its hairball-shaped fortress of solitude. 

After several weeks, the adult emerges from the cocoon. This stage of the insect is responsible for mating and laying eggs. Adults typically do not feed. The male is usually orange in coloration, and the female is orange with pink patches in the center of the hindwing. After mating, the female lays eggs on many weeds, grasses, and herbs. After the eggs hatch, the smaller larvae eat the host plant, and crawl towards the plants mentioned in the above paragraph. On occasion, woolly bears have eaten garden vegetables like lettuce, spinach, and broccoli seedlings; however, they are not usually pests of economic importance. They repeat this cycle twice per year, with the first generation hatching in May, and the second hatching in August.

Based on the mobility of woolly bear larvae, it is easiest to see them when they wander onto nature trails or roads. They should be getting active now that the weather is heating up, but if you miss them, you’ll have another chance to see them in the fall. From May to September, the adults are found at night, and will respond to light. If you have a UV lamp, that would be a highly effective tool for luring them out of the woods. 

Humans are not the only animals that seek out woolly bears. Parasitoid flies and wasps, as well as birds, are known to attack woolly bears. Fortunately for the woolly bears, they can tolerate naturally occurring toxins, such as pyrrolizidine alkaloids. These toxins are definitely harmful and oftentimes deadly to the wasps and flies that lay their eggs inside the caterpillars.  Chickens and ducks that eat woolly bears are at risk of getting sick and possibly dying from pyrrolizidine alkaloids, if they eat too many woolly bear larvae. 

Although this article has focused on the woolly bear known to scientists as P. isabella, it is worth mentioning the existence of another insect commonly referred to as a woolly bear. This moth, Grammia incorrupta Edwards (Lepidoptera: Erebidae), actually increases its consumption of plants containing pyrrolizidine alkaloids, such as rattlepods, ragworts, and flowering plants in the borage family.  For more information about this process, consider the paper by Michael Singer, “Self-Medication as Adaptive Plasticity: Increased Ingestion of Plant Toxins by Parasitized Caterpillars,” published in the journal PLoS One, in 2009. Alternatively, Ed Yong wrote a nice synopsis of the paper in his article, “Self-medicating caterpillars use toxic plants to kill parasites,” written for National Geographic.

Woolly bear caterpillars will soon be on the march in the Adirondacks. If you see one, feel free to post your picture in the comment section below. Happy hunting!

Picture of woolly bear larvae taken north of Broome County, NY, on Sept. 28, 2018. The moth shown is the adult of the woolly bear species Pyrrharctia isabella Smith (Lepidoptera: Erebidae). The image credit belongs to Andy Reago and Chrissy McClaren. The picture was taken Aug. 8, 2014. 

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Shane Foye is an entomologist volunteering with the Cornell Cooperative Extension of Essex County Agriculture Program. Learn more about Shane and the on-farm projects he's working on at the CCE Essex Agriculture page: http://essex.cce.cornell.edu/agriculture/biocontrol-projects

3 Responses

  1. Mark Friden says:

    With regard to the old saying you mention in your opening, in my family we’ve always said that, here in the Adirondacks, “May comes in like a lion and goes out like a lamb.” I still always have the snow shovel next to the front door until Mother’s Day, and even with that, have been caught by over a foot of snow falling after that holiday!

  2. Jackie Woodcock says:

    Great Article. I really enjoyed this!

  3. Karen E Linder says:


    Thank you for a fabulous article. I just retired from a long career and am focusing energy on expanding my skills as the Nature Lady. I learned so much from your article. It will be passed on with joy. Query. Do they bask on asphalt? Or are they just more obvious on the road? I will be watching next spring.