Banded Woolly Bear / Isabella Tiger Moth
| Scientific Name | Pyrrharctia isabella |
| Survival Strategy | Freeze tolerance |
| Conservation status | Secure |
Spring
The banded woolly bears that survived the winter become active in early spring, spin cocoons, and after emerging, take flight as Isabella tiger moths. In May, these moths mate and deposit clusters of 50 or more eggs on any plants their larvae use for food. The adult moths die shortly after mating, and the eggs that hatch in 5 to 12 days are the beginning of the year’s first generation of Isabella tiger moths.
Summer
The first-generation woolly bear larvae (caterpillars) go through five instars, growing to about five centimeters (2in) in length. They will find a suitable place to spin a cocoon (pupate) from a mixture of body hair and silk. The adult moths will emerge from the cocoon after 1 to 3 weeks. By August, their offspring will become the second generation of Isabella tiger moths.
Fall
In late fall, the larvae of the second generation will seek protective cover under leaf litter. This search for winter shelter is why you often see wooly bears “wandering” this time of year.
Winter
Wooly bears freeze solid and wait out the winter weather.
Behavior
Imagine a Cross Between an Armadillo and a Porcupine
Woolly bears will curl into a ball if a predator or a curious 5-year-old gets too close. The stiff hairs protect the larvae from wasps and other predators but make them even more attractive to 5-year-old entomologists.
Physiology
Quiescence
When cold weather sets in, larvae enter a state of dormancy called quiescence (think hibernation in insects). A January thaw, or the warmth of spring, will stimulate them to become active again.
Freeze Tolerance
The water in woolly bear larvae freezes solid during winter. Water is one of the few fluids that expands as it freezes. Anyone who has put a beverage in a freezer to cool quickly knows what will happen if they forget to take it out. The expanding water destroys the can or bottle!
So how do woolly bear larvae keep ice crystals from ripping their cells to shreds? They use two tricks of biochemistry. First, they produce a chemical (ice-nucleating protein) in the fluid between cells that make ice crystals form slowly and stay small. The slow formation of ice reduces the pressure differences on either side of the cell membrane so that the ice will only do minor damage.
For their second trick, the woolly bear larvae produce two antifreeze proteins, glycerol and sorbitol (cryoprotectants), which lower the larvae’s blood (hemolymph) and the fluids inside the cells.
Diet
Woolly bear larvae eat a wide variety of leafy plants. These include dandelions, goldenrod, plantain, and many types of tree leaves. Woolly bears will occasionally resort to cannibalism.
The adult tiger moths feed on nectar from flowers using their siphoning mouthpart called a proboscis. The proboscis works like a straw in a juice box.
Lifespan and Mortality
The adult tiger moths live for 1 to 2 weeks, during which they mate, lay eggs and die.
Predators
Predators of the woolly bear larvae include parasitic wasps, skunks, birds, and flies. The adult moths are preyed upon by birds, spiders, bats, and other insectivores.
Climate Vulnerability
Repeated freeze-thaw cycles have become more common as the climate warms. One study of woolly bear larvae found an increase in mortality and tissue damage after repeated freezing compared with caterpillars that remained frozen.
Never stop learning
Cocoons vs. Chrysalises
Cocoons and Chrysalises both protect the larvae during the last stage of metamorphosis. Cocoons are created by moths and enclose the pupae. Moths use their spinneret, found just below their mouth, to squeeze out a liquid that hardens quickly in the air to become silk thread. Chrysalises are the pupae stage of butterflies. The outer “shell” of the chrysalis is the exoskeleton made during the last molt.
References
| Layne, Jack R., et al. “Cold Hardiness of the Woolly Bear Caterpillar (Pyrrharctia Isabella Lepidoptera: Arctiidae).” The American Midland Naturalist, vol. 141, no. 2, University of Notre Dame, 1999, pp. 293–304, http://www.jstor.org/stable/2426919. |
| Rozsypal J. The role of water, ice nucleators, and inoculation in insect cold survival. Open Access Insect Physiology. 2015;5:21-30 https://doi.org/10.2147/OAIP.S64952 |
| The Infinite Spider: The Woolly Bear Caterpillar in Winter https://infinitespider.com/the-woolly-bear-caterpillar-in-winter/ |
| Marshall KE, Sinclair BJ. The sub-lethal effects of repeated freezing in the woolly bear caterpillar Pyrrharctia isabella. J Exp Biol. 2011 Apr 1;214(Pt 7):1205-12. doi: 10.1242/jeb.054569. PMID: 21389206. https://pubmed.ncbi.nlm.nih.gov/21389206/ |
| Butterflies and Moths of North America: Isabella Tiger Moth or Banded Woolly bear https://www.butterfliesandmoths.org/species/Pyrrharctia-isabella |