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Out There: A Silkmoth Story

By Ed McMackin, biologist by profession and naturalist by nature
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A Polyphemus silkmoth hangs from the cocoon in which it spent the winter - 5.5 inch wingspan, Big Bear FSR, late June. It was previously seen pictured from a reader in our “Weekly Wildlife” section. (Photo by Ed McMackin)

By Ed McMackin, biologist by profession and naturalist by nature

In North America, there is a group of large moths called “silkmoths”. Locally, in the Kootenay/Columbia Region, there are three species of silkmoths. Generally eastward of, and including, the Selkirk Mountains, is Glover’s Silkmoth and westward and southward in the Pacific States, in the dry regions, is the California (Ceanothus) Silkmoth. Sometimes hybrids occur where these two species converge. The third and one of the largest of the silkmoths, is the Polyphemus (Antheraea polyphemus), which is found all across North America, from the Pacific to the Atlantic Coast. Then there are silkmoths in South America and across Europe and Asia.

The name “silkmoth”, I believe, came originally from an Asian species, Bombyx mori (Chinese Silkmoth). The Chinese, many years ago, mastered the separating of the silk fibers from the cocoons of Bombyx mori. The process was also tried with North American silkmoths, especially the Ailanthus Silkmoth (which has an eight inch wingspan), of the Atlantic Seaboard of the U. S. These attempts to unwind the silk thread, in the long term, were not successful, as the silk strands were not continuous. The Chinese Silkmoth caterpillar weaves its cocoon from one continuous thread.

Silk from the Chinese Silkmoth was, at one time, very valuable because it could be woven into very fine fabric, including a fine silk velvet. Queen Elizabeth II wore a royal blue, velvet gown woven from Chinese silkmoth cocoons. Probably other royal garments were also woven of Chinese silk. The silk was so valuable that the “silk trains”, carrying silk from pacific ports, to eastern cities, were heavily guarded. Significant use of Chinese silk ended with the introduction of other less expensive fabrics.

North American silkmoths, in addition to the Polyphemus, were the Cecropia, Columbia, Luna, and Promethea silkmoths. In the last 50 years, occurrences of the Columbia and Promethea has dropped significantly. The Columbia Silkmoth caterpillar, which feeds on eastern tamarack, like the Glover’s Silkmoth and the Cecropia Silkmoth caterpillar attaches the cocoon, the full length, along a twig or stem of the host plant or nearby.

The Promethea moth caterpillar spins the cocoon in a rolled-up leaf which dangles all winter from the shrub. The Luna and the Polyphemus moths make globular cocoons that hang by the top-end from a twig near the ground or in a clump of dead grass. Where the winters are not so severe they may hang their cocoons from a small branch in a tree. Once, in southern Washington, I found 15 or so cocoons hanging from the branch tips of a weeping birch. I have also found the cocoons of the Polyphemus low down near the base of the food shrub or tree.

Some people get alarmed when they learn that those large moths lay eggs on shrubs or trees or find one of the fat green caterpillars, as large as a thumb, munching on leaves of their maple or birch tree. Those big Polyphemus moths lay their eggs individually as much as 200 feet apart.

When I was a bit younger in the Maritimes, one of my past times in the fall and winter, was to tramp along old roads, around edges of woods and alder swamps looking for cocoons, Understandably, the cocoons were easy to spot. I did find, in one bush, 13 cocoons of the seldom seen Promethea Moth.

Finding cocoons means applying some effort and is probably most successfully done, after learning what occurs in your area, what the cocoon looks like, and by looking for them along trails, on town and country walks, in or at the bottom of isolated shrubs or trees, on stems or in dry grass (Don’t stare into someone’s yard too long without explaining). As for the silvery-gray Glover’s silkmoth cocoons, check out the edges of forest roads after most of the snow is gone.

If you spot a cocoon, take a photo and leave the cocoon it in its found environment. Or if you do collect a cocoon with whatever supports it, keep it outside until the end of June in a shaded spot, in a cage to protect it from birds and mice. Keep a close eye on it and wait for it to emerge, starting in May.

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