With a wingspan of up to six inches, the adult Polyphemus Moth is the largest species of giant silk moth in Montana …so it should come as no surprise that their caterpillars are pretty massive too (3 to 4 inches in length when fully grown). They are found all across the lower 48 states (except Arizona and Nevada) and southern Canada. The hungry caterpillars can eat 86,000 times their own weight over the two months it takes to grow until pupation, happily munching on a variety of trees and shrubs including oak, willow, maple, and birch.

Cathy also included a photo of the silk cocoon it spun, where it will spend winter in the pupal stage. Typically, they are attached to a tree branch, or among leaves or grasses on the ground. Squirrels, woodpeckers, and mice are among the potential predators. 

Polyphemus Moths emerge in June, but have to do a little work first. Mary Holland, author of the blog Naturally Curious with Mary Holland, explains, ”Unlike most other giant silk moths’ cocoons, the Polyphemus Moth cocoon lacks an escape “valve” at one end. In order to emerge (as an adult) from the cocoon the summer after it spins it, the moth secretes an enzyme that digests and softens the silk at one end. Then it moves about the cocoon in a circular pattern, tearing the softened silk with two spurs located at the base of each wing on its abdomen. Eventually it escapes by splitting the silk and pushing the top up.”

Size: 3 to 4 inches when fully grown 

Photos by: Cathy Houle Barrows on 8/16/22 and 8/17/22 in Arlee, MT