What is it? Polyphemus Moth cocoons are generally not so elongated and hang from the branch of the food plant. Luna Moths incorporate leaves in their cocoons, which are hidden in the litter on the ground rather than attached to the host plant as this one is. The Columbia Silk Moth prefers to feed on American Larch in the east and this pupal covering is a long way from the nearest larch. Based on how the cocoon is attached along the length of this Boxelder twig I'm going to go with a Cecropia Moth or Robin Moth (Hyalophora cecropia). Time will tell ...
