You may have seen these caterpillars during their earlier stages, with a bunch of their buddies in a silken tent in a host tree. When the caterpillars reach their later stages of development, they will break up the group, heading off individually to continue feeding and eventually pupate. In Montana, the adult moths are usually present from mid to late summer. They can be found from southwestern Canada and the western US to northern Mexico.
Photo by Pat Little on 6/1/20 on Mount Jumbo near Missoula, MT