As you can see, unlike many caterpillars that feed alone, these guys like company and spend a large part of their caterpillar life with a bunch of friends in a silken tent. The Western Tent Caterpillar can be found from southwestern Canada and the western US to northern Mexico. During years where their population is high, the caterpillars can completely defoliate their 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 found from mid to late summer.
Photo by Carolyn Taber on 5/6/20 in Clinton, MT.