The Rocky Mountains divide the US geographically. Crossing the Rockies means driving over high mountain passes. At higher elevations, the climate and the ecosystems change. Insect species that can survive at low elevations, may not be able to survive the climate at higher elevations. Thus, the divide can be a barrier to insect movement.
East of the Rockies, Tent Caterpillars are primarily Malacosoma americanum, the Eastern Tent Caterpillar. West of the Rockies, the primary species is Malacosoma californicum, the Western Tent Caterpillar. At one time, these species may have had a common ancestor. However, when populations are separated geologcially, they become genetically isolated. The populations east of the Rockies become better adapted to the eastern conditions and the populations west of the Rockies become better adapted to the western conditions. Over time, the populations accumulate enough genetic differences such that they can no longer interbreed. Then they are separate species.
Because they are closely related, they maintain some of the same habits. The Eastern Tent Caterpillars will build silk tents for protection in the crotches of tree branches. The Western Tent Caterpillars will build silk tents that look very similar. The coloration of the caterpillars is similar.
The Western Tent Caterpillars will feed on aspen, willow, and other tree species. There were a number of Western Tent Caterpillar tents on the trees in Capital Reef National Park in Southern Utah. Homeowners typically dislike the tent caterpillars because they ruin the aesthetic quality of the trees. However, in natural areas, the tent caterpillars thin the foliage and allow sunlight to reach the seeding trees and help them grow.