Malacosoma californicum

Malacosoma californicum, the western tent caterpillar, is a moth of the family Lasiocampidae. It is a tent caterpillar. The Western Tent Caterpillar is found in southern Canada, the western United States, and parts of northern Mexico. There are currently six recognized subspecies of M. californicum. Western tent caterpillars are gregarious and will spend a large portion of their time with other caterpillars in silken tents constructed during their larval stage. Western tent caterpillars are univoltine, going through a single generation per year. Adults emerge in the late summer to copulate and lay eggs. Adult moths will preferentially lay their eggs on the sunny side of their host trees. Eggs will lay in diapause over the winter and hatch the following spring. Population sizes of western tent caterpillar can reach outbreak proportions, where populations reach very high numbers and large scale defoliation occurs. Severe outbreaks can cause defoliation of host trees however, damage to trees is minimal and most trees will grow their leaves back quickly.

from Wikipedia