On 7 May 2016, I retrieved a large caterpillar from a valley oak tree. I attempted to raise it, but the caterpillar died in the cocoon stage. Dr. Lynn Kimsey (thanks!) identified it as a Pacific tent caterpillar, Malacosoma constricta. It was a solo individual, and no netting or tenting was seen. Below are photos of the frass and the caterpillar. For image of moth, see this page.
Distribution of the Pacific Tent Caterpillar