Precise information about endangered species, in particular identifying their resources requirements, is needed to identify areas that might support populations. Little is known about the endangered Mount Hermon June Beetle (Polyphylla barbata) found only within Zayante soils region of Santa Cruz County, California. We investigated the beetle’s host plant selection, habitat association and mating behavior between June 2004 and September 2005. We identified angiosperm and Pteridophyta phyla, and fungi within the frass pellets of Mount Hermon June Beetle larvae demonstrating that they are not specialist feeders but are microhabitat specialists. Larval species was confirmed by DNA analysis. Significant differences were found in vegetation assemblages between regions where the Mount Hermon June Beetle did and did not occur for Chorizanthe pungens var. hartwegiana, and bare ground.