The polyphagous shot hole borer, Euwallacea sp., and Fusarium euwallaceae, a fungus it carries, are an exotic pest complex causing dieback and mortality to numerous native and ornamental hardwood trees and shrubs in southern California. The ambrosia beetle carries several symbiotic fungi, including Fusarium euwallaceae which was newly identified as a species associated with recent tree dieback symptoms. Injury occurs when the ambrosia beetle bores into a tree to create tunnels (galleries) for its eggs and larvae and, in the process, inoculates the tunnels with its symbiotic fungi that subsequently clog the surrounding water conducting tissue, or xylem. These pests in combination have killed several hardwood species in urban areas and on National Forest lands. This ambrosia beetle attacks over 20 species as hosts, including bigleaf maple, Acer macrophyllum, California box elder, Acer negundo var. californicum, California sycamore, Platanus racemosa, coast live oak, Quercus agrifolia, castorbean, Ricinus communis, red willow, Salix laevigata, valley oak, Q. lobata, and white alder, Alnus rhombifolia, all of which are found on public land. The large number of hosts increases the likelihood that this pest complex could spread to other areas.