Using molecular tools, we identify Grammia ursina Schmidt, 2009 to be present at high densities on San Clemente Island, California. As a generalist herbivore, G. ursina has many hosts and is a potential pest of rare and endemic plants. We present a list of host records of G. ursina, including rare plants on San Clemente Island. Conservation efforts to support rare plants on the island include outplantings, which have been negatively affected in G. ursina outbreaks. We suggest continual monitoring of G. ursina populations to predict future outbreaks and population peaks and to monitor its effects on endemic plant species and restoration efforts.