Type: report

Article abstract: The Hermes copper (Tharsalea hermes) is a rare butterfly endemic to San Diego County and northern Baja California. This species is Federally listed as Threatened, with its decline attributed to urbanization, wildfires, and drought throughout its range in the United States. Since most individuals and the two (known) remaining large populations are found in the southern portion of San Diego County, one large fire could extirpate the species in this country. Past efforts have contributed to our understanding of the distribution of the Hermes copper, so it is fairly well understood. This includes wildfires in 2003 and 2007 causing several extirpations with few recolonizations, and more recent droughts further restricting the distribution of this butterfly. We conducted widespread surveys in 2018 with the goal of detecting unknown populations; however, conditions were suboptimal due to below average rainfall. Efforts in 2019 and 2020 followed winters with closer to average precipitation. In 2018, one large population (Roberts Ranch South) was discovered to be larger than previously documented. Since this discovery, no new populations were documented and there was no evidence of recolonization within the 2003 or 2007 wildfire at selected sites. The objective of this project (2025 surveys) was to further assess the distribution and annual population sizes of the Hermes copper butterfly. We conducted surveys in 2025, similar to the 2024 efforts and more sites than sampled during 2019 – 2023. The 2025 status of Hermes copper populations is similar to the last couple years, with few and continued observations at Lyons Valley and Lawson Peak. Two Hermes copper adults were observed west of Potrero at a site not having been documented for this species previously. Given these observations, as well as recent observations at the Potrero BLM transect and past observations at a location between the two, there may be relatively widespread occupied habitat near Potrero. However, given these data, the long-term viability of the species still appears to be highly dependent on the Roberts Ranch South-Bell Bluff area.