The Xerces blue (Glaucopsyche xerces) is an extinct species of butterfly in the gossamer-winged butterfly family, Lycaenidae. The species lived in coastal sand dunes of the Sunset District of the San Francisco Peninsula in California. The Xerces blue is believed to be the first American butterfly species to become extinct due to loss of habitat caused by urban development. The last Xerces blue was seen in 1941 or 1943 on land that is now part of the Golden Gate National Recreation Area.
Taxonomy
Genetic analysis of museum specimens published in 2021 confirmed that the Xerces blue represented a distinct species within the genus Glaucopsyche, with its closest relatives being Glaucopsyche australis and Glaucopsyche pseudoxerces, with this group most closely related to Glaucopsyche lygdamus (the silvery blue).right|thumb|Specimens in [[California Academy of Sciences]]
Extinction and ecology
The species was first described and documented in 1852. The butterflies fed on vegetation belonging to the genus Lotus and Lupinus.
Reestablishment efforts
Efforts are ongoing to reestablish related butterflies in the Xerces blue's former habitat. The Palos Verdes blue (Glaucopsyche lygdamus palosverdesensis), which is considered a Los Angeles cousin of the Xerces, is being reared in labs. A new Xerces-like subspecies of the silvery blue (Glaucopsyche lygdamus), which the Xerces blue was thought to be a subspecies of has been discovered as well, and has been analyzed as a potential way to revive the Xerces blue. Successful reintroduction efforts of the silvery blue occurred in 2024 in a collaboration between the Presidio Land Trust, National Park Service, and California Academy of Sciences, among other groups. In 2025, silvery blue offspring were documented in the Lobos Creek Valley Dunes, implying successful reproduction within the Presidio. Releases of silvery blues have continued in 2025.