Xerces Blue

The first to go

The Xerces blue butterfly is significant for an unwelcome reason: it was the first North American butterfly species formally documented as extinct due to human activity. Last verified sightings were in the early 1940s; the species was declared extinct in 1943.

Its habitat — coastal sand dunes near San Francisco — was eliminated by urban development. The Sunset District, Lake Merced area, and Presidio sandhills, all once covered with native dune vegetation including the deerweed (Acmispon glaber) that Xerces caterpillars depended on, were paved over for housing through the 1920s and 30s. The butterfly disappeared along with its host plants.

A specific habitat dependence

The Xerces blue had specialized requirements:

• Sandy, sparsely-vegetated coastal dunes within fog belt.
• Specific host plants (deerweed, lotus species).
• Mutualistic relationship with certain ant species that protected the caterpillars in exchange for sugary secretions.

Each requirement made the butterfly more vulnerable. As soon as urban development began destroying the dune ecosystem, the butterfly’s survival became impossible. There was no other habitat for it.

The Xerces Society

In 1971, conservationist Robert Pyle founded the Xerces Society for Invertebrate Conservation in honor of the lost butterfly. The Society is now the largest invertebrate-focused conservation organization in the world, working on:

• Pollinator conservation (bees, butterflies, moths)
• Endangered species recovery (especially butterflies and freshwater mussels)
• Habitat restoration for invertebrates
• Scientific research and education

The Society’s name is a permanent memorial to a species that wasn’t saved — a reminder that small, specialized invertebrates can disappear with their habitat.

Possible de-extinction

In 2021, researchers at the California Academy of Sciences sequenced ancient DNA from museum-preserved Xerces blue specimens. The species turned out to be genetically distinct enough to confirm it was a true species (some had argued it was a variant of the closely-related silvery blue, Glaucopsyche lygdamus).

The DNA sequencing has spawned discussions of using gene-editing or selective breeding to potentially revive Xerces-like populations from related extant species — though no active de-extinction project is underway.