Xerces Blue

I always thought there were more.

When entomologist W. Harry Lange was collecting insects on March 23, 1941 at the former military base Presidio in San Francisco, he unknowingly caught the Xerces blue that is now regarded as the last of its kind. Lange later commented on this and said: “I always thought there were more. I was wrong.”

There are differing accounts of when exactly the butterfly from the gossamer-wing family (Lycaenidae) was last seen. The book California Butterflies (1986), as well as Kathy Keatley Garvey’s article (2011) from the UC Davis Department of Entomology and Nematology, name 1941. By contrast, California Insects (1980) gives the year 1943. It makes little difference, because one thing is certain: the Xerces blue is extinct.

The species was scientifically described in 1852 by a French lepidopterist and botanist named Jean Baptiste Boisduval. Boisduval named the butterfly after the Persian kings Xerxes I and II from the 5th century BC.

Xerces blue – fact sheet

scientific names	Glaucopsyche xerces, Glaucopsyche lygdamus xerces
original range	San Francisco, California (USA)
time of extinction	1940s
causes of extinction	habitat loss, disappearance of the main food plant
IUCN status	extinct

With the urbanization of the Sunset District, the Xerces blue disappeared

Scientists assume that the Xerces blue was the first American butterfly to go extinct as a result of habitat loss caused by urban development. In an article published in 1956 in Lepidoterist’s News, lepidopterist James W. Tilden lamented the loss of the Xerces blue. He wrote that the butterfly species died out shortly after the Second World War:

“Only a few years earlier it had been the most characteristic butterfly of the coastal sand dune region known as the Sunset District, but the complete settlement of the area left no habitat for the Xerces blue.”

As late as the 1930s, the Xerces blue, Tilden continues, could still be found in undeveloped parts of the Sunset District and at Lake Merced. “Some individuals survived for years at Fort Funston, but these also disappeared when the region was leveled. At present (1956), the former habitat of Xerces is almost 100% built over.”

Tilden also linked the butterfly’s extinction with the disappearance of the bird’s-foot trefoil species Acmispon glaber. This sand dune plant was highly sensitive to changes in the soil. Tilden observed that the plant disappeared in some places before the Xerces blue did.

Presumably the Xerces caterpillars, which were supposed to later become butterflies, were specialized on the sand dune plant and could not develop without it. With that, the reproduction of the Xerces blue was doomed. The butterfly was indeed also seen on lupines, but observations showed that this plant genus was evidently unsuitable for the development of Xerces caterpillars.

Xerces blue—a population of the silvery blue?

Until the end, Felix Grewe of the Field Museum of Natural History in Chicago hoped that the Xerces blue was not a distinct species, because then it would not now be extinct. In a research paper published in July 2021, Grewe and his colleagues investigated whether the Xerces blue was merely an isolated population of the more widespread and very similar silvery blue (Glaucopsyche lygdamus).

For this, the researchers analyzed DNA taken from a museum specimen and compared it with samples of the silvery blue. The genetic material showed hardly any similarities, so it must be assumed that the Xerces blue was indeed a distinct species—and is therefore extinct.

Mutations: the Xerces blue wasn’t always lavender-blue

The Xerces blue basically had blue wings with white dots, but what was special about this butterfly species was that five different forms or mutations existed. The different forms bore the names xerces, polyphemus, mertila, antiacis and huguenini. Among other things, they differed in the size of the white spots and in the blue coloration.

This partly very different appearance led some scientists to initially assign the butterflies to different species, which later turned out to be incorrect. These different forms are also not subspecies of the Xerces blue. Rather, they show in an especially striking way the effects genetic mutations can have on a small population.

The fate of the Xerces blue strongly recalls that of the Miller moth or of Sloan’s urania. The caterpillars of both moth species were likewise no longer able to develop because of destroyed vegetation. Another extinct moth belonging to the gossamer-wing family is Morant’s blue. It was endemic to South Africa and was not seen again after 1879.

Presidio: restoring the former ecosystem

There had long been efforts aimed at re-establishing butterfly species similar to the Xerces Blue in its former habitat. These included, for example, the Palos Verdes Blue (Glaucopsyche lygdamus palosverdesensis), a kind of cousin of Xerces from Los Angeles that is bred mainly in laboratories. It is regarded as one of the rarest butterflies in the world.

In 2024, as part of a reintroduction project by wildlife experts on the western edge of San Francisco, a group of silvery blues was released. This project for the restoration of the sand dunes in Presidio National Park began 30 years ago and included the reconstruction of the lost flora and fauna. The newly established silvery blues are intended to play a key role in pollinating native plants in the Presidio and in forming a basic link in the food chain.

To close the resulting gap in the ecosystem, what was first needed was a butterfly very similar to the Xerces blue. It had to adapt to the local climate and feed on the bird’s-foot trefoil species Acmispon glaber, which is now once again thriving between the dunes.

The search for a replacement butterfly began at the California Academy of Sciences, where researchers analyzed the DNA of long-preserved museum specimens of the Xerces blue and compared it with the genomes of modern populations of blue butterflies throughout the San Francisco Bay Area. Their goal was to identify the closest living relatives of Xerces, which turned out to be a group of silvery blues native around 160 kilometers south of San Francisco.

The project in Presidio National Park shows that the regeneration of destroyed ecosystems can be possible, but it does not bring back the Xerces blue.

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