Where the Xerces Blue Butterfly Was Lost, Its Closest Relative Is Now Filling In

A Silvery Blue butterfly.

A Silvery Blue butterfly. Gayle Laird

More than 80 years after the iconic Xerces Blue butterfly vanished from San Francisco, researchers have analyzed century-old specimens to track down its closest living relative, the Silvery Blue. Last week, they released a handful of Silvery Blues on the western edge of the city, where Xerces Blues once thrived.

In 1943, the Xerces Blue became the first U.S. butterfly to be driven to extinction by development after builders destroyed the sand dunes where it made its home. Thirty years ago, officials began to restore a swath of those dunes in the Presidio, a former military base, and as part of that project they sought to bring back native wildlife, including rough-skinned newts, western pond turtles, and San Francisco forktail damselfies. But they had no Xerces Blues. To fill the gap, they would need a very similar butterfly, one that was suited to the cool, foggy climate and would feast on the deerweed that grows among the dunes.

A Xerces Blue specimen.

A Xerces Blue specimen. Gayle Laird

The search for a replacement began at the California Academy of Sciences, where researchers excavated long-preserved Xerces Blues from a vast collection of specimens, analyzing their genes to identify their closest living relatives. Those turned out to be a group of Silvery Blues living 100 miles south of San Francisco.

Durrell Kapan, a researcher at the California Academy of Sciences, said the newly relocated Silvery Blues will help pollinate native flowers and form a critical link in the food chain. “This isn’t a Jurassic Park-style de-extinction project,” he said, “but it will have a major impact.”


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