Bay Nature magazineOctober-December 2007


Elfin Butterfly

October 1, 2007

To us, the San Bruno elfin butterfly, with its one-inch wingspan, seems small, but to the ants that protected it during its larval stage, it must seem a giant. On rocky north-facing outcrops on San Bruno Mountain in South San Francisco, female elfins deposit their eggs in spring, coinciding with the flowering of stonecrop, its host plant. Elfin caterpillars feed on stonecrop leaves and flowers, and they excrete a liquid called honeydew along the way. In exchange for the honeydew, the ants groom the larvae and guard them against predators.

This past spring, longtime San Bruno Mountain protector David Schooley came upon the elfins and their guardian ants in a privately owned quarry on the mountain. “It’s miraculous, really,” he says. “After all these years of our trying to protect the elfin, they found the perfect spot and quietly doubled their population all by themselves.”

The elfin, endemic to the Bay Area, is just one of three endangered butterflies (along with Mission blue and Callippe silverspot) that inhabit this island mountain surrounded by houses and industrial parks. The top of the mountain was protected by a first-of-its-kind Habitat Conservation Plan in the 1980s, but the protection came at the price of allowing development on the mountain’s skirt. Now, a plan by Brookfield Homes to build 71 homes on Northeast Ridge threatens habitat for the endangered Mission blue and Callippe silverspot butterflies.

The owners of the abandoned quarry where Schooley discovered the thriving elfin population plan to build an industrial park nearby. Schooley envisions a different use of the land: “It would be a perfect place to showcase San Francisco’s native plants in a botanical garden.”

San Bruno Mountain Watch has obtained permission to lead hikes through the quarry to show people the elfin in its reclaimed habitat. For more information, go to San Bruno Mountain Watch.

The butterflies and their human advocates will get full film treatment in The Story of Butterflies and Bulldozers: The Fight for San Bruno Mountain, a documentary chronicling the grassroots fight to protect the mountain. Look for a premiere in early 2008. To find out more, go to Butterflies and Bulldozers.

About the Author

Writer Aleta George trained as a Jepson Prairie docent in 2009. In addition to writing Bay Nature's Ear to the Ground column, she has written for Smithsonian, High Country News, and the Los Angeles Times.