U.S. Fish & Wildlife Service

Sacramento Fish & Wildlife OfficeServing the people, conserving the fish, wildlife, and plants of California

A Unit Of The Pacific Southwest Region

Species Information

Photo of San Bruno Elfin Butterfly

Photo Credit: Ken Gardiner / USFWS

San Bruno Elfin Butterfly

Basic Species Information

STATUS

Endangered. This species is in danger of extinction throughout all or a significant portion of its range.

DESCRIPTION

The San Bruno elfin is a small brownish butterfly in the Lycaenidae family. They are very sedentary and live only about a week.

FOOD SOURCES

Elfin butterflies feed on other flowers in addition to their host plant, stonecrop (Sedum spathulifolium). Stonecrop is a low growing succulent associated with rocky outcrops that occur at 274 to 328 m (900 to 1075 feet) elevation.

Adult food plants have not been fully determined. Montara Mountain colonies are suspected to use Montara Mountain manzanita (Arctostaphylos montaraensis) and huckleberry (Vaccinium ovatum).

HABITAT

The San Bruno Elfin Butterfly inhabits rocky outcrops and cliffs in coastal scrub on the San Francisco peninsula.

MATING

The adult flight period is late February to mid-April, with the peak flight period occurring in March and early April. Eggs are laid in small clusters or strings on the upper or lower surface of stonecrop (Sedum spathulifolium). Larvae hatch from the eggs within 5 to 7 days after they are deposited on the plant.

Young larvae start to feed immediately by tunneling into the swollen succulent leaves. The first and second instars feed in this manner until they molt into the larger third instar. Third and fourth instars move up to the flowers of the food plant and feed while they are tended by several species of ants that protect them from predators and, to a lesser extent, parasites.

These ants also groom the larvae and feed on a honeydew substance produced by the larvae to attract the ants. Pupation and pupal diapause (a dormant stage) take place in the loose soil and litter at the base of the larval food plant from June until February of the following year. The adults then emerge and mate.

RANGE

The San Bruno elfin is found in coastal mountains near San Francisco Bay, in the fog-belt of steep north facing slopes that receive little direct sunlight. All known locations are restricted to San Mateo County, where several populations are known from San Bruno Mountain, Milagra Ridge, the San Francisco Peninsula Watershed and Montara Mountain.

THREATS

Present or threatened destruction, modification, or curtailment of the habitat or range of the San Bruno elfin butterfly due to private development projects no longer pose as serious of a threat to the species as they did at the time of listing. However, public infrastructure development projects remain a significant threat. All San Bruno elfin butterfly populations found on Golden Gate National Recreation Area properties are relatively safe from development activities that would destroy, modify or curtail habitat.

WHAT YOU CAN DO

Consider planting a butterfly garden.

Last updated: December 1, 2017