The San Bruno elfin is a small butterfly with a wingspan of 2.0 to 2.4 centimeters. In adults of both sexes, the wings are brown on the upperside and reddish brown on the underside with a whitish, irregular median line. The adult flight period is late February to mid-April. Eggs are laid in small clusters or strings on the upper or lower surface of the larval hostplant, broadleaf stonecrop (Sedum spathulifolium). Typical habitat is coastal grassland and low scrub of north-facing slopes within the fog belt where the larval host plant grows. All known locations are restricted to San Mateo County, California, where several populations are known from San Bruno Mountain, Milagra Ridge, the San Francisco Peninsula Watershed and Montara Mountain.
The butterfly was listed as endangered on June 1, 1976.
The butterfly continues to be threatened by:
- Public infrastructure development
- Climate change
- Illegal collection
- Habitat modification by non-native plant species
- Parasitism and predation of larvae potentially exacerbated by non-native Argentine ants
- Small populations
- Trampling larval host plants during population monitoring
Land on which the natural dominant plant forms are grasses and forbs.
The land near a shore.
The San Bruno elfin is a small butterfly with a wingspan of 2.0 to 2.4 centimeters.
In adults of both sexes, the wings are brown on the upperside, and reddish brown on the underside with a whitish, irregular median line. Larvae are dichromatic, either red or yellow.
Adults are highly sedentary and move less than 100 meters on average and no more than 800 meters from their larval host plant. Larvae emerge in five to seven days and feed on the stonecrop host plant. The developing larvae have a mutualistic relationship with native ants that defend the larvae from predation and parasitism in return for honeydew secreted by the larvae. However, this mutualistic relationship with native ants may be disrupted by the presence of non-native Argentine ants resulting in increasing rates of predation and parasitism of larvae. Larvae complete their development in late May or early June and enter the pupal phase. Pupation occurs among the loose soil and litter of the host plant stonecrop’s roots. The pupal stage lasts from June until the following March.
The adult flight period is late February to mid-April. Eggs are laid in small clusters or strings on the upper or lower surface of the larval hostplant, broadleaf stonecrop (Sedum spathulifolium).
The butterfly’s larval host plant is broadleaf stonecrop (Sedum spathulifolium). The preferred nectar source for adults is common lomatium. Adults may drink the nectar of early-blooming coastal flowers, especially hog fennel, using a long tube called a proboscis that extends from the underside of the head.
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