Status: Endangered

Description: On the upperside of the wings, the adult female is brown with some blue, and the male is light blue. Both have blackish wing edges. The underside of the wings is off-white with two rows of irregularly shaped black spots. The larva (caterpillar) is light green with diagonal white bars on each segment.

Size: Adults are about the size of a quarter (21 – 33 mm). Larvae are very small and rarely seen.

Habitat: The Mission blue requires a host plant and appropriate nectar plants in a coastal grassland habitat. The host plants utilized by the Mission blue are silver lupine (Lupinus albifrons), summer lupine (Lupinus formosus), and varicolor lupine (Lupinus variicolor). Nectar plants include various composites (Asteraceae) that grow in association with the lupines.

Range: Remaining populations of Mission blue butterfly are found in only a few locations around the San Francisco Bay area in California: the Marin Headlands (Marin County), the Skyline ridges (San Mateo County), San Bruno Mountain (San Mateo County), and possibly at Twin Peaks (San Francisco County). The historical distribution of the species probably encompassed much of the coastal scrub/grassland habitat of the northern San Francisco peninsula and Marin County.

Life Cycle: The eggs are usually laid singly on the dorsal side of new lupine leaves, but may also be laid on stems, flowers, and seed pods of the lupine. Eggs hatch within 6 – 10 days, and the first and second instar larvae feed on the mesophyll of the host plant. Approximately three weeks after eclosion, the second instar larvae begin diapause, usually in the litter at the base of the host plant. Diapause begins at about the same time that the host lupine shifts its energy from leaf maintenance to flower and seed production. Larvae remain in diapause for the rest of the summer and through the winter, and emerge to continue feeding in the spring. Pupation, which lasts about a week, occurs in the duff at the base of the host plant or other plants. The adult flight period lasts from March to June. Adults live for approximately one week, and females lay eggs on the host plant. The complete life cycle of the Mission blue lasts one year.

Food Source: Mission blue larvae will feed only on the leaves of the three host lupines: L. albifrons, L. formosus, and L. variicolor. Adults may drink the nectar of composite flowers (sunflower family), using a long tube called a proboscis that extends from the underside of the head.

Population: The San Bruno Mountain population is estimated at 18,000 adults. The Skyline Ridges support approximately 2,000 adults, and there may be as many as 500 at Twin Peaks. The Marin Headlands population has not been estimated, but is significant.

Behavior: Mission blue larvae spend most of their time feeding on lupine leaves in a variety of weather conditions. Adults spend their short lives mating, laying eggs, and nectaring. To locate a mate, adults patrol around in patches of host plant, rarely straying far from the lupine habitat. On rainy, cool, windy, or foggy days, adults hide out underneath vegetation.

Survival Threats: The main threat to the Mission blue is habitat loss due to agricultural and urban expansion. Various parasites and predators threaten the Mission blue during its life cycle. Eggs and larvae are parasitized by other insects, such as wasps and flies. Rodents prey upon both larvae and pupae, and many pupae die due to desiccation. . The invasion of exotic plant species, such as ice plant, pampas grass, broom, gorse, and eucalyptus, also threatens habitat. Trampling of host plants, larvae, and pupae by humans and dogs is also a problem in some areas.

Legal Protection: The Mission blue was added to the Federal Endangered Species List in 1976, and is protected under the Endangered Species Act.