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NATURAL HISTORY

ISLAND MARBLE BUTTERFLY } Euchloe ausonides insulanus
FAMILY: Pieridae

DESCRIPTION: The island marble is white and greenish in color, with a marbled texture under the hind wing and a wingspan of approximately 45 millimeters. The flight of the butterfly is straight, fluttering, and usually fast.

HABITAT: Coastal shoreline and adjacent prairie on San Juan Island are vital habitat for the survival of the only known viable population of the island marble butterfly. These prairies have declined to less than 3 percent of their historic extent.

RANGE: Historically, the island marble butterfly was only known to occur on Vancouver Island and the Canadian Gulf Islands off the coast of British Columbia. The butterfly was last observed on Gabriola Island, Canada in 1908 before a small population was found within the San Juan Island National Historical Park, off the coast of Washington State, almost a century later in 1998. 2005 and 2006 surveys found butterflies at 11 new locations, although most sites contained fewer than five butterflies. The vast majority of the butterflies are within the San Juan Island National Historical Park. Most likely, many of the individuals found at the new locations are strays from this main site.

MIGRATION: This species is nonmigratory.

BREEDING: Adults take flight from mid-April to mid-June. The female lays a single egg on the flower bud of a host plant.

LIFE CYCLE: Eggs hatch shortly after they are laid. The larva, or caterpillar, is striped lengthwise in greenish gray and yellow and feeds only on buds, flowers, and fruit. The pupa overwinters inside its chrysalis until it emerges as an adult.

FEEDING: Little is known about the butterfly’s feeding habits, although biologists believe wild mustard is the preferred host larval plant. This plant provides a place for the eggs to hatch and food for the larvae.

THREATS: Most likely, sheep and/or cattle grazing on the larvae’s host plant(s) caused the butterfly’s initial decline. Today, its habitat is threatened by development as the San Juan Island National Historical Park, which now supports the only known remaining island marble population, is increasingly ringed in by adjacent private land development. The realignment of the road through the park and the construction of roads throughout the historic range of the island marble are also threats. Pesticide use (particularly that of Btk, used to control gypsy moths) within the park and vicinity also threatens the island marble.

POPULATION TREND: Although several small populations have been found in recent years, there is only one potentially viable population of approximately 200 island marble butterflies.

Photo © Bill Yake