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Island Marble Butterfly

Island marble butterfly adult

Scientific name: Euchloe ausonides insulanus

Status: Endangered

Critical Habitat: Designated in San Juan County, Washington

Latest Update: The final rule listing the butterfly as an endangered species under the Endangered Species Act, and designating critical habitat in Washington, published in the Federal Register May 5, 2020.

  • Description and Life History

    The island marble is a member of the “white” family of butterflies (Pieridae), primarily consisting of white and yellow butterflies, and is a subspecies of the large marble (Euchloe ausonides). The island marble is 1.75 inches long, which is slightly larger than other subspecies of marble butterflies. The upper (dorsal) side of the island marble butterfly’s wings are creamy white with black markings while the yellow-green marbled pattern on the lower (ventral) side of their wings are what give the subspecies its name.

    Island marble butterflies are most visible in the spring when they are winged adults, but for the rest of the year they are present as either eggs, larvae, or chrysalises (the butterfly version of a cocoon). Island marble butterflies spend most of their year-long life as a chrysalis waiting for the right environmental cues to undergo metamorphosis and emerge as a winged adult. One island marble butterfly was documented as being a chrysalis for 334 days.

    After an island marble butterfly emerges from its chrysalis, it immediately mates and lays eggs, living only a few days before dying. The freshly laid eggs of the island marble are barely a millimeter tall and are initially a faint bluish-green to cream-colored, changing to reddish-orange just prior to hatching. Upon hatching, island marble butterfly caterpillars rapidly progress through five distinct developmental larval stages known as instars and each instar is characterized by specific color patterns, all of which make these caterpillars nearly impossible to see when they’re feeding. Immediately prior to developing into a chrysalis, island marble butterfly caterpillars wander around searching for a safe location to over-winter in chrysalis form, where they will stay until emerging as adults the following spring. The island marble butterfly only flies and lays eggs in the spring of the year.

    Historical Status and Current Trends

    Until 1998, the island marble butterfly was only known from historical collections made on Vancouver Island and the Canadian Gulf Islands in British Columbia, Canada (14 specimens collected from 1861 to 1908). These islands are part of the same geologic formation as the San Juan archipelago located immediately to the south of the Canadian Gulf Islands. The island marble butterfly (island marble) was last collected on Gabriola Island, Canada, in 1908 and was considered extirpated throughout its known range before it was rediscovered on San Juan Island, Washington, in 1998. Although the island marble was not previously known to exist on the American San Juan Islands, it has likely been present in low numbers throughout the last century. Extensive surveys were conducted from 2005-2010 that included 6 northern counties and 16 islands in Washington State. During this time period, twenty-six locations were determined to be occupied, representing five core populations, all on San Juan and Lopez Islands.


    The island marble butterfly does not migrate and is only known from the San Juan Islands in Washington State. It lives its entire lifecycle in upland prairie-like habitat, sand dunes, or coastal lagoon habitat. Adults fly for a short period in the spring and utilize primarily nonnative plants of the mustard family as larval food plants. The island marble butterfly appears to have switched from a native mustard plant as a source of food for larvae to nonnative mustard plants that readily colonize the open prairie-like habitat and sand dunes. Regardless of how this shift occurred, the use of nonnative mustard plants such as Brassica and Sisymbrium has contributed to the survival of the island marble in upland habitat and is expected to continue to play a significant role in the species continued existence.

    Reasons for Decline

    There are multiple threats to the island marble butterfly. One of the big ones is habitat loss due to the plants island marble caterpillars need to survive being eaten by deer, snails, and rabbits. Habitat for the island marble is further degraded by natural succession from prairie-like habitat to woody plants and trees. Invasive plant species that crowd out the plants the butterfly needs to survive are also a threat to butterfly habitat. Additionally, many animals eat island marble butterflies, either as caterpillars or as adults. Spiders and wasps are predators that have a strong negative effect on the butterfly, while deer unintentionally devour the butterfly’s eggs and larvae when they eat the same flowers of the plants where the caterpillars are feeding. Because there are so few island marble butterflies left (fewer than 200 adult butterflies were counted in 2017), they are particularly vulnerable to anything that affects their population, such as unusual storm events, or other chance events that increase mortality or reduce the butterfly’s ability to reproduce.

    Conservation Efforts

    The National Park Service is the owner and steward of the largest parcel of suitable habitat (approximately 600 acres) and the last known remaining population of the island marble butterfly. This population is centered on American Camp, San Juan Island National Historic Park, in Washington. In 2006, the National Park Service worked with USFWS to develop a Conservation Agreement for the island marble butterfly that has guided all conservation efforts for this species on lands administered by the National Park Service, and specifically for implementation of prairie restoration activities at American Camp. These activities include the removal of invasive vegetation through the application of herbicides and use of prescribed fire, and the planting of native species beneficial to the butterfly. Additionally, deer have been excluded from some areas known to be occupied by the butterfly, a captive rearing program has been developed, and experimental work to establish habitat for the island marble butterfly has been ongoing at the park since 2014.

    USFWS and Washington Department of Fish and Wildlife worked together to develop information to help local landowners identify island marble butterflies on their property and to provide voluntary guidelines for producing and managing island marble habitat. USFWS, Bureau of Land Management, USDA Natural Resources Conservation Service, Washington Department of Fish and Wildlife, Washington Department of Natural Resources, the San Juan County Land Bank ,and the San Juan Preservation Trust, the San Juan Islands Audubon Society Chapter, the San Juan Islands Native Plant Society Chapter, and other local volunteers are working on San Juan and Lopez islands to expand the network of local citizens searching for butterflies and establishing and managing island marble butterfly habitat.

    References and Related Pages Links

    Federal Register 90-day Finding 08/19/2014 (79 FR 49045)

    Federal Register 12-month Finding 04/05/2016 (81 FR 19527)

    Federal Register Proposed Listing and Critical Habitat 04/12/2018 (83 FR 15900)

    Federal Register Final Listing and Critical Habitat 05/05/2020 

    San Juan Island National Historic Park (NPS) Island Marble butterfly information

    Washington Department of Fish & Wildlife (WDFW) Island Marble butterfly information

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