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Euchloe ausonides insulanus, Island Marble Butterfly

Before its rediscovery in 1998 at the National Historical Park American Camp, San Juan Island,WA; the Island Marble had been believed extinct for 90 years. Belonging to the subfamily Pierinae, family Pieridae,it is one of three subspecies of large marble butterflies. It is white and greenish, characterized by a marbled texture under the hind wing, and a wingspan of about 2in. (45 mm). Female Island Marbles are often yellowish and may reflect ultraviolet light. This butterfly is a single-brood species with spring-active larvae that are striped grey and yellow and presumably feed on wild mustard.

Historically found in British Columbia where it inhabited coastal grasslands in Garry Oak woodland and foraged in adjacent prairies, this subspecies is completely isolated, and is now only found on San Juan Island. Extensive survey work conducted in 2005 revealed only three new populations, all with less than 10 individuals. However, there is a remote possibility that it still occurs at other sites in the San Juan Islands.

The most likely cause of the loss of the Island Marble Butterfly is the elimination of the larval food plant from its habitat through grazing by sheep or cattle. Livestock grazing also may cause adverse impacts to butterfly populations by (1) trampling eggs, larvae, pupae, and adults, (2) selectively eating larval and adult food sources, and (3) disturbing the soil, which allows weeds to invade. Substantial areas of the Garry oak and coastal prairie habitats, which are essential for the Island Marble, have been cleared for housing and urban development throughout the historic range (Canada, Vancouver Island, and the Gulf and San Juan Island): less than 5% of the region's original ecosystem coverage remains. Commonly used herbicides harm larval and adult food sources. Invasive species of plants may also displace their main food source.

Overcollecting the Island Marble for commercial, recreational, scientific, or educational purposes is a threat because of its small, localized population. Also, insecticide applications, specifically Btk (Bacillus thuringiensis var.kurstaki), could threaten populations as it is a Lepidoptera-specific. Because the only known viable population is found in a national park that receives many visitors, recreation could pose a risk to the Island Marble’s habitat if not properly managed. It is also very vulnerable to normal factors of insect population dynamics such as weather, predation, and disease. There are no existing state or local regulations in place to protect these butterflies or their habitat, so without immediate attention, this species could go extinct.

Since the population was found experts have been trying to work with park officials to keep them from spraying herbicides, but they have shut off contact, refusing to talk about moving forward with site restoration. A group called the FRIENDS of the San Juans has been taking measures since 1979 to protect the character and resources of the islands and the Puget Sound. They have helped protect development of the shoreline and lead citizen outreach efforts to educate and train the community on properly managing the land and its animals. Recently, FRIENDS developed co-petitions with the Center for Biological Diversity for the Endangered Species act for species of orca, herring, salmon, and the Island Marble. On February 13, 2006, the U.S. Fish and Wildlife Service released a positive 90-day finding determining that protection may be warranted. A status review of the species is pending.


Written by: Sade McDougal, 2006

Updated: Eric Denemark, 2008

Image credit: The Xerces Society