Draft Status Report for the Island Marble Butterfly in Washington

The island marble butterfly (Euchloe ausonides insulanus), a subspecies of the large marble (Euchloe ausonides) was designated as a Washington State candidate species in 2002. It was listed as a Species of Greatest Conservation Need in Washington’s 2005 and 2015 State Wildlife Action Plans. In 2002, U.S. Fish and Wildlife Service (USFWS) received a petition to emergency list this butterfly as an endangered species filed by The Xerces Society, Center for Biological Diversity, Friends of the San Juans, and Northwest Ecosystem Alliance (Black & Vaughan 2002). The subsequent USFWS 12-month finding concluded that the petition was not warranted (USFWS 2006). The USFWS was petitioned again in 2012 to list the island marble, the subsequent 90-day finding found that the island marble may be warranted for listing. On May 5, 2020, the USFWS issued a final rule listing the island marble as a federally endangered subspecies and designated 812 acres of critical habitat on San Juan Island.

The island marble butterfly is a medium-sized butterfly, with a wingspan of approximately 1.75”. The most easily recognized traits are the expanded marbling on the ventral hindwings which are strongly covered with yellow scales and hair-like structures. The island marble butterfly was thought to be extirpated until its rediscovery in 1998 at the American Camp Unit of the San Juan Island National Historical Park (SAJH) on San Juan Island. The rediscovery occurred during a two-year Puget Prairie Butterfly Survey effort led by the Department of Natural Resources (DNR) Heritage Program, Department of Fish and Wildlife (WDFW), and Nature Conservancy (TNC) with funding assistance through U.S. Fish and Wildlife. The island marble was last documented in 1908 on Gabriola Island, British Columbia, Canada prior to its rediscovery. After significant search efforts across historic sites on Vancouver Island and surrounding islands in Canada, the island marble is considered extirpated in British Columbia.

Today, the range of island marble is restricted to a single complex of prairie, coastal dune, coastal bluffs and lagoon habitats located on the southern end of San Juan Island. Island marble uses three annual mustard species as larval host plants, tall peppergrass (Lepidium virginicum var. menziesii), a native species and two introduced species, field mustard (Brassica rapa) and tumble mustard (Sisymbrium altissimum). Island marble produce one brood per year and adults emerge in the early spring through early summer. Larvae (caterpillars) feed and develop on host plants through their last developmental stage, then leave its host plant to pupate (form a chrysalis) and overwinter for 11 months until emerging as adult butterflies in the early spring.

After its rediscovery, approximately 100 sites were surveyed in the San Juan Islands and surrounding mainland areas between 1998 and 2004. No new sites were found. Intensive survey efforts designed to determine the butterfly’s range and distribution led by WDFW commenced in 2005 and continued through 2012. Island marble was detected at 52 sites during this time across San Juan and Lopez Island, many of the sites occurring on private land. Island marble was last detected at a single site on Lopez Island in 2012. 2015 survey efforts on San Juan Island (134 surveys across 48 sites) confirmed that the distribution of island marble shrank from its previous expansion, now restricted to the southern end of San Juan Island.

Annual transect monitoring at the American Camp Unit of SAJH has occurred since 2004. Butterfly encounter rates dropped significantly between 2004-2009 and 2013-2014 and have not rebounded on the most consistently monitored transects.  The decline of adults in 2013 triggered the need for a formalized captive rearing program to prevent extinction. Since 2014, between 74-248 adult butterflies have been released annually at American Camp Unit of SAJH to augment this last remaining population.

Washington State defines “Endangered” as any wildlife species native to the state that is seriously threatened with extinction throughout all or a significant portion of its range. We conclude that the island marble is at a high risk of extinction given that: 1) only one population remains; 2) annual transect monitoring from 2004-2023 at the core population shows a consistent decline in encounter rate; and 3) local extirpation has occurred at all previously occupied sites on San Juan and Lopez Islands except for the remaining population located on the southern end of San Juan Island. Therefore, it is recommended that the island marble butterfly be classified as an endangered species in Washington state.