Island Marble Butterfly

The green and white Island Marble Butterfly resting on a green plant.
The rare Island Marble butterfly has reappeared on the American Camp prairie and elsewhere on San Juan and Lopez islands after more than 100 years.

Karen Reagan/USFWS


The reappearance of the rare Island Marble butterfly (IMB) on San Juan Island after nearly 100 years continues to intrigue researchers and enthusiasts throughout the country.

In 1998, the Island Marble (Euchloe ausonides insulanus), thought to be extinct since 1908, was discovered during a prairie butterfly survey at American Camp. The only known specimens had previously been found on Vancouver Island and Gabriola Island in British Columbia. Scientists now believe American Camp, along with scattered locations on San Juan and Lopez islands, to be the only viable population in the world. This finding is based on multiple studies and monitoring over a number of years since 2008.

In 2008, a mark-release-recapture (MRR) study by Merrill Peterson, a professor/researcher in the Biology Department at Western Washington University established a baseline from which to understand IMB population. The park conducted another MRR in spring 2009 during the flight period, and monitored usage of test plots of native host plants beginning in 2010. Also in 2009, the U.S. Fish and Wildlife Service (USFWS) sponsored a survey, in partnership with San Juan Island National Historical Park, to learn more about the natural history of the butterfly, including how far it flies, how long it lives, and whether gender ratios vary in different areas. Since this early work, the park has established a captive breeding program at American Camp where butterflies are reared and then reintroduced into the praries. During these efforts, a separate track worked on developing and submitting a petition to USFWS for the listing of the IMB as an endangered species under the Endangered Species Act (ESA).

In the spring of 2020, based on this body of work, and the developed data and findings, as well as the determined rarity of the species and the threats posed to it, this wonderful and beautiful butterfly was listed as endangered by the USFWS which administers such isting. The IMB is now protected throughout the United States and is entitled to all of the advantages afforded by the ESA.

An island marble butterfly clinging to a mustard plant.

Karen Reagan/USFWS

The park, USFWS and the Washington Department of Fish and Wildlife continue to work with and assis private landowners outside the park with conservation strategies and habitat protection for the Island Marble butterfly.

Additionally, the park continues to investigate if the IMB can adapt to feeding and laying eggs on native rather than the nonnative mustards it is currently utilizing at American Camp. This will be critical in determining whether or not the park continues to incorporate native mustard plants into its ongoing prairie restoration plan.

The Island Marble is white and pale green with a mottled pattern of greenish-yellow under its hind wings. Look in the grassy prairie near wild mustard plants. Don’t confuse it with the more common Cabbage White, which is mostly white with a yellow underside and feeds on the same plants.


Protection status: Washington state Candidate (under review for listing as state Threatened or Endangered) species. The Federal government under the Endangered Species Act simply lists it as endangered. To read an article the National Park Service recently wrote about this journey, head here!

Last updated: August 26, 2020

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