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.


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!


Island Marble Butterfly History

San Juan Island National Historical Park is home to the only remaining population of the federally endangered island marble butterfly (Euchloe ausonides insulanus). This pollinator is one of the rarest butterflies in North America, and appears fuzzy with marbled green, yellow, white, and black wings that have a span smaller than 2 inches1. Scientists previously documented the island marble butterfly (IMB) on Vancouver and Gabriola Islands in Canada, but believed the species became extinct in 1908. Ninety years later, the butterfly was rediscovered further south on San Juan Island and Lopez Island. Scientists are still working to understand why the species disappeared during that time.

Today, the few hundred IMBs left in existence only inhabit a small area on San Juan Island, as they have been extirpated from Lopez Island. In 2009, NPS staff began working with the United States Fish and Wildlife Service (USFWS), Washington Department of Fish and Wildlife (WDFW), and local conservation partners to recover the IMB through rearing efforts of collecting IMB eggs from the wild, raising them in captivity, and then releasing adult butterlfies back into the wild. Other efforts include creating, enhancing, and protecting IMB habitat.

After many years of petitioning, the IMB received federal protection under the Endangered Species Act in 2020. The federal government designated approximately 812 acres of land as critical habitat for the IMB, 744 acres of which are located at American Camp within the park.

Life Cycle

IMBs begin their life cycle in the spring as eggs for about 10 days before hatching as tiny caterpillars. They then spend 4-5 weeks eating host plants and growing through five stages, or instars, before finding a low-hanging plant to metamorphosize in their chrysalis (pupate) for 10-12 months! This pupation period is extremely long, especially compared to the monarch butterfly, which is only in its chrysalis for 8-14 days2.

After their long overwinter, IMBs emerge from their chrysalis as mature adults in the spring. These busy butterflies only fly for 6-9 days, so the IMBs get down to business once they emerge. In this quick period, the butterflies spend their time feeding on the sweet nectar of flowering plants (pollinating in the process), finding their mates to fertilize eggs, and then laying the eggs of the next generation of IMBs. Interestingly, this small and fuzzy butterfly adapted to use two non-native mustards as their primary host plants to lay eggs on, field mustard (Brassica rapa) and tumble mustard (Sisymbrium altissimum). Virginia pepperweed (Lepidium virignicum) is the only native plant the IMBs will successfully lay eggs on. It is approximated that only 5% of eggs laid each year will survive to become adult butterflies.

Park Efforts

Since the butterflies have such a low survival rate in the wild, careful efforts by NPS resources staff and other federal, state, and local conservation partners are critical to ensure the IMB’s viability. Each spring, NPS biologists collect 100-300 butterflies from the wild while they are in their egg and caterpillar life stages, and bring them into the rearing lab for pupation. In contrast to survival rates in the wild, 80-90% of IMBs raised in the rearing lab survive and are released as flying adults. During the butterfly flight season (April-June), park staff monitor the butterflies weekly along transects bisecting the island. These data inform population numbers and future release locations.

In addition to raising butterflies, park resource specialists are also enhancing the habitat for the IMB by growing host plants and collecting seeds to distribute in the fall. While most parks aim to get rid of non-native or invasive species, San Juan Island National Historical Park is unique in their efforts to encourage non-native mustard plants in areas IMBs are known to occupy. After all, the butterfly is only found on this single island, so efforts to ensure it has viable habitat are critical.

Park staff also install temporary deer fencing each spring to provide protection for the butterfly in certain parts of their habitat. Federal, state, and local agencies continue to conduct research projects to support the IMB, including DNA analysis, habitat distribution mapping, and experimental habitat enhancement. This includes considerable efforts by local island residents in coordination with the USFWS and WDFW to increase habitat elsewhere on the island.

How can you do your part?

To be a better steward of the IMB when exploring the park, remember to follow Leave No Trace principles, stay on park designated trails, keep pets on leash, and be mindful in IMB habitat.

Once thought to be extinct, this rare butterfly is a symbol of hope. The nimble efforts by NPS staff in collaboration with the many IMB partners show how science can be coupled with care to restore our environment and its unique inhabitants.


  1. “Island Marble.” Washington Department of Fish & Wildlife,

  2. Monarch Butterfly Biology.

  3. Natural History: Island Marble Butterfly


Last updated: November 13, 2022

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