For more information on
wildlife recovery and management, please contact
the Wildlife Program.

Phone: 360-902-2515


Island Marble butterfly
Susan Vernon photo

A New Discovery

The Island Marble, a beautiful white butterfly with green ‘marbling’ on the underside of the wings, was discovered by early Canadian lepidopterists (butterfly specialists) in 1861 on Vancouver Island and it was last seen in 1908 on nearby Gabriola Island. No one saw it again for 90 years. It had never been found in the United States. In 1998, zoologist John Fleckenstein of the Washington Department of Natural Resources collected a butterfly at American Camp on the south end of San Juan Island. He was intrigued; he didn’t know what it was. It looked like a species of marble butterfly, but they were not known to occur in western Washington. It was only after he took the specimen to experts that the Island Marble, a butterfly believed to be extinct for almost 100 years, was correctly identified and officially “re-discovered.”

The excitement of this discovery brought Canadian and American lepidopterists to San Juan Island to survey for and study the Island Marble. After hundreds of surveys at potential locations in the San Juans, Gulf Islands, Vancouver Island, Olympic Peninsula, and northern coastal Puget Sound, only a few small populations were found on San Juan and Lopez Islands. Because of its rarity and small population numbers, the governments of British Columbia, Washington, and the United States have identified it as a species of conservation concern.

Island Marble larvae
Susan Vernon photo

Island Marble Life Cycle

The life cycle of a butterfly is a fascinating four-stage process: a transformation from egg to caterpillar to chrysalis and finally winged adult. After Island Marble butterflies mate in spring, the females carefully lay their eggs on the flower buds of mustard plants. About 10 days later, the eggs hatch into tiny larvae (caterpillars) and begin to feed on the flower petals of the mustard plants. As they grow and feed on other parts of the mustard plants, they shed their skin (molt) five times, with each stage larger than the last. After about 35 days, the caterpillars leave their host plants and travel a short distance. They select a spot low in the vegetation and shed their skins one last time to form a chrysalis. This process is usually completed by mid- July. The Island Marble overwinters as a chrysalis and emerges as an adult butterfly the following spring.

Identification Guide

A Creamy-white Butterfly

The Island Marble is a medium-sized creamy white butterfly that measures approximately 1.75 inches (4.5 centimeters) wing tip to wing tip. It is white above, with black-patterned wingtips, and a fine black rectangle mid-wing. It has a mottled pattern of greenish-yellow on the underwings. The Island Marble’s flight pattern is straight, fluttering and fast. Marbles may feed and perch with wings either folded or flat.

Other White Butterflies In The San Juans

The Island Marble is similar in appearance to the common and widespread Cabbage White butterfly, which uses the same host and nectar plants. Other white butterflies (members of the Family Pieridae) are found in flight during spring and early summer, as well. It may be difficult to tell them apart – especially on the wing. Look carefully at perched individuals and note the subtle differences in their markings both on the top of their wings (dorsal) and below (ventral).

Cabbage White butterfly
Kelly McAllister photo
Sara's Orangetip butterfly
Aaron Barna photo
Pine White butterfly
Bob Barber photo
Cabbage White (Pieris rapae)
Ubiquitous – our most common butterfly; white above with charcoal wingtips; note the black dot(s) on the mid-outer dorsal forewings; pale white to yellow below with no marbling or veins. Introduced from Europe.
Sara Orangetip (Anthocharis sara )
Males are white, females yellowish; the orange tips of its dorsal forewings are diagnostic; green marbling and yellow veins below; strong, straight flyer that remains close to the ground; rarely remains still.
Pine White (Neophasia menapia)
White above with patterned black wingtips; note black veins on white below; found in woodlands fluttering out of the conifers; rarely remains close to the ground; flight usually begins in June.

Key Plants and Critical Habitats

Island Marbles and Mustard Plants

The Island Marble butterfly is only known to occur in the San Juan Islands of Washington State. It primarily inhabits grasslands including fields and pastures, disturbed sites, and the margins of saltwater lagoons. The marble’s host plants – where the butterfly lays its eggs and the caterpillars (larvae) are able to feed and develop – are members of the Mustard Family. The most important host plants are listed below. Research is ongoing to identify additional native mustard species that may also host the Island Marble, possibly including Hairy Rockcress (Arabis hirsuta).

Field Mustard (Brassica campestris)
Brassica is an introduced annual weed common throughout the islands, that grows to 1 m tall. It is easily identified by bright yellow flowers and basal leaves that clasp the upper stem. Field Mustard grows in large stands on grasslands, along roadsides, and often as an invader of island gardens.
Field Mustard
Susan Vernon photo
Field Mustard
Amy Lambert photo
Tall Tumble Mustard (Sisymbrium altissimum)
This rangy plant (2-3 ½’ tall), also introduced, commonly grows in disturbed habitats, grasslands, sand dunes, and sandy soils often with bracken, lupine, and naturalized poppies. An Island Marble caterpillar may spend its entire larval stage on one plant before over-wintering as a chrysalis.
Tall Tumble Mustard
Susan Vernon photo
Tall Tumble Mustard
Amy Lambert photo
Tall Peppergrass (Lepidium virginicum)
This native mustard has tiny white flowers and flattened seed pods called silicles that nourish marble larvae. The plant grows to 12 inches tall along the margins of tidal marshes.
Tall Peppergrass
Amy Lambert photo
Tall Peppergrass
Susan Vernon photo

Nectar Plants

Island Marble butterflies take nectar from a variety of native and introduced plants in addition to their host plants. These plants help sustain them during the winged adult phase of their life cycle, which runs from early April to late-June.

Three additional nectar plants are shown below:

American Searocket
Susan Vernon photo
Field Chickweed
Susan Vernon photo
Small-flowered fiddleneck
Susan Vernon photo
American Searocket
(Cakile edentula)
Field Chickweed
(Cerastium arvense)
Small-flowered Fiddleneck
(Amsinckia menziesii)


Threats to Island Marble habitat include conversion for development and encroachment by trees and shrubs. Mustards, the larval host plants for the Island Marble, germinate and grow after ground disturbing activities like plowing, burning, and digging by animals and humans. While these activities can help create habitat for the Island Marble, they can be damaging once the mustards are present. Mowing, grazing, trampling of vegetation, burning, plowing, and herbicides can harm or kill mustard plants. Pesticide use, including the biocide Btk (Bacillus thuringiensis var. kurstaki), on and around plants can kill butterflies and larvae. Deer predation, by way of eating the mustard flowers where eggs are deposited and larvae develop, is common and presents a threat to the butterfly.

Legal Status

The Island Marble is listed by the Washington Department of Fish and Wildlife as a state candidate species for possible listing as endangered, threatened, or sensitive. The U. S. Fish and Wildlife Service lists the Island Marble as a species of concern.

Conservation and Research

Many partners are working together to conserve and protect Island Marble butterflies. The U.S. Fish and Wildlife Service and the National Park Service have entered into a Conservation Agreement that implements conservation measures specifically addressing the needs of this butterfly ( sajh/parkmgmt/upload/finalimbca.pdf). Biologists are conducting annual surveys for the Island Marble at known and potential sites. The National Park Service is controlling non-native invasive shrubs in Island Marble habitat and has used prescribed fire to control native shrubs and trees. Partners in Island Marble Conservation include the U.S. Fish and Wildlife Service, Washington Department of Fish and Wildlife, San Juan Island National Historical Park, Washington Department of Natural Resources, The Xerces Society, KWIAHT (Center for the Historical Ecology of the Salish Sea), San Juan County Land Bank, the San Juan Preservation Trust, private landowners, and other organizations and individuals.

Research by the University of Washington and Western Washington University is also underway to learn more about the population size, life history, and habitat needs of the Island Marble.

What You Can Do to Help

The continued survival of the Island Marble hangs in the balance. People who live and work in the San Juan Islands are encouraged to get involved in the conservation of this rare butterfly. Here are some things you can do to help conserve the Island Marble.

Report Island Marble Sightings

If, after close-up observation of a perched individual, you believe you have sighted an Island Marble in an area other than the south end of San Juan Island, please report it to one of the contacts below.

Washington Department of Fish and Wildlife:
Ann Potter (Wildlife Biologist)
Phone: 360.902.2496

Ruth Milner (Wildlife Biologist)
Phone: 360.466.4345 ex. 265

U.S. Fish & Wildlife Service:
Karen Reagan (Ecologist)
Phone: 360.753.9440

Protect The Island Marble

It is illegal under Washington State law to collect this butterfly, even for catch and release, without a state permit. The National Park Service must approve any research, handling, or other study of the Island Marble on National Park property.