The island marble butterfly (Euchloe ausonides insulanus) is one of the rarest butterflies in North America — and its only remaining population lives at San Juan Island National Historical Park in Washington. It is a small, fuzzy pollinator, with marbled green, yellow, white and black wings spanning less than 2 inches.
The island marble butterfly. Photo by the National Park Service
Lost and found
| Early records | Vancouver and Gabriola islands, Canada |
| Presumed extinct | 1908 |
| Rediscovered | ninety years later, farther south on San Juan and Lopez islands |
| Today | a few hundred, in a small area of San Juan Island only — gone from Lopez |
| Recovery work begins | 2009 |
| Endangered Species Act protection | 2020, after years of petitions |
| Critical habitat | about 812 acres, 744 of them at the park's American Camp |
Why it vanished for so long is still a mystery.
A long wait in the chrysalis
| Stage | How long |
|---|---|
| Egg (spring) | about 10 days |
| Caterpillar, through five instars | 4–5 weeks of eating |
| Chrysalis | 10–12 months — against 8–14 days for a monarch |
| Adult (spring) | flies for just 6–9 days: feed on nectar (pollinating as it goes), mate, lay eggs |
Oddly, it relies mainly on two non-native mustards to lay its eggs — field mustard (Brassica rapa) and tumble mustard (Sisymbrium altissimum). The only native plant it uses successfully is Virginia pepperweed (Lepidium virginicum). In the wild, only about 5% of eggs become adults.
The five caterpillar stages, left to right. Photo by the National Park Service
From a distance, adults look like — and in flight are often mistaken for — cabbage whites; the green-and-white marbling under the wings gives them away. Photo by the National Park Service
What threatens it
- Wasps and spiders prey on it.
- Deer eat its host plants — and the eggs on them.
- Development, especially roads, in its habitat.
- People and pets walking through its habitat can knock caterpillars off plants and crush caterpillars, chrysalises or host plants.
How the park helps
The Park Service works with the U.S. Fish and Wildlife Service, the Washington Department of Fish and Wildlife and local partners.
- Rearing: each spring, biologists bring 100–300 eggs and caterpillars into the rearing lab. There, 80–90% survive to be released as adults — against about 5% in the wild.
- Monitoring: during the April–June flight season, staff walk transects across the island weekly, counting butterflies to guide population estimates and release sites.
- Habitat: staff grow host plants and collect seed to spread in the fall. Unusually for a park, San Juan Island encourages non-native mustards where the butterfly lives — it has nowhere else to go.
- Deer fencing goes up each spring around key habitat.
- Research: DNA analysis, habitat mapping and experimental habitat work — with island residents helping to create habitat elsewhere on the island.
What you can do
In the park, follow Leave No Trace, stay on designated trails, keep pets on a leash, and tread carefully in butterfly habitat.
Sources
Based on "Rearing the Rare Island Marble Butterfly at San Juan Island National Historical Park," National Park Service; a work of the United States government in the public domain. The source's "Lepidium virignicum" is corrected to Lepidium virginicum.
In these publicationsSan Juan Island National Historical Park
Licence: CC0 1.0 (public domain) · Adapted from www.nps.gov
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