Rearing the Rare Island Marble Butterfly at San Juan Island National Historical Park

A small marbled yellow, white, and black fuzzy butterfly with yellow-green eyes and an antennae sits on a thin green plant with budding white flowers.
Island marble butterfly

NPS Photo

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 butterflies 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.
5 individual images depicting each of the 5 instars of the island marble butterfly from a tiny brown caterpillar to a yellow-green caterpillar with black spots. The caterpillars are sitting on host plants with green stems.
The five instars of the island marble butterfly from left to right.

NPS photo

A small butterfly perched on a plant with its wings flat. The wings are white with black and pale yellow markings.
IMB adults look like small white butterflies from a distance, and when flying can often be confused with cabbage whites. They can be distinguished by the green and white marbling on the underside of their wings.

NPS photo


The main threats to IMBs include predation by wasps and spiders, deer eating their host plants (and inadvertently their eggs), development within IMB habitat, and humans. By walking through IMB habitat, humans and their pets can unintentionally knock caterpillars off their host plant and crush caterpillars, chrysalids, or the host plants themselves. Additionally, road development within IMB habitat poses a serious threat to these rare butterflies3. Unfortunately, these actions result in butterfly mortality and help explain why only 5% of IMB eggs laid survive to be adults in the wild.

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

San Juan Island National Historical Park

Last updated: June 11, 2024