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.
Source: U.S. Department of the Interior, National Park Service
