Raising the Island Marble Butterfly at San Juan Island

The island marble butterfly — named for the marbled green streaks on its white wings — has a remarkable story. Thought to be extinct for 90 years, it was rediscovered on San Juan Island in 1998. Today it lives only on that island, and almost entirely within San Juan Island National Historical Park, apart from a single slope just outside the park boundary.

The island marble butterfly. NPS photo.

Park biologist Jenny Shrum jokes that if the island marble had a sitcom, it would be a juicy one: a mysterious past, a naughty dependence on non-native plants, and promiscuous habits.

Heading toward the endangered list

The butterfly’s numbers have kept falling. When this article was written, the U.S. Fish and Wildlife Service had just proposed listing it under the Endangered Species Act. The reasons, as Shrum summarized them: a shrunken range, dwindling numbers, lost habitat, and threats that persist — among them development, agriculture, and deer grazing the plants the butterflies lay their eggs on. The park did not wait for the decision.

The rearing program

The park began a small captive rearing program in 2013. As in other such efforts, caterpillars grow in a controlled setting, free of their usual threats — predators and weather. In the lab they feed on their host plants, which are non-native mustards; nobody knows what plant the butterfly relied on before those mustards reached the island.

Shrum arrived at the park in 2014 expecting to work with youth groups, and on her second day was told she would be running the rearing lab instead. She was not pleased, she admits — she did not want to be stuck in a lab with a silly butterfly. But the work changed her mind. It was a chance to slow down, and unusually hands-on for the Park Service, which usually keeps its distance to avoid disturbing anything; she came to know the bond between caterpillar and host plant intimately. For the butterfly, it offered a fighting chance.

NPS photo.

Clues in the frass

Now leading the park’s on-the-ground conservation of the island marble, Shrum is using the program for research as it enters its fifth official year. Collecting old chrysalises and caterpillar droppings, called frass, allows DNA analysis without harming the insects. The results may answer the park’s questions: How distinct are populations that feed on different host plants? Are butterflies loyal to particular plants? How much genetic diversity is there — and how does it compare with specimens in collections in Canada?

Integrated resource manager Sara Dolan, new to the park, recalled her joy when Shrum showed her the first butterfly of the season to emerge. “It’s great to see something good that is working,” she said.

For Shrum, the island marble’s role in the prairie — pollinator, prey — matters less than what it stands for: a small creature fluttering madly around the prairie, striving to survive, and a team that has decided it must be protected.

Sources

Based on "How to raise your endangered butterfly," San Juan Island National Historical Park, National Park Service; a work of the United States government in the public domain.