Island Marble Butterflies on San Juan Island

Thought to be extinct for 90 years, the Island Marble Butterfly surprised many with its rediscovery on San Juan Island in 1998. Last documented in Canada, the species originally had a historic range far greater than its current one, which is contained almost entirely within the American Camp section of San Juan Island National Historical Park. Upon rediscovery, the population was in decline, and until only very recently the population was shrinking steadily from year to year.

The Island Marble Butterfly has a life span of only one year. Eggs are laid one-by-one on mustard plants in early spring, caterpillars hatch and eat their way through the next few weeks, chrysalises develop and overwinter in the fields, and finally adult butterflies emerge the following spring. These adults then lay eggs, and the process begins anew. 

An island marble butterfly perched on top of a lupine.

The National Park Service, with assistance from other organizations, has undertaken monitoring and restoration efforts to study the remaining individuals and ensure the species’ survival. Results from initial studies showed that the butterfly has a low survival rate as caterpillars (less than 5% of wild caterpillars live to form chrysalises), and so in 2013 captive rearing of caterpillars began as a way to curb the population’s decline.

Island Marble Butterflies in the captive rearing program.

The Center for Natural Lands Management recently partnered with the US Fish and Wildlife Service and National Park Service to keep the captive rearing program running. So far this year, over 200 caterpillars have been reared in the lab. Although not all will survive, we estimate that the majority will live to adulthood and be ready for release out on the prairie next year.

Preparing the butterfly habitat in the rearing program area.

As a native resident of Puget Sound prairie habitat, this species’ historical decline and current struggle says a lot about the health of the local ecosystem. But with new found attention and organizational partnerships, the butterfly has become a symbol of hope for many local residents. As we continue to study and rear this remarkable species, we look forward to a future when the prairie is dotted with little white butterflies each spring, living their full lives out in the fields without any need for our intervention.

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