Conserving the Island Marble, One Poke in the Rear at a Time

"One Sunday morning (in the lab) the warm sun came up and – pop! – out of the egg came a tiny and very hungry caterpillar…"

An island marble butterfly caterpillar in its fifth, and final, instar. In the wild, it is approximated that only 5% of eggs laid each year will survive to become adult butterflies.

NPS Photo/Bolin

JULY 2023

As an intern for San Juan Island National Historical Park's captive rearing program, my workday starts with the sight of tiny caterpillars devouring mustard plants. Not a day passes that I don’t immediately think of Eric Carle’s “The Very Hungry Caterpillar,” but sadly we don’t keep chocolate cake and cherry pie on the menu. These caterpillars are on a strict green-leaf diet.

This ravenous feasting is what transforms the miniscule caterpillars (less than 2mm at hatching) into the plump, wriggling specimens who greet me every morning. As days pass, they shed their skin and grow even bigger, eventually reaching the size of a fork prong. They then adopt a pink hue and begin exploring the corners of their cage in search of a suitable spot to hang their chrysalis, where they will remain throughout the fall and winter. Next summer, they’ll emerge as island marble butterflies (Euchloe ausonides insulanus)—an endangered species found only on San Juan Island.

Island marble butterflies primarily depend on two non-native plants as hosts: field mustard and tumble mustard. They also use native Virginia pepperweed. Researchers bring fresh cuttings of the host plants indoors to ensure the caterpillars never go hungry.

NPS Photo/Crawford

This ravenous feasting is what transforms the miniscule caterpillars (less than 2mm at hatching) into the plump, wriggling specimens who greet me every morning. As days pass, they shed their skin and grow even bigger, eventually reaching the size of a fork prong. They then adopt a pink hue and begin exploring the corners of their cage in search of a suitable spot to hang their chrysalis, where they will remain throughout the fall and winter. Next summer, they’ll emerge as island marble butterflies (Euchloe ausonides insulanus)—an endangered species found only on San Juan Island.

The island marble butterfly is a subspecies of the large marble butterfly (Euchloe ausonides), found throughout western North America. The island marble was believed to be extinct for nearly a century, until two researchers from the Washington State Department of Natural Resources stumbled across this remaining population during a survey of San Juan Island’s American Camp in 1998.

However, its existence remains precarious, particularly due to the scarcity of its host plants. With a limited range and numerous threats, including habitat loss and predation, the species teeters on the brink of extinction. The total population is estimated at about 300 individuals. That is why every summer, the captive rearing program collects over 100 eggs to raise in safety and release the following year.

Island marble butterfly chrysalises hang in a row in the captive rearing lab on San Juan Island. After the butterflies emerge researchers release them into the wild to mate and lay eggs, laying the foundation for the next generation.

NPS Photo/Galloway

Our role as researchers is to ensure that these caterpillars are well-fed and content. We provide them with cuttings of plants collected from outside, but they never last long. The plants that aren’t devoured end up wilting on the cage floor, and so we gently coax the caterpillars off the dying plants and onto fresh buds. It can be a laborious process, requiring delicate pokes to the rear as the caterpillar twists and teeters, attempting to remain on its old familiar plant. Sometimes their stubbornness causes them to fall, which always sends me into a panic. Most of the time the caterpillars recover swiftly, but not always. Even while they resist assistance, these fragile and vulnerable creatures could use all the help they can get. This thought places a lot of weight behind every nudge and poke.

I was able to join this year’s program with the National Park Service via the Mosaics in Science Environment for the Americas program. I’ve quickly learned the well-studied and varied methods for promoting the butterfly’s continued existence, such as cultivating more host plants and fencing off sensitive habitat. We can’t collect too many eggs, as this could lead to a genetic bottleneck or leave them susceptible to disease, so nudging the species towards a brighter future requires a very delicate hand.

My favorite part of the project is observing the sheer delight the butterfly brings to park visitors, such as the Butterfly Conservation tour group that we encountered during a release of two captively reared individuals. The group had low hopes of seeing the butterfly due to the limited population and were overjoyed to witness the release. They even stopped strangers on the trail to show them the butterflies and educate them on the plight of the island marble.

Releasing a reared butterfly and witnessing its majestic flight is a breathtaking experience, symbolizing resilience and transformation—the future we strive for in safeguarding the island marble butterfly. I am looking forward to next season. In May and June of 2024, these very hungry caterpillars will emerge from their chrysalises, inspiring a whole new group of humans to fight for their survival.

Saani Borge is a Mosaics in Science intern working with the San Juan Island National Historical Park captive rearing program during the summer of 2023. Read more blog posts from her season on the island.

Further Information

• Read more about the captive rearing program at San Juan Island.
• Watch a short video on the rearing program from Outside Science (Inside Parks)

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