Raising the Island Marble Butterfly, One Caterpillar at a Time

Written from a July 2023 post by Saani Borge, a Mosaics in Science intern with San Juan Island National Historical Park's captive rearing program.

"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 caterpillar in its fifth and final instar. In the wild, an estimated 5% of eggs laid each year survive to become butterflies. NPS photo / Bolin.

The hungriest caterpillars

The day in San Juan Island's captive rearing program starts with tiny caterpillars devouring mustard plants — straight out of Eric Carle's The Very Hungry Caterpillar, minus the chocolate cake and cherry pie. They're on a strict green-leaf diet.

• Hatching: less than 2 mm long.
• Growing: they shed their skin again and again until they're the size of a fork prong.
• Ready: they turn pink and roam the cage for a spot to hang their chrysalis, where they stay through fall and winter.
• Next summer: they emerge as island marble butterflies (Euchloe ausonides insulanus) — an endangered species found only on San Juan Island.

The butterflies depend mostly on two non-native plants, field mustard and tumble mustard, and on native Virginia pepperweed. Fresh cuttings come indoors so the caterpillars never go hungry. NPS photo / Crawford.

Back from "extinction"

The island marble is a subspecies of the large marble butterfly (Euchloe ausonides) of western North America. It was thought extinct for nearly a century, until two Washington State Department of Natural Resources researchers found this last population during a survey of San Juan Island's American Camp in 1998.

It is still on the brink: its host plants are scarce, its range is tiny, and it faces habitat loss and predators. The whole population is about 300. So each summer the program collects over 100 eggs to raise in safety and release the next year.

Chrysalises lined up in the rearing lab. Once the butterflies emerge, they're released to mate and lay eggs for the next generation. NPS photo / Galloway.

A poke in the rear

The job is to keep the caterpillars fed and content. Cuttings don't last: uneaten plants wilt on the cage floor, so the caterpillars have to be coaxed onto fresh buds — often with gentle pokes to the rear as they twist and cling to their old plant. Sometimes they fall. Most recover, not all, and every nudge carries weight.

A delicate balance: the park also grows more host plants and fences off sensitive habitat. It can't collect too many eggs, which could create a genetic bottleneck or leave the butterflies open to disease.

Worth it

The best part, the author writes, is the delight the butterfly brings visitors — like a Butterfly Conservation tour group that had little hope of seeing one, then watched two captive-raised butterflies released. They were so thrilled they stopped strangers on the trail to show them and explain the island marble's plight.

Watching a released butterfly take flight is a symbol of resilience and transformation. In May and June 2024, these very hungry caterpillars would emerge from their chrysalises and, she hoped, inspire a new group of people to fight for them.

More: the captive rearing program, a short video from Outside Science (Inside Parks), and the author's other posts from the season.

Sources

Based on "Conserving the Island Marble, One Poke in the Rear at a Time," by Saani Borge, published by the National Park Service and in the public domain; rewritten in hubnx's own words. The author's portrait is not reproduced.