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Island marble butterfly – once thought extinct and only known to exist in Washington – listed as endangered

UPDATED: Tue., May 5, 2020

An island marble butterfly adult on its host plant. (US Fish and Wildlife Service)
An island marble butterfly adult on its host plant. (US Fish and Wildlife Service)

For most of the 20th Century, the island marble butterfly was thought to be extinct.

Then the green-and-white winged insect that had disappeared from Vancouver Island and some other nearby islands was discovered on San Juan and Lopez islands, where some of the mustard plants it likes to eat are found.

With its habitat shrinking, the federal government is trying to make sure island marble, formally known as Euchloe ausonides insulanus, doesn’t go extinct for real. On Monday the butterfly, which now exists only on San Juan Island, was listed as an endangered species. It’s main home in the island’s national park was listed as a critical habitat.

While the discovery of the island marble butterfly in the San Juans was a surprise in 1998, it’s possible the butterfly was always there, said Karen Reagan, an endangered species biologist with the U.S. Fish and Wildlife Service.

“A lot of times, we don’t see what we’re not looking for,” Reagan said. “Nobody was looking for it.”

The island marble can be confused with the more common cabbage white at a distance, she said.

But it was “rediscovered” on San Juan and Lopez islands during a 1997-98 survey of butterflies, where its main food source was a mustard plant that grows at the edge of the shoreline. Since then, however, that habitat has declined from various types of development and because of larger storm tides in the winter.

“Butterflies have a great capacity to change their diet,” Reagan said. Fortunately for the island marble, there are two other species of mustard plants on San Juan found in the island’s National Historical Park. The park holds a large prairie area once occupied by early settlers who planted field mustard and the butterflies have adapted to that.

The butterflies lay eggs in the mustard flowers, which hatch in 10 to 12 days, grow into a caterpillar that lasts about a month before spinning a chrysalis that can last for 11 months, a long time for a chrysalis, Reagan said. The winged adults emerge in the spring, live about six to nine days, during which they lay eggs and the process starts all over.

One problem for maintaining the species is that the flowers where the adults like to lay eggs are a favorite food of the island deer. Another is the chrysalis is very similar to a dead blade of grass and can get trampled.

There are fewer than 200 island marble butterflies in the park, which was designated a critical habitat in the species’ long-term development plan. Recovery of the island marble is a joint effort of federal and state agencies, local landowners and conservation groups, Fish and Wildlife Supervisor Brad Thompson said in a news release announcing the endangered species listing.

A butterfly that was thought to be extinct for some 90 years and is now so rare as to be endangered could prompt collectors to come to the park in an effort to catch one the remaining few, Reagan said.

“That’s always a concern. We really have to count on people doing the right thing,” she said.

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