Reintroduction of the Oregon silverspot butterfly on the Oregon Coast, Tillamook County, Oregon, USA

Summary

The Oregon silverspot butterfly (Speyeria zerene hippolyta), federally threatened since 1980, had dwindled from 20 historical Pacific coast locations to five populations by 2016, with two drought years further driving sharp declines across all of them. Building on a captive-rearing program run by the Oregon Zoo and Woodland Park Zoo since 1999-2000 that had already augmented four of the five wild populations, the US Fish and Wildlife Service restored 12 hectares of coastal prairie at Nestucca Bay National Wildlife Refuge in Oregon, planting 56,000 early blue violets, the caterpillar's sole host plant, plus thousands of nectar plants between 2015 and 2017. In August 2017, roughly 900 captive-reared caterpillars, mostly larger instars for better predation and starvation resistance, were released onto violet patches in two batches, designated a nonessential experimental population under the Endangered Species Act; adults were observed flying within three weeks, mating and laying eggs. About 8% of released caterpillars were later detected as adults during weekly transect counts, slightly below the roughly 12% rate seen with a caged-pupae release method used elsewhere. A second reintroduction is planned for 2018 at Saddle Mountain State Natural Area, unoccupied since 1973 but now supporting restored habitat. Because the new population currently descends from just 16 wild females, well short of the 50 needed for full genetic diversity, further incremental releases are planned. The project rates itself successful, citing confirmed post-release breeding and reusable husbandry and habitat-restoration methods, while flagging degraded-habitat restoration and precise caterpillar-release timing as the most persistent difficulties.