Oregon Silverspot Butterfly

Speyeria zerene hippolyta
PROFILE Silverspot 520x289

Few things convey the fleeting loveliness of summer better than a butterfly on the wing, flitting from flower to flower, rarely settling in one spot for long. The Oregon Silverspot Butterfly, a federally threatened species found only at a handful of sites in Oregon and California, is an especially poignant example. In its delicate flapping there is a sense of frantic impermanence, of fragile, ephemeral vim. The butterfly, like the flowers and the lucent sun, will last only the season, and so must get right to work.

Oregon Silverspots depend entirely on the early blue violet (Viola adunca) to nuture their miniscule young. This violet, similarly modest in mien, grows best on undisturbed coastal prairies and montane grasslands, habitat types fast disappearing from the Pacific Coast.

The historic range of the Oregon Silverspot extends from Westport, Grays Harbor County, Washington, south to Del Norte County, California. Most of these populations are restricted to the immediate coast, centered around salt-spray meadows, or within a few miles of the coastline in similar meadow-type habitat. Unfortunately for the butterfly, these habitat types are threatened themselves, either lost to development or in rapid decline. Within its range, the butterfly is known to have been extirpated from at least 11 colonies (two in Washington, eight in Oregon, and one in California). In 1980 (when the butterfly was listed as threatened under the Endangered Species Act), the only viable population known was in the Siuslaw National Forest in Tillamook County, Oregon. Additional populations have since been discovered at Cascade Head, Bray Point, and Clatsop Plains in Oregon; on the Long Beach Peninsula in Washington; and in Del Norte County in California. The status of these isolated colonies remains tenuous at best.
 
The life history of the Oregon Silverspot revolves around its obligatory host plant, Viola adunca. Females lay, or oviposit, some 200 eggs among salt-spray meadow vegetation near the host plant, usually in late August and early September. Approximately 16 days later the eggs hatch, and the larvae wander short distances to find a suitable site for diapause (suspended growth for overwintering). Diapause ends sometime in early spring and the larvae begin to feed on violet leaves. In quick succession the larvae pass through five molts before pupating, whereupon they enter the intermediate stage between larval and adult forms. Approximately two weeks later the butterflies emerge. Adult emergence starts in July and extends into September; mating takes place soon after, in relatively sheltered areas through the end of summer.

The decline of the Oregon Silverspot is primarily due to habitat destruction and modification by development, agriculture, invasion of nonnative vegetation, and natural succession of grasslands—that is, grassland giving way to forest. Additionally, excessive use of salt-spray meadows by grazing animals or off-road vehicles has directly eliminated habitat. 

The coastal prairie habitat on which the Oregon Silverspot is dependent becomes dominated by shrubs or forest if left unmanaged. Natural processes such as wildfires and wildlife grazing likely functioned to maintain open grasslands in the past. On Cannery Hill at Nestucca Bay NWR, the Institute for Applied Ecology (IAE) is leading a habitat restoration effort to actively maintain grassland structure preferred by the butterflydespite there being no Oregon Silverspots currently present at the site. Introduced grasses were burned and native ones planted in their stead, along with nectar-rich forbs and the all-important Early Blue Violet. 
 
USFWS biologist and silverspot expert Anne Walker calls these efforts "an experimental process." Nestucca Bay NWR is within the known historic range of the butterfly, she points out, although they haven't been seen there in decades. Through IAE’s restoration plan, best-known practices are being applied to re-establish native coastal prairie vegetation and reduce encroachment by trees and shrubs. The fact that butterflies aren’t present actually affords flexibility in grass-control measuressuch as the application of herbicidesthat would otherwise be dangerous to the Oregon Silverspot. Ultimately, however, only time will tell if native flowers can once again take hold on this remnant of historic coastal prairie habitat. If they do, butterflies captive-raised by the Oregon Zoo in Portland and the Woodland Park Zoo in Seattle will be introduced to re-populate the prairie.

Facts About Oregon Silverspot Butterfly

-Listed as a federally Threatened Species with critical habitat in 1980

-Found in coastal prairies and montane meadows at six known sites, one in California and five in Oregon

-Larvae can feed and develop on only one plant species: Early Blue Violet

-Each larva requires 200-300 violet leaves to pupate