Oregon Silverspot Butterfly

Speyeria zerene hippolyta

Description

The Oregon silverspot butterfly, Speyeria zerene hippolyta, is a medium-sized orange and brown butterfly with black veins and spots on the hind wings, and a yellowish band and bright metallic silver spots on the forewings. It has a forewing length of about 1.1 in (2.9 cm); females are typically slightly larger than males.

The species S. zerene consists of 15 subspecies, divided into five major groups. The Oregon silverspot belongs to the bremnerii group consisting of five subspecies. Compared to its relatives, the Oregon silverspot is slightly smaller and darker at the base of the wings—these are adaptive traits derived from its persistently windy and foggy environment.

Behavior

Adult butterflies emerge from early July to early September. Mating occurs during adult flights, and females deposit 200 or more eggs in the vegetation near the violet host plant in late August or early September. Eggs hatch in about 16 days, and the larvae seek out suitable places for overwintering. In spring, the larvae feed on violet leaves for two months, then enter pupation for two or three weeks. Some adults emerge during periods of sunny, relatively calm weather.

Habitat

This subspecies is found only in the salt spray meadows along the Pacific Coast of Washington and Oregon. The climate is characterized by mild temperatures, heavy rainfall, and fog. The most important feature of the habitat is the presence of the western blue violet (Viola adunca ), the larval host plant.

Distribution

The Oregon silverspot was historically found in 17 different locations between Rock Creek and Big Creek, about 15 mi (24 km) north of Florence along the central Oregon Coast, and in the vicinity of Westport, south of Grays Harbor, Washington, which is the northern extent of the range.

By 1980, when it was listed by the U. S. Fish and Wildlife Service (FWS) as a threatened species, the silverspot was known from only one site, which was located on the Siuslaw National Forest along the central Oregon Coast. Since then, the species has been found at six other small sites on federal, state, and private land. The butterfly is not abundant anywhere, and in a typical year there are fewer than 4,000 individuals distributed along 350 mi (560 km) of coastline.

The FWS considers the species Threatened in California and Oregon.

Threats

The main threats to this butterfly are increased housing development and recreational use of the coast. Natural fire patterns have been suppressed, allowing non-native plants to intrude and change the mix of plants in the habitat. There have also been rapid successional changes in the native plant community.

Conservation and Recovery

Critical habitat was designated for an area in Lane County, Oregon, where a healthy population of the butterfly exists. In 1983, the Siuslaw National Forest, in consultation with the FWS, began to restore about 100 acres (40 hectares) of meadow habitat. Efforts included burning, introducing violet seeds and plants, mowing grass thatch, and removing invading trees and shrubs by machine or hand. A cautious approach was used. Treatments were confined to small plots outside of prime habitat where there was little risk of killing butterfly larvae.

Mowing several times a year (every fourth or fifth year), particularly after the initial surge of growth in late spring or early summer, reduces grass thatch and often produces spectacular stands of blooming blue violets. Removing scattered stands of invading woody plants and maintaining shelter areas in the forest fringe has been relatively easy, and has opened up more areas for mowing. As of 1994, burning was restricted largely to removing mowing residue and to clearing steep slopes where mowing was impossible.

Although efforts have not always been successful, results so far have exceeded expectations. Adult silverspots heavily use many of the renovated areas, and by 1994, three populations were reasonably secure within the Siuslaw National Forest. A fourth, introduced population had maintained itself at a low level for four years. Overall, it seems that the species is on the way to recovery in Oregon.

Contact

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Complex
911 N. E. 11th Ave.
Portland, Oregon 97232-4181
Telephone: (503) 231-6121
http://pacific.fws.gov/

References

Arnold, R. A. 1988. "Ecological and BehavioralStudies on the Threatened Oregon Silverspot Butterfly." Report. U. S. Fish and Wildlife Service, Olympia, Wash.

Hammond, Paul C., et al. 1980. "Ecological Investigation Report: Oregon Silverspot Butterfly (Speyeria zerene hippolyta ) Mt. Hebo Supplement." U.S. Forest Service.

Howe, W. H. 1975. The Butterflies of North America. Doubleday, Garden City, N.Y.

U. S. Fish and Wildlife Service. 1982. "The Oregon Silverspot Butterfly Recovery Plan." U. S. Fish and Wildlife Service, Portland, Oregon.