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Behren's Silverspot Butterfly
Speyeria zerene behrensii

General Information

Official Status: Endangered, the Behren's silverspot butterfly is federally listed under the Endangered Species Act as endangered.

Date Listed: December 5, 1997; Federal Register: 62 FR 64306.

Critical Habitat:Critical habitat for Behren’s silverspot butterfly has not been designated.

Recovery Plan: A Draft Recovery Plan (pdf, 650 KB) for Behren’s Silverspot Butterfly (Speyeria zerene behrensii) was published in January 2004.


Behren's Silverspot Butterfly, Photo Credit: Gordon Pratt
Behren's Silverspot Butterfly

Photo Credit:Gordon Pratt

arrow button Photo Gallery for the Behren's Silverspot Butterfly

Identifying Characteristics:

The Behren’s silverspot butterfly is a medium-sized butterfly with a wingspan of approximately 5.5 cm (2.2 inches).  The upper surfaces are golden brown with numerous black spots and lines.  Wing undersides are brown, orange-brown, and tan with black lines and distinctive silver and black spots.  Basal areas of the wings and body are covered with fine hairs.  The Behren’s silverspot is a member of the family of true fritillary, or silverspot butterflies, of which 13 species occur in North America.  The species Speyeria zerene, sometimes known as the Zerene Fritillary, includes a number of subspecies, of which eight occur in the Pacific Northwest and on the California coast.

The Behren’s silverspot butterfly is similar in appearance to two other coastal subspecies of Speyeria zerene, the Oregon silverspot butterfly (Speyeria zerene hippolyta) and Myrtle’s silverspot butterfly (S. z. myrtleae), both of which are also federally listed. The Behren’s silverspot differs from the Oregon silverspot primarily by its darker basal suffusion on the upper sides of the wings and its relative larger size.  The Myrtle’s silverspot is larger in size and lighter in color than the Behren’s silverspot.

Current Geographic Range:

The historic range of Behren’s silverspot butterfly is based on six known locations which extended from near the City of Mendocino, Mendocino County, south to the area of Salt Point State Park, Sonoma County (USFWS 2003).  The current known range of the Behren’s silverspot butterfly is limited to a small number of sites located from the Point Arena-Manchester State Park area south to the Salt Point area.  South of Salt Point in coastal Sonoma County, populations of Zerene Fritillary occur, which have similarities to both the Behren’s and Myrtle silverspot subspecies.

Life History:

The best available information on the life history of the Behren’s silverspot butterfly comes from studies of a closely-related coastal subspecies, the Oregon silverspot butterfly.  Those studies found that females lay their eggs in the debris and dried stems of the larval food plant, the early blue violet (Viola adunca).  The early blue violet is a small, native, perennial herb with pale to deep violet flowers.  This violet typically blooms in late spring to early summer and dies back to the perennial rhizome during winter.  Early blue violets occur widely in western North America; within the Behren’s silverspot’s range, they are associated with coastal grasslands.

Upon hatching, the caterpillars (larvae) wander a short distance and spin a silk pad upon which they pass the fall and winter in diapause (dormancy).  The larvae are dark-colored with many branching, sharp spines on their backs.  Upon ending diapause in the spring, the larvae immediately seek out the violet food plant.  During the spring and early summer they pass through five instars (stages of development) before forming a pupa within a chamber of leaves that they draw together with silk.  The adult butterflies emerge in about two weeks and live for approximately three weeks, during which time they feed on nectar and reproduce.  Depending upon environmental conditions, the flight period ranges from about July through August or early September.

Behren’s silverspot butterfly flight behavior is moderately erratic and swift in windy places, 0.3 to 1.8 meters (2 to 6 feet) above ground surface.  Flights usually occur by late morning when temperatures are above about 60 degrees Fahrenheit.  Adults may feed on nectar as long as 5 minutes, returning to the same plant repeatedly.  Butterflies may rest on bare ground, in grasses, or on ferns [bracken] and other foliage.

Adult Behren’s silverspot butterflies feed on nectar, which is their only food source, besides internal reserves present when they emerge from the pupae.  Observations of nectar feeding are few, but based on observations of this and closely related silverspot subspecies, plants in the sunflower family (Asteraceae) dominate as nectar sources, including thistles (Cirsium spp); gumplant (Grindelia stricta); goldenrods (Solidago spp); tansy ragwort (Senecio jacobaea), California aster (Aster chilensis), pearly everlasting (Anaphalis margaritacea), seaside daisy (Erigeron glaucus), and yarrow (Achillea millefolium). Reported nectar species from other plant families include: yellow sand verbena (Abronia latifolia), sea-pink (Armeria maritima) and western pennyroyal (Monardella undulata).

General Habitat Characteristics:

The Behren’s silverspot butterfly inhabits coastal terrace prairie habitat west of the Coast Range in southern Mendocino and northern Sonoma Counties, California.  This habitat is strongly influenced by proximity to the ocean, with mild temperatures, moderate rainfall, and frequent summer fog.  An occupied or potential site must have two key resources: 1) caterpillar host plants; and 2) adult nectar sources.  Distribution of the Behren’s silverspot butterfly is highly dependant on these resources.  Coastal terrace prairie is a dense grassland dominated by perennial grasses, on sandy loam soils on marine terraces below about 1,000 feet elevation and within the zone of coastal fog.  In addition to perennial and annual grasses, the coastal prairie vegetation includes bracken ferns and woody shrubs and trees such as coyote brush, red alder, salal, and conifers.  Violets, in particular the early blue violet, need to be present, as they are the butterfly’s larval host plant.  Nectar sources need to be available to foraging adults during the summer flight period.  In addition to availability of violets and nectar plants, areas with shelter from wind may affect habitat suitability.  The coastal prairies within the species’ range are frequently windy during the butterfly flight season, with winds predominantly from the northwest. Trees and large shrubs, as well as topographic features, can provide sheltered pockets, where microclimates are more favorable to butterfly flight and essential activities during windy periods.

Population and Habitat Status:

Little is known about the amount and distribution of suitable habitat for the Behren’s silverspot.  We know that coastal prairie habitat is highly fragmented by agricultural and residential use, roads, and other human development.  Within this fragmented landscape, suitable coastal prairie habitat (with violets and nectar plants) is patchy and further fragmented.  Recent surveys indicate that few remaining coastal prairie sites have sufficient early blue violet populations to support populations of the butterfly.  The population status of Behren’s silverspot is not well known, but surveys suggest that numbers are very low.  The species persists at three of six known historic locations, but the area of potential habitat, and likely populations, appears to be small at two of these locations.  The third area, near Point Arena, appears to have more a widely-distributed population, with sightings of the species at multiple locations within an area of several square kilometers, although initial surveys suggest sparse and localized distribution within this area.

Threats:

The primary threats to the Behren’s silverspot butterfly, cited at the time of listing, are overcollecting, and habitat destruction, fragmentation and degradation due to urban development, alien plant invasion and competition, and excessive livestock grazing.  Other factors include potential genetic problems associated with small populations, the lack of natural, periodic fires to maintain coastal prairie habitats, and the inadequacy of existing regulatory mechanisms to protect the species and its habitat.

Conservation Needs:

Immediate conservation needs of the species include preventing the further loss and degradation of known and suitable habitat within the species range, combined with protection of areas to provide for long-term viability of the species and its habitat. 

A conservation strategy for the Behren’s silverspot butterfly is provided in the draft recovery plan, (pdf, 650 KB) and includes two key priority actions: 

1. location and protection of existing Behren’s silverspot butterfly populations and their habitat, and

2. identification of suitable habitat that can be managed for the conservation of the Behren’s silverspot butterfly should reintroduction be warranted. 

In addition, augmentation may be warranted to maintain existing metapopulations, as may be reintroduction to establish new metapopulations in areas of existing suitable and/or restored habitat within the historic range.  The recovery strategy also calls for management and monitoring of protected habitats to address continuing and persistent threats.

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Last updated: April 11, 2011

Arcata Fish and Wildlife Office, 1655 Heindon Road, Arcata, California 95521, USA
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