Silverspots: Myrtle’s silverspot (Speyeria zerene myrtleae)
(Lepidoptera: Nymphalidae: Argynninae)
Profile prepared by Scott Hoffman Black and Mace Vaughan, The Xerces Society for Invertebrate Conservation
Formerly widespread on the San Francisco and Marin peninsulas, Myrtle’s silverspot is now only known from four populations in northwestern Marin County and southwestern Sonoma County. Habitat loss due to residential and commercial land development has extirpated this butterfly from parts of its range and may threaten some of the remaining populations. Changes in natural fire patterns, introduction of exotic plants, successional changes in the plant community, and grazing have reduced the availability of host plants. silverspot butterfly larvae are also extremely sensitive to pesticides.
Xerces Red List Status: Critically Imperiled
Canada – Species at Risk Act: N/A
Canada – provincial status: N/A
USA – Endangered Species Act: Endangered
USA – state status: None
IUCN Red List: N/A
The current population status is unknown. No comprehensive range-wide surveys or monitoring have been conducted and surveys are needed to confirm population status. Surveys conducted in the early 1990s estimated populations of more than 1,000 at Point Reyes National Seashore and 2,500-5,000 at a nearby site that at the time was proposed for development.
Myrtle’s Silverspot is listed as a Federal Endangered Species (Federal Register 57:27848-27859; June 22, 1992).
Recovery Plan (ESA): Recovery Plan for Seven Coastal Plants and the Myrtle’s Silverspot Butterfly (September 30, 1998).
Critical Habitat (ESA): None designated.
Myrtle’s silverspot is a medium sized butterfly in the family Nymphalidae (brush-footed butterflies). Wingspan is approximately 2.2 inches. The dorsal surfaces of the wings are golden brown with numerous black spots and lines. The ventral surfaces are brown, orange-brown and tan with black lines and distinctive silver and black spots. Larvae are dark-colored with many sharp branching spines on their backs. Myrtle’s silverspot is larger in size and also lighter in color than the closely related Behren’s silverspot (Speyeria zerene behrensii; see Red List profile for details).
Speyeria zerene myrtleae dos Passos & Grey, 1945.
Females are single-brooded and lay their eggs in the debris and dried stems of violets (typically hookedspur violet, Viola adunca), the larval food plants. After hatching, the caterpillars wander a short distance and spin a silk pad upon which they pass the winter. At the end of their diapause in the spring, the caterpillars immediately seek out the food plant. After seven to ten weeks, the larvae form their pupa within a chamber of leaves drawn together with silk. Adults may emerge in about two weeks and can live for three weeks. The adult flight season may range from late June to early September. Adults feed on nectar from flowers including hairy gumweed (Grindelia hirsutula), coastal sand verbena (Abronia latifolia), mints (or monardella) (Monardella spp.), bull thistle (Cirsium vulgare), and seaside fleabane (Erigeron glaucus).
Myrtle’s silverspot is found in coastal dune or prairie habitat. Populations were formerly found in dunes and bluffs from San Mateo County north to the mouth of the Russian River in Sonoma County. The populations south of the Golden Gate apparently have been extirpated by urban development. Four populations are known to inhabit coastal terrace prairie, coastal bluff scrub, and associated non-native grassland habitats in western Marin and southwestern Sonoma counties, including the Point Reyes National Seashore. Adult butterflies are typically found in areas that are sheltered from the wind, below 810 feet (250 meters) elevation, and within 3 miles of the coast.
This butterfly is the southern-most member of a clade of closely related subspecies that live on the Pacific coast. Behren’s silverspot (Speyeria zerene behrensii; see Red List profile for more details) ranges along the coast of northern California and the Oregon Silverspot (Speyeria zerene hippolyta; see Red List profile for more details) had a historic range from northern California to southern Washington.
The major threat is loss of habitat due to development or vegetation change. The entire region in which this butterfly is found is under pressure for both residential and commercial development. Such development has extirpated this butterfly from parts of its range and may threaten some of the remaining populations. Changes in natural fire patterns, introduction of exotic plants, and successional changes in the plant community and grazing have reduced the availability of host plants. Silverspot butterfly larvae are also extremely sensitive to pesticides.
Both larval hostplants and adult nectar plants appear to be important. Obviously, without hostplants, the larvae will not survive. Nectar plants are required to provide females with adequate nutrition for egg development. Measures for habitat improvement may include eradication of invasive exotics such as iceplant (Mesembryanthemum spp.) and European beach grass (Ammophila sp.).
This butterfly is in serious need of action on its behalf by the public, including working with private landowners on whose land most of the populations survive. It is likely that given the specific habitat requirements of this subspecies and the history of environmental change brought on by human activities, that simply setting aside land for this butterfly will not ensure its future. Active management, whether by selective grazing or some other method, of Myrtle’s silverspot habitat will be necessary.
On going surveys to establish the distribution and population status are needed. Studies into the impacts of land management activities and the most appropriate way to manage sites would be valuable
Hammond, P. C., and D. V. McCorkle. 1983. The decline and extinction of Speyeria populations resulting from human environmental disturbances (Nymphalidae: Argynninae). Journal of Research on the Lepidoptera 22(3): 217-224.
Launer, A. E., D. D. Murphy, J. M. Hoekstra, and H. R. Sparrow. 1992. The endangered Myrtle’s silverspot butterfly: present status and initial conservation planning. Journal of Research on the Lepidoptera 31 (1-2): 132-146.
Steiner, J. 1990. Bay area butterflies: The distribution and natural history of the San Francisco region Rhopalocera. Master’s thesis. California State University at Hayward.
Thelander, C. (ed.). 1994. Life on the edge: a guide to California’s endangered natural resources. BioSystem Books. Santa Cruz, CA. p 436-437.
U.S. Fish and Wildlife Service. 1998. Recovery Plan for Seven Coastal Plants and the Myrtle’s Silverspot Butterfly. USFWS Region 1, Portland, Oregon. Available online (Accessed 4/12/05)
U.S. Fish & Wildlife Service; Threatened and Endangered Species System: Myrtle’s Silverspot Butterfly (Accessed 9/17/08)
U.S. Fish & Wildlife Service, Sacramento Fish & Wildlife Office; Endangered Species Division: Myrtle’s Silverspot Butterfly (Accessed 4/12/05)
NatureServe Explorer (Accessed 9/23/08)
University of California at Berkeley; Essig Museum of Entomology: California’s Endangered Insects, Myrtle’s Silverspot Butterfly (Accessed 4/12/05)
Black, S. H., and D. M. Vaughan. 2005. Species Profile: Speyeria zerene myrtleae. In Shepherd, M. D., D. M. Vaughan, and S. H. Black (Eds). Red List of Pollinator Insects of North America. CD-ROM Version 1 (May 2005). Portland, OR: The Xerces Society for Invertebrate Conservation.