Myrtle’s Silverspot Butterfly

Speyeria zerene myrtleae

Overview

Formerly widespread on the San Francisco and Marin peninsulas in California, the butterfly 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 these butterflies from parts of their 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.

Description

Myrtle’s silverspot (Speyeria zerene myrtleae) is a medium sized butterfly in the brush foot family (Nymphalidae). Wingspan is approximately 2.2 inches (5.6 cm). 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).

Life History

Females are single-brooded and lay their eggs in the debris and dried stems of violets (typically 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. The caterpillars immediately seek out the food plant at the end of their diapause in the spring. After 7-10 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 gumplant (Grindelia rubicaulis), yellow sand verbena (Abronia latifolia), mints (Monardella spp.), bull thistle (Cirsium vulgare), and seaside daisy (Erigeron glaucus).

Distribution

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 820 feet elevation, and within three miles of the coast.

Conservation Status

No comprehensive range-wide surveys or monitoring have been conducted. Surveys are needed to confirm stable population status given in 1996 report – current status is unknown. As noted in a 1998 report, land ownership has changed at one site; the Service does not have information regarding the status of the site. No new information has been submitted for 2001 supporting a change in status.

Conservation Needs

Measures for habitat improvement may include eradication of invasive exotics such as iceplant (Mesembryanthemum spp.) and European beach grasses.

Working with landowners

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 population survive.

Education

Education sheets available at zoological facilities (e.g. zoos, aquariums, botanical gardens with butterflies, butterfly houses, natural history museums) or events at which BFCI partners participate are a valuable way to disseminate information about imperiled butterflies.

When schools and other youth organizations study biodiversity and species extinctions they typically use examples of charismatic megafauna (e.g. bald eagles) or exotic creatures from the tropics. However, vulnerable species found within the state or ecoregion in which students live provide an excellent opportunity to develop curricular materials with a direct link to the students’ home region. In addition to classroom studies, students may be able to visit sites to see the butterflies, as well as talk to the scientists and land managers involved in the species’ conservation.

Educational activities that school students and community members could do include:

  • studying butterfly (insect) life stages;
  • researching the special habitat needs of the Myrtle’s silverspot;
  • corresponding or meeting with the biologists managing current Myrtle’s silverspot sites;
  • visiting Myrtle’s silverspot sites during adult flight season;
  • visiting captive breeding programs;
  • assisting scientists with on-site habitat management;
  • propagating and growing host plants for planting at butterfly sites or use in captive breeding programs; and
  • writing letters to decision makers to ensure that the Myrtle’s silverspot receives adequate resources and protection.

Research

Insufficient information available at time of publication.

Captive Rearing

Insufficient information available at time of publication.

Recovery Plan

Recovery Plan for Seven Coastal Plants and the Myrtle’s Silverspot Butterfly (10.6MB pdf), September 30, 1998.

The Myrtle’s silverspot butterfly is included in a recovery plan for seven coastal plant species that was finalized in 1998. As these species share the same habitat in many instances, the recovery goals are shared by all of the species.

According to the recovery plan the Myrtle’s silverspot can be reclassified to threatened status when:

  • The habitat of the northwestern Marin County/southwestern Sonoma County population of this species is protected in perpetuity.
  • Two new populations have been discovered or re-introduced at suitable sites that have been protected in perpetuity.
  • Adequate funding for management of all sites is assured and adaptive management plans have been developed and are being implemented.
  • Annual monitoring has shown the five populations (three existing, two new) cumulatively to have a total of more than 10,000 adults in each of ten years, with no individual population having fewer than 200 adults in any year and no recent severe declines.

The delisting criteria for the Myrtle’s silverspot are as follows:

  • Nine populations of the species have been established (three existing, six discovered or re-introduced) on habitat protected in perpetuity. If appropriate sites have been identified in the screening and prioritization process, at least two of these populations should be south of the Golden Gate.
  • Adequate funding for management of all sites is assured and adaptive management plans have been developed and are being implemented.
  • Annual monitoring has shown the nine populations cumulatively to have a total of more than 45,000 adults in at least eight years, no fewer than 10,000 adults cumulatively in any year, no individual population having fewer than 100 adults in any year, and no recent severe declines.

Recovery Priorities

The recovery strategy for the Myrtle’s silverspot butterfly includes the following measures:

  • Protect habitat where remaining populations occur.
  • Identify and establish vegetation management that benefits the native ecosystem of larval host plants and adult nectar sources.
  • Re-introduce populations of the butterfly to prioritized areas.
  • Control illegal collecting.
  • Conduct or fund research to identify critical recovery needs or actions.
  • Monitor existing populations and survey historic and un-surveyed locations.

In order to accomplish the interim objectives and formulate primary objectives, the following recovery activities are listed in the plan:

  • Preserve and protect populations at all known, new, and reestablished sites.
  • Establish three new, self-sustaining viable populations on suitable secure habitats of at least two hectares.
  • Conduct ecological studies to develop management recommendations, determine larval and adult host plants, physiological requirements, demographics, and other biological/ecological studies, and to determine criteria for declassifying and delisting.
  • Develop and implement public information and education programs.
  • Enforce laws and regulations prohibiting illegal take and enforce land use plans and ordinances.

More Info

  • USFWS Recovery Coordinator/Contact: Jim Browning, Sacramento Fish and Wildlife Office 2800 Cottage Way, Room W-2605, Sacramento, California 95825. Phone (916) 414-6600
  • Entomological Consulting Services, Ltd.: Richard Arnold, 104 Mountain View Court, Pleasant Hill, California 94523-2188. Phone (925) 825-3784

References

  • 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, 1994. 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. pp. 436-437.
  • U.C. Berkeley, Essig Museum of Entomology. California’s Endangered Insects Myrtle’s Silverspot page.
  • U.S. Fish & Wildlife Service. 1988. Recovery Plan for Seven Coastal Plants and the Myrtle’s Silverspot Butterfly (in PDF). Portland, OR.
  • Sacramento Fish and Wildlife Office Endangered Species Division Myrtles silverspot resources
  • USFWS Myrtles silverspot resources

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