Myrtle's Silverspot Butterfly

Speyeria zerene myrtleae

Description

Myrtle's silverspot, Speyeria zerene myrtleae, is a medium-sized butterfly with a wingspan of 2.1-2.3 in (5.3-5.8 cm). The upper surfaces of the wings are golden brown and possess many black spots and lines. The undersides are brown to orange-brown matriculated with tan and black lines and conspicuous silver and black spots. The basal areas of the wings and body are densely pubescent.

Overall, Myrtle's silverspot is similar in size and appearance to Behren's silverspot, but can be distinguished by the center of the underside of the hind wing, which is reddish-brown with yellowish-green overscaling, and a bright yellow submarginal band.

Behavior

During the summer flight season, female Myrtle's silverspot butterflies lay their eggs singly on or near dried leaves and stems of violets. Eggs of the closely related Oregon silverspot butterfly (S. z. hippolyta ) are milky white at first, then gradually change to pale yellow, then to brownish-grey or pinkish-bronze with pearly longitudinal lines. About five days before hatching, the head capsule can be seen as a dark purple tip, with the remainder of the egg lightening to white or gray.

Within a few weeks after the eggs are laid, the larvae or caterpillars emerge. These caterpillars, which are less than 0.2 in (5 mm) long, crawl a short distance into the surrounding foliage or litter, and spin a silk pad on which they spend the fall and winter. The fall and winter period of inactivity is a physiological resting state called diapause, during which no feeding occurs. The larvae may be able to extend their diapause for more than one year. Upon termination of diapause in the spring, the caterpillar finds a nearby violet and begins feeding. Feeding is difficult to observe, and apparently occurs at dusk and possibly at night. The larval feeding stage lasts about seven to 10 weeks, after which the larvae form their pupal chamber out of leaves spun together with silk.

The adult butterfly emerges from the pupa after about two weeks. Emergence typically occurs from mid-June to mid-July. The timing of adult emergence is probably related to photoperiod and to weather, especially temperature and sunlight, which may result in annual differences in the timing of peak emergence of as much as a few weeks. Although Myrtle's silverspot adults live for only about two to five weeks, because of individual variation in emergence time, the species has a two-to three-month flight period, ranging from mid-June to early October. Adult activity is closely tied to weather conditions: the butterflies are active during calm weather and inactive during windy periods.

Both sexes are good flyers and can trave miles in search of nectar, mates, or violets; if, however, all these resources are available in topographically restricted valleys or basins that are sheltered from strong winds, most movements are short. Males emerge earlier than females and patrol widely for females, a behavior that may tend to bias survey counts in the males' favor. In related species, both sexes produce pheromones that apparently function in mate-finding and courtship. Males of most Speyeria species transfer a mating plug at the end of copulation, and most females therefore mate only once.

Little is known about impacts of disease, parasitism, or predation on Myrtle's silverspots. About 40% of 11 Oregon silverspot eggs observed in the field died before hatching, due to predation or disease. The larvae have spines and an eversible (can be protruded) ventral scent gland, similar to structures that in other species are used to ward off predators. Silverspot larvae may be vulnerable to predation by ants, ground beetles, spiders, and shrews.

Adult feeding on nectar is very important to the reproduction of the Myrtle's silverspot. In a related species, S. mormonia, a strong correlation exists between the amount of nectar consumed by female butterflies and the number of eggs they produce. Males of Myrtle's silverspot also feed on nectar, possibly to fuel their patrolling and mating activities. Ideally, a spectrum of plants that flower across the flight season of the butterfly is used.

Populations of Speyeria butterflies are known to exhibit large fluctuations in numbers of individuals, appearing at times to be virtually on the brink of extinction, and then rebounding to substantially higher numbers the following year. Such wide population fluctuations—changing by a factor of 10 or more in a single year—are typical of insects with little overlap among generations, and of annual plants, and stand in contrast to most vertebrate populations. High and variablemortality during the immature life stages, and corresponding high fecundity to counterbalance the high mortality, are characteristic of these butterflies. The annual variability of California climate, even at the relatively moderate, maritime-influenced locations inhabited by the Myrtle's silverspot, contribute to extreme population fluctuations through effects on development, mortality, and fecundity, either directly or mediated through effects on host plant growth and survival. Myrtle's silverspot butterfly occurs in separate populations whose long-term persistence may depend upon intercolony movement. Habitat degradation resulting in the loss of intervening populations, larval food plants, and adult nectar sources may make movements between populations more difficult.

Habitat

Myrtle's silverspot inhabits coastal dunes, coastal prairie, and coastal scrub at elevations ranging from sea level to 1,000 ft (300 m), and as far as 3 mi (4.8 km) inland. The adult butterflies prefer areas protected from onshore winds, but can be observed in exposed areas when winds are calm. Temperatures in this region are moderated by fog, which keeps summers relatively cool and winters relatively warm compared to inland habitats. The fog also provides moisture to vegetation, in addition to the ample winter rains.

Critical factors in the distribution of the Myrtle's silverspot include presence of the presumed larval host plant,Viola adunca (western dog violet), and availability of nectar sources for adults. Although alternate larval host plants have neither been confirmed nor ruled out for the Myrtle's silverspot, other subspecies of S. zerene and other species of Speyeria can feed on more than one species in the genus Viola. Seeds of Viola are often dispersed by ants. Violets sometimes bear self-pollinating flowers, and are also cross-pollinated by insects. Selection of habitat for oviposition (egg laying) has been observed in the Oregon silverspot. Gravid (fertilized egg-bearing) females were attracted to areas of low vegetation height (15 in [38 cm] or less), where they would sometimes perform a searching flight characterized by low hovering or dipping. Females flew past areas with deep thatch (covered by plant litter). They were more likely to land and perform a walking search and more likely to lay eggs in areas with higher violet density and cover. Other, unknown factors, however, must also affect the selection of oviposition sites, since some portions of the study sites with low vegetation height and high violet cover were used less frequently than expected. It has been therefore suggested that managing for violet density alone would not necessarily enhance silvrspot oviposition.

Much of the coastal prairie in this species' range has been grazed for more than a century, and is now characterized by a mixture of non-native annuals and forbs and native prairie plants. In the upland grasslands, this butterfly has been observed obtaining nectar from non-native species such as bull thistle and rarely Italian thistle. In dune scrub habitat, these butterflies seek nectar from several native species such as gum plant, western pennyroyal, yellow sand verbena, seaside daisy, and mule ears. Other flowers might serve as good nectar sources for the opportunistic adults, such as brownie thistle and groundsel. Myrtle's silverspot does not use the flowers of the invasive non-native iceplant or sea fig for nectar.

Distribution

Myrtle's silverspot occupies the southernmost range of all the coastal S. zerene silverspot butterflies. It was recorded from coastal San Mateo County as far south as Pescadero in 1950, north to the vicinity of Black Point in northern Sonoma County. There were only 10 specimens collected south of the Golden Gate, and by the late 1970s, populations of Myrtle's silverspot south of the Golden Gate Bridge were believed to be extinct and extant populations were known only from Marin County at the Point Reyes National Seashore. In 1990, an additional population was discovered at a site in coastal Marin County, near Estero de San Antonio, on property proposed for golf resort and residential development. This discovery led to more surveys of the current and historical range of Myrtle's silverspot butterfly. The proposal for the golf course was withdrawn and later replaced with a proposal for low-density residential development and open space at the same site. At the private site in coastal Marin, the number of Myrtle's silverspots was estimated to be between 2,500 and 5,000 adults in 1991. Two apparently separate populations in Point Reyes National Seashore were estimated at less than 5,000 individuals and several hundred individuals, respectively, in 1993. As of the late 1990s, this butterfly was known from three occurrences, with a probable total of less than 10,000 individuals. Population sizes can be expected to fluctuate drastically.

Threats

The coastal San Mateo population was last documented at Pescadero in 1950, and was probably extirpated by loss of habitat to urbanization, agriculture, and invasion of non-native plants. Historical occurrences at Valley Ford and Bloomfield may have been extirpated by farming and grazing pressures and by invasion of non-native plants.

Overcollection is a threat to Myrtle's silverspot. Specimens of Myrtle's silverspot butterfly are known to have been illegally collected in Point Reyes National Seashore. Although collectors generally do not adversely affect the healthy, well-dispersed populations of many butterfly species, a number of rare species, highly valued by collectors, are vulnerable to extirpation from collecting. Collection of females dispersing from a colony also can reduce the probability that new colonies will be founded. Butterfly collectors pose a threat because they may be unable to recognize when they are depleting colonies below the thresholds of survival and recovery, especially when they lack appropriate biological training or they visit the area for only a short time.

Inadequate nectar resources appear to be an ongoing problem for several Myrtle's silverspot populations. Butterflies without adequate nearby nectar plants may be forced to expend time and energy reserves searching for nectaring areas, reducing the number of fertilized eggs laid, and at the same time exposing them to predation, winds, and road mortality. Overgrazing of properties within the range of Myrtle's silverspot may have reduced the abundance of native nectar sources, and could be contributing to the regional decline of this species. The reduction in native nectar sources may have been offset, at least partially, by the silverspot's use of non-native thistles as nectar sources. Prolonged, intensive grazing disturbance reduces the vigor of native plant species and disturbs the site, allowing the establishment of invasive non-native weedy plant species. One such weed is the invasive, non-native iceplant, a competitive threatto several native plant species that provide this butterfly with nectar. This plant and other non-native grasses and forbs have undoubtedly displaced larval and adult food plants of the silverspot and contributed to the overall degradation of habitat quality.

Conservation and Recovery

Substantial areas of habitat and potential habitat for Myrtle's silverspot have been conferred a degree of long-term protection at Point Reyes National Seashore. The national seashore has conducted or commissioned a number of studies of the status and biology of the butterfly, and the recent management plan for the Tule Elk Range in Point Reyes National Seashore contains provisions to assess elk grazing effects on butterfly habitat.

The recovery strategy for the Myrtle's silverspot butterfly includes the following measures: (1) protect habitat where remaining populations occur; (2) identify and establish vegetation management that benefits the native ecosystem of larval host plants and adult nectar sources; (3) reintroduce populations of the butterfly to prioritized areas; (4) control illegal collecting; (5) conduct or fund research to identify critical recovery needs or actions; and (6) monitor existing populations and survey historic and unsurveyed locations.

Habitat protection is essential to the recovery of Myrtle's silverspot butterfly. Two populations of the Myrtle's silverspot are protected at Point Reyes National Seashore, but this national seashore does not yet have a management plan for the butterfly. Under the agreement that established Point Reyes National Seashore, much of its area is leased for grazing, mostly by cattle.

More needs to be known about vegetation management practices that would benefit the Myrtle's silverspot. While heavy grazing is thought to have adverse impacts on nectar plants for the butterfly, and possibly also on the larval host plant Viola, complete absence of grazing may also have adverse effects. Heavy growth of non-native grasses and other plants and accumulation of dead plant litter on top of the ground can result in overgrowth or shading of Viola.Little is known about how to balance these factors in California coastal prairie or dune scrub. Fire is another vegetation management tool that needs further investigaion. A study of Midwestern tallgrass prairie found that populations of threeSpeyeriaspecies were all immediately reduced by fire, but that over the longer term two species benefited or were unaffected, while the third was depressed.

Myrtle's silverspot lives in an ecosystem that has been greatly changed, perhaps forever, by introduced plants, and certain non-native plants have now taken over vital roles in the butterfly's life cycle. In particular, bull thistle (Cirsium vulgare) is an important nectar resource, especially in the late season, and should not be eradicated in the absence of a comparable replacement.

Reintroduction of populations is likely to be a useful tool to increase the number of Myrtle's silverspot individuals and populations, and thus reduce extinction risk. Reestablishing Myrtle's silverspot populations should be done in its historic range, on protected public lands and private lands with the full permission and cooperation of the landowners.

One high-priority area of potentially suitable but apparently unoccupied habitat exists around and south of Dillon Beach, extending to south of Tom's Point. Much of this area overlaps or adjoins areas targeted for plant recovery actions. Protecting this habitat, restoring habitat if needed, and reintroducing the Myrtle's silverspot butterfly is the highest priority. Captive rearing of Myrtle's silverspots to produce large numbers for reintroductions may be appropriate.

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

Hammond, P. C., and D. V. McCorkle. 1983. "TheDecline 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., et al. 1994. "The Endangered Myrtle'sSilverspot Butterfly: Present Status and Initial Conservation Planning." Journal of Research on the Lepidoptera 31 (1-2): 132-146.

U. S. Fish and Wildlife Service. 22 June 1992. "Six Plants and Myrtle's Silverspot Butterfly from Coastal Dunes in Northern and Central California Determined to Be Endangere