Myrtle’s Silverspot Butterfly

Speyeria zerene myrtleae dos Passos & Grey, 1945
Date of listing: 1992
Federal Status: Endangered
State Status: None

Unlike its relative, the Oregon Silverspot, the future of Myrtle’s Silverspot is far from certain. Formerly widespread on the San Francisco and Marin peninsulas, the butterfly is now only known from a few sites in northern Marin County most of which are under constant threat of development.

Adults of Myrtle’s Silverspot appear throughout the late summer and early fall. Adults live for about three weeks during which time they feed on nectar, mate and lay eggs. The eggs are laid only on species of Viola, possibly only Viola adunca. The eggs hatch in the fall. Most of the larval feeding, however, occurs in the spring when most of the violet’s growth takes place. The adults emerge following a two week pupation.

Populations of Myrtle’ Silverspot are restricted to areas immediately adjacent to the coast: dunes, scrub, and grasslands. All of these have become increasingly popular as sites for residential development. There are only four populations known, each isolated from one another by human habitations. Where formerly a metapoulation model (see essay Populations Genetics and Endangered Species) may have operated in maintaining populations of this species, each population is now on its own. In addition to habitat loss, invasive nonnative plants are also a serious threat. Coastal dunes, an important habitat for Myrtle’s Silverspot are being stabilized by European beach grasses and ice plant. A golf course project in Marin County now threatens to completely wipe out one of the largest of the remaining populations. This butterfly is in serious need of action on its behalf by the public including a commitment to its protection from private landowners who now hold the fate of most of the populations in their hands.

For further reading:
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.


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