Hairstreaks belong to the large worldwide family Lycaenidae, which also includes all blues, coppers and metalmarks. Hairstreaks are small butterflies, often with tail-like extensions or “hairstreaks” on their hind wings, particularly in members of the genus Satyrium. In Washington State there are eighteen species of hairstreaks, six of them in the genus Satyrium. Butterflies in the genus Satyrium fly in mid-summer, peaking in July, and are highly prized by butterfly enthusiasts.

The reported flight period in Washington extends from early June to mid-August. The Sylvan Hairstreak is relatively uncommon, and is confined to well-defined colonies where it can be found year after year. Typically colonies are found where Coyote Willow (Salix exigua), the preferred larval host plant, occurs together with a good nectar source such as Showy Milkweed. As Coyote Willow usually occurs along waterways and wet ditches, so too does the Sylvan Hairstreak.

The Sylvan Hairstreak occurs in two forms, a light form which is placed in subspecies sylvinus, and a darker form in subspecies nootka. S. s. sylvinus flies a little earlier in the year, typically in late June to mid-July, and is clearly associated with willows, especially Salix exigua. S. s. nootka flies as late as mid-August at higher elevations; its larval food plants are surely willows as well but the species are not known with certainty.

Satyrium sylvinus is a strictly western North America species, occurring from British Columbia south to Baja California, and east into the western parts of the Great Plains. Within Washington State S. sylvinus is mostly restricted to the east side of the Cascade Mountains, occurring in the foothill and mountain areas surrounding the Columbia Basin, but also sparsely in a few colonies within the west part of the Basin itself. This species does also occur in western Washington, but is limited to small areas in the Olympic Mountains, also in Mason County, and along the west slopes of the Cascades in the southern part of the state.

Eggs are laid on the host plant, usually Salix exigua, in July. The eggs have a normal Lycaenid (“flattened tennis ball”) shape, but are unusual in that they have a very dark color and are attached to the host plant with a tough, shiny, glue-like substance. The eggs are placed in a protected location such as a narrow crotch in the smaller twigs of the host plant. Eggs remain unhatched until the following spring, whereupon the larvae eclose and leave the egg shells uneaten. The larvae feed on tender new deciduous host plant leaves, eating holes clear through. About

50 days after hatching the larvae reach a length of about 17 mm and pupate. After an additional 12 days or so the adult butterfly ecloses, and the cycle is repeated.

Adult Sylvan Hairstreaks are easily separated from all Washington hairstreaks except Satyrium californica, the California Hairstreak, with which the dark S. s. nootka form is often confused. The Sylvan Hairstreak is generally less heavily marked on the ventral hindwing than is S. californica. S. sylvinus has fewer marginal orange spots above the blue ‘thecla’ spot (usually only one, sometimes a second pale one) than californica which has several such spots. Both species have a second row of black spots lying proximally (toward the body) from the orange band, but in californica these spots are more chevron- shaped than in sylvinus. Most notably the black spot proximal to the blue ‘thecla’ spot is distinctly chevron-shaped in californica but nearly straight or flat in sylvinus. Interestingly, while the adults may be confusing, S. californica and S. sylvinus are distinct and easily separated at all immature stages.

Adults of S. sylvinus sylvinus are often found in the higher shrub-steppe, and up to the lower timber line. Watch for this species in July wherever you find blooming Showy Milkweed and Coyote Willow (the larval host plant and a good nectar source) together.

All photos this page by David Nunnallee.