Our species profile for this issue is the Thicket Hairstreak, Mitoura spinetorum. Note however that many lepidopterists currently place all Mitoura species in the genus Callophrys.

Hairstreaks belong to the large worldwide family Lycaenidae, which also includes all blues, coppers and metalmarks. Hairstreaks are small butterflies, often with taillike extensions or “hairstreaks” on their hindwings. In Washington State there are eighteen species of hairstreaks, three of them in the genus Mitoura. All Mitoura are generally rather scarce and highly sought after by butterfly enthusiasts. All three species have a close association with evergreen trees, the larvae of one (Cedar Hairstreak) feeding on cedars or junipers, and the other two (Thicket and Johnson’s Hairstreaks) feeding on dwarf mistletoe growing on evergreens. All three Mitoura species have distinct white lines on their ventral hindwings, contrasting with a darker ground color of various brown or purplish tones.

This species is found only in close proximity to its larval host plant Arceuthobium campylopodum, a type of dwarf mistletoe. Arceuthobium is a common parasite of evergreens and A. campylopodum has a strong preference for yellow pine, occurring most frequently in foothills and lower mountainous areas.  The Thicket Hairstreak is most likely to be seen basking or nectaring along forest roads where the dwarf mistletoe is common. Dwarf mistletoe dies back each year, but when the bright orange parasitic plant begins growing in the spring the flight period of the Thicket Hairstreak will not be far behind. The mistletoe will be in a relatively early growth stage when M. spinetorum begins flying, so the observant butterflier may want to note areas with luxurious Arceuthobium infestations during mid summer and check back during the next spring to find the best populations of the Thicket Hairstreak. While its reported flight period in Washington extends from mid April to late August the Thicket Hairstreak is relatively scarce, never abundant, and in some years can be quite difficult to find at all.

Mitoura spinetorum is a western North America species, ranging from British Columbia to Baja California, and from the coastal mountains to the Rockies of southern Alberta, Colorado and New Mexico. Primarily a mountain species M. spinetorum is absent from unforested lowlands and arid areas. Within Washington State M. spinetorum is almost entirely restricted to the east side of the Cascades, spilling over the divide a bit only in the vicinity of Mt Rainier. In eastern Washington, M. spinetorum is found in mountainous areas along the east flanks of the Cascades, across the northern tier mountains from Okanogan to Pend Oreille Counties, in the far eastern mountains of Spokane County and in the Blue Mountains.

The Thicket Hairstreak life cycle is closely tied to its larval host plant Arceuthobium campylopodum, a species of dwarf mistletoe which grows as a parasite on evergreen trees, particularly yellow pine. Eggs are laid singly, directly on the dwarf mistletoe, often tucked deep into the many crevices of the plant. On hatching the tiny yellow- orange larvae are well camouflaged against the similarly-colored host plant, and the later instars become beautifully patterned in blending yellows and greens. The larvae are slow-moving, clinging tightly to a bud or fruiting head of mistletoe and moving on only after it has been largely consumed. On maturity the larvae move to an area of dense needles in or adjacent to the Arceuthobium where they pupate, anchoring themselves with a single thin silk girdle thread. In Washington there is a second brood, however this may be only a partial (opportunistic) brood, as late-flying individuals are seen less frequently than first-brood butterflies.   Mitoura spinetorum overwinters in the chrysalis stage.

The Thicket Hairstreak, with its white, extremely distinct W-shaped line across the ventral hindwing, contrasting with a reddish-brown ground color, can be confused only with the two other species of Mitoura. The Thicket Hairstreak overlaps in range with the Cedar Hairstreak in Washington’s northeast mountains and in the Blue Mountains. In these areas the Cedar Hairstreak can be distinguished on its ventral hindwing by its generally less distinct and less angular white line, and by an infusion of distinct purplish color which is never present in the Thicket Hairstreak. Beware however that the purplish color fades in worn Cedar Hairstreak adults. Also, the Thicket Hairstreak could potentially be found with the Johnson’s Hairstreak west of the Cascade crest in the vicinity of Mt Rainier, and possibly also in Skamania and Whatcom Counties. In these areas the Johnson’s Hairstreak can be distinguished by its brown dorsal wing color as opposed to the steely blue color of the Thicket Hairstreak. Also, on the ventral hindwing M. johnsoni has a brown ground color overlain by an incomplete row of peripheral black dots, while the Thicket Hairstreak is reddish-brown and has a complete row of peripheral black dots.

All Photos by Dave Nunnallee

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