Half-moon hairstreak (Satyrium semiluna) COSEWIC assessment and status report: chapter 2

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Executive summary

Half-moon Hairstreak
Satyrium Semiluna

Species information

The Half-moon Hairstreak, named after its type locality at Half-Moon Ranch in Wyoming, is a small drab hairstreak butterfly in the family Lycaenidae. The adult butterflies are brown or blackish brown on the dorsal wing surfaces and grayish brown on the ventral wing surfaces with variable amounts of white and black maculation. There are two distinct populations in Canada, one in British Columbia and the other in Alberta, and these entities may represent different subspecies.


Distribution

The Half-moon Hairstreak is found from extreme southern interior British Columbia south to central California and east to eastern Wyoming and northern Texas. In Canada the species occurs as two disjunct populations, one on either side of the continental divide. The British Columbia population is known from a few locations in the southern Okanagan Valley from the Canada/United States border north to White Lake. On the east side of the Rocky Mountains, Alberta has one known population in Waterton Lakes National Park, which is also disjunct from the nearest known populations in the United States.


Habitat

In British Columbia, populations of the Half-moon Hairstreak have been found at elevations from 600 m to 1100 m in a number of scattered sites in sagebrush/grassland habitat where lupines are present. The Alberta population is restricted to an alluvial fan, a grassland habitat with abundant lupines, at 1300 m elevation in Waterton Lakes National Park.


Biology

The larval food plants of the Half-moon Hairstreak have not been confirmed in Canada, but lupines are thought to be used because of the association of the butterfly with lupines at all known sites in Canada and the confirmed use of lupines as the host plant in the United States. The Half-moon Hairstreak is reported to have one annual brood. Adults of the British Columbia population have been found from 20 May to 4 July and Alberta adults have been found in late June 25 and in the last two weeks of July. The likely association of the immature stages of the butterfly with ants may be a significant factor in the biology (and hence distribution) of both Canadian populations. The presumed lupine larval food plant is an abundant and widespread species in British Columbia and its presence clearly does not determine the species’ range in the province.


Population sizes and trends

Although no quantitative surveys were done in 2003, the Alberta population of the Half-moon Hairstreak was thought to be anywhere from several thousand to less than ten thousand adults. In 2004, only 250 adults were observed over several days of intensive searching. The size of the British Columbia population is unknown, but a plausible range is 5 000 to 15 000 adults. Population trends are unknown.


Limiting factors and threats

The Half-moon Hairstreak requires unforested areas with lupines. This butterfly’s larvae may depend on a mutualistic relationship with ants. If that is the case, the presence of the appropriate species of ants will also be a limiting factor. Intensive livestock grazing, especially on private land, conversion of native sagebrush grasslands to agriculture and habitat loss to development are considered to be the main threat to British Columbia populations.

The Alberta population may be limited primarily by a combination of habitat flooding and possibly by severe spring frosts. This population is also threatened by knapweed invasion and control.


Special significance of the species

The Half-moon Hairstreak is one of a group of butterflies that reach their northern distribution limit in western Canada. The Alberta population is disjunct from populations in the United States and in British Columbia.


Existing protection

The Waterton Lakes National Park population is protected under the National Parks Act. The British Columbia population is not protected under any statute although some of its habitat is protected in some lands acquired for conservation purposes. The species is considered to be secure in the United States.

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