Western Tiger Swallowtails are large yellow butterflies with black stripes running from front to back across the wings. All the marginal spots on the dorsal hindwings are yellow, including the first one. The ventral surface of the hindwings frequently has extensive blue and orange spots or areas, especially in females, but much less so than in the Canadian Tiger Swallowtail.
The eggs are green, hemispherical in shape, and smooth. Mature larvae are smooth, hairless, and green, with two sky blue dots on each side of each segment. The head is orange, with a yellow "collar" behind it. The eyespots of mature larvae consist of a large main eyespot with a smaller satellite spot below. The larvae have orange osmeteria. The prepupal larvae turn brown, the blue spots fade, and a prominent eyespot (orange with a black centre) replaces the eyespots on the third segment. The pupae are brown and have a girdle, and are fastened head up on the side or underside of logs, branches, fences, or house siding (CSG).
The nominate subspecies of the Western Tiger Swallowtail (TL: restricted to near Belden, Plumas Co., CA [Emmel et al.1998b]) occurs in BC. The subspecies name arcticus Skinner, 1906, which has sometimes been considered to apply to a northern subspecies of Western Tiger Swallowtail, is a synonym of the Canadian Tiger Swallowtail (Hagen et al. 1992).
Linnaeus divided butterflies into several groups. The first group was the swallowtails, which were called equites or knights. Those with red on the thorax were Greek heroes, those with no red on the thorax were Roman heroes (Emmet 1991). Papilio, which is Latin for butterfly, was the original generic name that Linnaeus used for all butterflies. The common name was first used in Britain in 1766 for "The Swallowtail," P. machaon (Bretherton 1990b), in reference to the resemblance of the tails on the hindwings to the tails of swallows. The name was later extended to include the entire genus. Gosse (1840) was the first to use the common name "swallowtails" in North America.
Swallowtails found in North America are large, brightly coloured butterflies with tails on their hindwings. Six of the eight species in BC are yellow with black stripes. In addition, Pale Swallowtails are white to very pale yellow with black stripes, and Indra Swallowtails are mostly black. Swallowtails also have an orange eyespot at the base of each hindwing tail, and orange and blue spots on the ventral hindwings.
Eggs are smooth and hemispherical, and are cream, yellow, yellow green, or green when laid. The egg colour darkens, and a red ring develops around the top before hatching. Young larvae are black with a white saddle, and resemble bird droppings. Larvae of all ages have well-developed osmeteria, extrusible Y-shaped glands on the top of the thorax that produce defensive chemicals in response to attack. Pupae have two small horns on the head and a point at the top of the thorax. A silk girdle holds them head up against a stem.
The eggs are laid on the leaves of the larval foodplants. On plants with large flat leaves, the eggs are laid on the top or occasionally just under the leaf edge. Both the top and bottom of small leaves are used. The pupae overwinter. In BC two species have more than one generation each year in some populations; the other six are univoltine.
Hancock (1983) split the genus Papilio into six genera, two of which (Papilio and Pterourus) are in BC. We treat Hancock's genera as subgenera of a single large genus, Papilio, as do most recent authors.
Higgins (1975) suggested that the North American populations of Old World Swallowtails may not be the same species as Papilio machaon. Eitschberger (1993) found that the rings of plates surrounding the egg micropyle are significantly different between one European machaon subspecies and subspecies aliaska, the North American subspecies closest to European machaon. European P. machaon has 3 rings with about 112 plates around the micropyle, while aliaska has 5 rings with about 142 plates. A second character used by Eitschberger to separate P. machaon from aliaska, the number of teeth on the harpe of the male genitalia, is not useful in separating species in North America. The difference in egg structure is insufficient to split the species without additional data, hence we continue to treat the North American populations as subspecies of Papilio machaon.
All the tiger swallowtails (subgenus Pterourus) hybridize in the wild to some extent. In southern BC there is a broad zone of hybridization between Canadian Tiger Swallowtails and Western Tiger Swallowtails from Manning Provincial Park east to Creston. In the areas where their ranges overlap, Western Tiger Swallowtails prefer low-elevation deciduous forest habitats whereas
Canadian Tiger Swallowtails prefer higher-elevation boreal forest habitats. Hybridization between Pale Swallowtails and Western Tiger Swallowtails is rare, but Wagner (1978) collected a perfectly intermediate male hybrid in the wild in Idaho. Jon and Sigrid Shepard found a male hybrid of the Pale Swallowtail and the Canadian Swallowtail 10 km south of Galloway, BC. lt is intermediate in appearance between the two species.
Similarly all the Old World swallowtails (subgenus Papilio) occasionally hybridize in the wild. The Old World swallowtail species are most easily distinguished by the overall coloration of the hindwing and by the colour of the eyespot at the base of the tail on the hindwing.
Western Tiger Swallowtails are univoltine in BC, but populations further south have two or three generations in a summer. They are in flight from late May to July, and until August at mid elevations. Eggs are laid singly on the upperside of the leaves of foodplants, with a female laying only one or a few eggs on a plant before flying on to find another. Eggs hatch in 7-10 days, and pupation occurs about 6-8 weeks later. There are five larval instars. The brown prepupal larvae, which spend a day or two wandering before pupating, are frequently found crossing roads and paths in August before finding a sheltered spot in which to pupate. Pupae hibernate, and adults emerge early the next summer. Males patrol forest openings and stream banks looking for females, and are frequently seen mud-puddling. Vehicles on roads frequently kill both adults and prepupal larvae (CSG). An individual Western Tiger Swallowtail has lived at least 39 days in the wild (Smith 1982).
Larval foodplants in BC are alder, cultivated apple, birch, bitter cherry, poplar, and the willows Salix hookeriana and S. scouleriana (Harvey 1908; Jones 1936; Sugden and Ross 1963; CSG). Outside BC additional foodplants include Salix exigua, S. lasiolepis, S. lucida ssp. lasiandra, S. babylonica, Alnus viridis, Populus angustifolia, P. tremuloides, and Platanus (Edwards 1874-84; Emmel et al. 1971; Emmel 1975; Scott 1992).
Western Tiger Swallowtails are common along the southern edge of BC from Bella Coola and Vancouver Island east to Creston. Populations are free of hybridization with the Canadian Tiger Swallowtail west of the Cascade and Coast mountains. They are found wherever their larval foodplants occur, but are particularly common in or near riparian habitats along streams and rivers, where their larval foodplants are most abundant. They are often quite common in residential areas due to the use of birches, willows, and poplars as boulevard and garden trees.
Western Tiger Swallowtails occur from southern BC south to CA and northern MEX, and east to the Rockies and NM.
Recommended citation: Author, Date. Page title. In Klinkenberg, Brian. (Editor) 2019. E-Fauna BC:
Electronic Atlas of the Fauna of British Columbia [efauna.bc.ca]. Lab
for Advanced Spatial Analysis, Department of Geography, University of British
Columbia, Vancouver. [Accessed:
2021-05-13 10:45:53 AM]
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