Pterourus, Scopoli, 1777

Pavulaan, Harry, 2024, Determination of a new spring-flying species of the Pterourus glaucus complex (Papilionidae) in southern New England, The Taxonomic Report of the International Lepidoptera Survey 12 (1), pp. 1-26 : 12-14

Pterourus near- canadensis (Rothschild & Jordan, 1906)

Canadian Tiger Swallowtail

Status: It is unclear which spring “Tiger Swallowtail” individuals are actually attributable to canadensis . What makes this difficult is that some spring individuals of the Eastern Tiger Swallowtail are virtually identical to the Canadian Tiger Swallowtail and some early “New England Tiger Swallowtails” may be rather on the small side. See “Comments” below for further information on distinguishing canadensis from glaucus . Canadensis is reliably found in western Massachusetts and northwestern Connecticut. If canadensis does occur in Rhode Island ( Figs. 18- 21 ), it is apparently rare or very infrequent, and more likely a stray from regions further north.

Habitat: An inhabitant of virtually all forested habitats, though the species is better associated with the transition zone forest of northern New England, western Massachusetts and northern Connecticut.

Larval Hosts: In Rhode Island, a captive female oviposited on, and larvae fed and matured on Black Cherry ( Prunus serotina ). Also reported on the same host in Connecticut and Massachusetts for all members of the glaucus complex, thus making this host not a good measure for distinguishing species. The species feeds on hosts that are documented to be toxic to glaucus larvae, such as Populus tremuloides (Quaking Aspen) and Betula papyrifera (Paper Birch) but there are no actual documented reports for use of these hosts in southern New England. Both hosts are, in fact, quite common in Rhode Island.

Habits: Males patrol back and forth along forest roads and along watercourses. In the White Mountains of New Hampshire, canadensis is known to be exceedingly abundant, often found in “enormous numbers”, such as “600-700 individuals” that were once observed along a damp roadside edge ( Kiel, 2003). In southern New England, no such large aggregations have been reported.

Broods: One brood. Confirmed early dates: May 20 (Connecticut); June 4 (Massachusetts); May 8 (Rhode Island). Confirmed late dates: July 2 (Connecticut); June 4 (Massachusetts); June 3 (Rhode Island).

Distinguishing features and variation: The smallest of the Tiger Swallowtails (though rivalled by some dwarfed individuals of spring brood glaucus ). Wingspan is given as “ 53-99 mm ” in Canada ( Layberry et al., 1998), though this likely reflects the smallest P. canadensis arcticus (Skinner, 1906) and the largest of the “Mid-Summer Tiger Swallowtails”. Ording (2008) measured the forewings of canadensis in Vermont, measuring 39-53 mm in length (averaging 45.1 mm), more applicable to southern New England canadensis . The Rhode Island specimens have an average wingspan of 70 mm. Several characters have been identified, which can be used for distinguishing canadensis from glaucus , but are not always reliable. The hindwing tends to be more angular (triangle-shaped) than in glaucus ( Figs. 18-21 , 37 , 38 ). Dorsally and ventrally, the black edge along the inner margin of the hindwing, adjacent to the thorax, is wide while in glaucus it is narrow. In the females, the blue chevrons on the dorsal hindwing are subdued, whereas in glaucus the blue chevrons tend to be broad and generally form into a solid band. Ventral characters are most reliable: The yellow submarginal band of the forewing is formed into a solid band, which replaces the row of yellow spots in glaucus ; the black line separating the yellow inner region of the hindwings from the blue submarginal chevrons is straight as opposed to scalloped in glaucus ; the marginal chevrons of the hindwing tend to be rectangular in shape (technically tilted quadrilaterals), whereas in glaucus they form crescents; and the outer edge of the hindwing is more akin to a slanted staircase pattern whereas in glaucus it is strongly scalloped. However, a percentage of glaucus individuals of the spring brood may take on all the characters of canadensis and can only reliably be differentiated by experimenting with host acceptance and voltinism. Most importantly, there is no black female form in canadensis .

One reliable way to distinguish canadensis from either glaucus and the Mid-Summer Tiger Swallowtail is to examine the first instar larvae. The larvae of canadensis are black with three white transverse saddle marks on the back. It is not known if this larval character difference holds up with the other two species: the New England and Mid-Summer Tiger Swallowtails. Schmidt (2020) provides a comparison between glaucus , canadensis and the “Mid-summer Tiger Swallowtail” in which he illustrates how the male clasper scales of canadensis bear sparse black scales. While the male claspers of glaucus , “New England Tiger Swallowtail” and the “Mid-summer Tiger Swallowtail” are generally clear yellow, most males of the glaucus spring brood from regions as far south as the Carolinas appear to have variable clear-yellow or black-dusted claspers. This character difference is difficult to ascertain from internet and published images.

Comments: In a 1983 rearing experiment, a female tentatively identified as canadensis in Rhode Island was confined on Prunus serotina , on which she readily oviposited. Approximately 30 ova were deposited on leaves. The first instar larvae displayed the three saddle marks characteristic of northern canadensis . The larvae were reared under identical conditions to captive (ex-ova) glaucus larvae. The canadensis larvae produced diapausing pupae that did not produce a summer generation, while the glaucus immatures produced a late summer generation of typical summer form glaucus . Unfortunately, the canadensis chrysalids were not exposed to winter cold. Rather, they were kept in warm indoor conditions in hopes of forcing a false second brood. These desiccated without producing adults in 1984, despite misting.

A study by Scriber et al. (2002) indicated that summer maximum temperatures in the range of 30-36℃ (86-97℉) over a period of four days were lethal to diapausing pupae of canadensis . Data from the Rhode Island Dept. of Environmental Management (accessed online, 2023) stated that interior portions of Rhode Island experienced an average of 8-10 days of high temperatures of 36℃ (97℉) annually, with some summers experiencing as many as 20 or more such warm days. Scriber et al. (2002) concluded: “Natural temperature-induced stress on diapausing pupae prevents canadensis from extending range south.” Interestingly, along the immediate coast, temperatures rarely ever exceed 90℉, due to the moderating effect of the ocean. This might account for records of canadensis -like specimens near the Rhode Island coast.

Distributional County Records: Confirmed photo or specimen records for this study. Connecticut: Litchfield , New Haven. Massachusetts: Berkshire [spring records throughout the remainder of Massachusetts appear to be confused with the “ New England Tiger Swallowtail ”]. Rhode Island: Kent, Washington.

Figures

Fig. 18. Pterourus “near-canadensis”, male. June 3, 1984, Quidnick, Kent Co., R.I. Leg. Harry Pavulaan. Dorsal view [abdomen removed for genome research].

Fig. 19. Pterourus “near-canadensis”, male. Same data as Fig. 18. Ventral view.

Fig. 20. Pterourus “near-canadensis”, female. May 8, 1983, Coventry, Kent Co., R.I. Leg. Harry Pavulaan. Dorsal view [abdomen removed for genome research].

Fig. 21. Pterourus “near-canadensis”, female. Same data as Fig.20. Ventral view.

Fig. 37. Holotype (male), Papilio canadensis Rothschild & Jordan, 1906. June 25, 1906, St. John’s, Newfoundland. Dorsal (left), ventral (right). Photo courtesy of Norbert Kondla.

Fig. 38. Pterourus canadensis (female). July 5, 2009, Mount Uniacke, Hants Co., Nova Scotia. Dorsal (left), Ventral (right).