ABOUT BOM SPECIES LIST BUTTERFLY HISTORY PIONEER LEPIDOPTERISTS METHODS
The Butterflies of Massachusetts
04 Canadian Tiger Swallowtail Papilio canadensis (Rothschild & Jordan, 1906)
It was only in 1991 that the Canadian Tiger Swallowtail was recognized as a species separate from the Eastern Tiger Swallowtail . Differences in adult size, wing coloration patterns, host plants, number of broods, and other heritable traits are the basis of differentiation (Hagen, et al. 1991; Sperling 1993; Scriber et al. 2003). In the field, tiger swallowtails can be identified to one species or the other only if they are seen very well. Observers in our area have been reporting them separately since 1993, but many identifications are still somewhat uncertain.
Most of Massachusetts except for the southeast coastal plain lies within a broad “hybrid zone” running from the Great Lakes through New England, a zone in which phenotypic and genetic traits between the two tiger swallowtail species vary clinally south to north. The differences and gradations between the two taxa in this zone are complex. Genetic differentiation and speciation are ongoing processes for these taxa in our area (Scriber et al. 2003), and what we now call "Eastern Tiger Swallowtail" (see that species account) may in fact be a hybrid between P. glaucus and P. canadensis. MBC members have photographed individuals which appear to be intermediate between the two species (e.g. http://boghnielsen.com/2010/0703/index.html ; E. Nielsen, masslep 7/19/2010).
Photo: Mt. Greylock, Mass. F. Model, May 30, 2008
It is probably P.canadensis to which Scudder refers when he writes in 1889 that “it appears to swarm in mountain valleys” and that “Travellers by the Canadian Pacific railway in July will hardly fail to notice the great flocks that arise from every heap of vegetable or animal refuse by the railway stations on the wilder part of the route east of Lake Superior.” He quotes D’Urban, a Canadian naturalist: “when the fragrant lilacs are in full bloom, it is a glorious sight to see the tiny hummingbirds flying over the blossoms in company with this splendid butterfly, which is very partial to the flowers of that plant.” (Lilac, of course is not native; this is another early example of our butterflies adopting a non-native nectar source.) D’Urban also reports that “this splendid butterfly frequently assembles in great numbers about wounds on the roots of trees from which sap exudes (1889: 1295-6).”
Throughout Canada today, Canadian Tiger Swallowtail is widespread and common, whereas what appears to be Eastern Tiger is usually found only in southern Ontario (Layberry et al., 1998). Massachusetts is at the southern edge of the Canadian Tiger Swallowtail's range (Cech 2004).
Little can be deduced about the early history of Canadian Tiger Swallowtail in Massachusetts, except that as a species which prefers edge and early successional habitat, it, like what we call the "Eastern Tiger Swallowtail," probably benefited from the timbering, burning, and grazing that took place between 1600 and 1850, as European settler agriculture expanded. Its numbers may have increased in those years (Table 1)
Possibly in an effort to sort out species or sub-species, an inordinate number (55) of Tiger Swallowtail specimens were taken in various Berkshire County towns in 1966-1968 by C. G. Oliver (specimens at Yale Peabody Museum from Becket, Hinsdale, Otis, Sandisfield, October Mountain/Lee, Washington, Dalton). Of these, 36 are now retroactively labeled Canadian, and 19 Eastern Tiger Swallowtail. The public may be forgiven for wondering why such a large number of these beautiful animals had to be killed. The main result, that Canadian Tiger Swallowtail can apparently now be documented for the Berkshires in those years, might have been achieved with much less loss of life.
Host Plants and Habitat
Range-wide, the preferred hosts for P. canadensis are aspen and birch; whereas for P. glaucus the preferred host is tulip tree. Larvae of each do not fare well on the other’s main host plant (Scriber et al., 2003). Black cherry is a host common to both species; the Connecticut Atlas found P. canadensis ovipositing on black cherry in the wild. Photographer Sam Jaffe found “tiger swallowtail” larvae on poplar in the Worcester area early in the season, but the exact species was not distinguished. In Canada, P. canadensis may also use willow and ash (Layberry et al, 1998). More research is needed on what hosts this butterfly is using in Massachusetts.
The Canadian Tiger Swallowtails host trees, aspen, birch, and black cherry, are early successional species in Massachusetts, re-growing quickly after a site has been cleared or burned, but then giving way to other tree species of the mature forest. As noted, Canadian Tiger Swallowtail may benefit from some limited forest clearing, and it likes dirt roads. It is not a species of mature, closed-canopy forests, so as re-growing forests have matured in many areas, habitat may have been lost. The Canadian's preferred habitat is open areas, lightly used roadsides, woodland edges, and streamsides.
Like the Eastern Tiger Swallowtail, the Canadian is “very fond of assembling in companies around spots of moist earth or on the edge of pools by the roadside after a rain (Scudder 1899: 1295)
Relative Abundance Today
According to 2000-2007 MBC sight records, swallowtails identified as Canadian rank among the Uncommon butterflies in Massachusetts (Table 5). The Canadian Tiger Swallowtail is much less frequently reported than the Eastern Tiger Swallowtail. Between 1992 and 2008, there were 6423 reports of Eastern Tiger Swallowtails and 5692 reports of “Tiger Swallowtail spp.” in MBC records, compared to only 1072 reports of Canadians.
The two tiger swallowtail species were not distinguished during the 1986-90 MAS Atlas, so comparisons with those years are not possible. And in MBC records, there is uncertainty about identification, so that reports of sighting frequencies are not very meaningful. However, the number of Canadians reported per total trip reports has increased greatly between 2000 and 2009.
State Distribution and Locations
Map 4 shows every town from which at least one Canadian Tiger Swallowtail has been reported. Some towns at the eastern and southern edges of the range-- for example, Ipswich and Topsfield in Essex County, and Uxbridge and Douglas to the south-- have only one older report, of one or two butterflies. Any new sightings reported from these areas should be carefully documented with photographs.
Canadian Tiger Swallowtails occur most frequently and reliably throughout central and western parts of the state. They have rarely been reported from towns around Boston, and never from southeastern towns, the Cape or the islands.
Map 4: MBC Sightings by Town, 1993-2009
Towns which should be added to Map 4 for 2010 are Arlington, Leverett, New Marlborough, North Andover (photo), Shutesbury, Ware, and Williamstown (photo). Canadian Tiger Swallowtail has been reported from 54 towns as of 2010.
Locations which have been especially productive for this species are Mount Greylock (max 235 on 6/5/1999, T. Gagnon); Pittsfield (max. 45 on 6/4/2009, B. Spencer); Peru (max. 38 on 6/12/2009, B. Spencer); Lee October Mountain SF (max. 5 on 7/10/1999, T. Dodd and B. Cassie); and Petersham Tom Swamp (max. 12 on 5/31/2006, B. Benner et al.).
The closest locations to Boston from which this species has been reported are Arlington (Great Meadows 4/21/2010, T. Whelan), and Acton/Concord Fort Pond Brook (1 on 5/16/2009, T. Whelan). Interestingly, there is an early (1937) specimen from Concord, Massachusetts, (no collector listed) in the Yale Peabody Museum, which has been retroactively identified as Canadian Tiger Swallowtail.
Despite these reports, it is well to remember that the taxonomic status of all tiger swallowtails in Massachusetts is somewhat uncertain, and that characters observable in the field may not be sufficient to determine species or hybrid status.
Broods and Flight Time
Throughout its range, the Canadian Tiger Swallowtail has only one generation, while the Eastern Tiger Swallowtail and the various hybrids have two or more. This is one of the key traits distinguishing Canadian Tiger Swallowtail, and thus flight time may be an aid in field identification. According to MBC records 1993-2008, the flight of Canadian Tiger Swallowtails begins in early May, and lasts only until mid-July (http://www.naba.org/chapters/nabambc/flight-dates-chart.asp ) The Eastern Tiger Swallowtail flies both earlier and later than the Canadian, based on these field identifications.
MBC reports of Canadian Tiger Swallowtail start to become more reliable from 1999 onward. In the 12 years 1999 through 2010, ten of the first sightings have been May 15 or earlier; the other two were in the last week of May. The earliest five sightings reported as Canadians are 4/21/2010 Arlington T. Whelan; 5/6/2000 Franklin Co., T. Gagnon; 5/6/2001 Worcester, B. Walker; 5/6/2002 Florence, T. Gagnon; and 5/7/2007 Florence, T. Gagnon. The first sighting dates for Canadians are later than those reported for Eastern Tiger Swallowtail.
The latest dates reported for Canadian Tiger Swallowtail in MBC records come from the Northern and Central Berkshire NABA Counts. These are held on a pre-determined day in mid-July, and so do not reflect searches over time. In addition, in the spring and through July, the Eastern Tiger Swallowtail is also flying, and sight reports from a distance cannot be considered reliable. With these caveats, the latest dates reported are 7/15/2001, Central Berkshire NABA, and 7/14/1999, Northern Berkshire NABA.
Outlook
In Canada, this swallowtail is widespread and common. Within its northern range limits, it adapts well to temperature and habitat variations. Cech (2004: 65) terms it an adaptable generalist within its range. It remains to be seen how far its southern range can stretch, and whether climate warming will lead to a range contraction northward, and thus a decline in Massachusetts. However, if climate change actually results in colder though shorter winters in Massachusetts, the Canadian Tiger Swallowtail could hold its own, while the classic southern Eastern Tiger Swallowtail (P. glaucus) could decline. NatureServe (1/2011) currently lists the Canadian Tiger Swallowtail’s status in Massachusetts as S5 or secure.
© Sharon Stichter 2011
page updated 3-24-2011
ABOUT BOM SPECIES LIST BUTTERFLY HISTORY PIONEER LEPIDOPTERISTS METHODS