Species Account. Giant Swallowtail / Grande Porte-queue (Papilio cresphontes)
University of Waterloo, Ontario, Canada N2L 3G1
Taxonomy: Class: Insecta, Order: Lepidoptera, Family: Papilionoidea, Genus: Papilio, Species: Papilio cresphontes. The Giant Swallowtail was first described by Cramer in 1777 (Layberry et al., 1998). The name Heraclides oxilus was proposed by Hübner (1819), but is not currently accepted as the official binomial name. It has been suggested that the northern-ranging Giant Swallowtail should be considered a subspecies termed Papilio cresphontes pennsylvanicus (Pennsylvanicus: meaning from Pennsylvania), but this has not yet been recognized taxonomically because of its weak phenotypic differentiation from the nominate subspecies (Papilio cresphontes cresphontes)(Klots, 1979; Layberry et al., 1998). The larval form is commonly called “orange dog” or “orange puppy” in the southern United States because of its damaging defoliation of citrus crops (Bullock and Pelosi, 1991). The vernacular name “swallowtail” refers to the tail-like prolongations at the end of the hindwings that are present in most swallowtail species (Borror et al., 1976). The designation “Giant” simply refers to its overall larger size compared to other swallowtail species (Layberry et al., 1998). The Black Swallowtail is a similar species found in the QUBS region, but this butterfly can be differentiated from the Giant Swallowtail by its smaller size and more extensive black coloration on both the top and bottom of the wings (Borror, 1976).
The Giant Swallowtail is the largest butterfly in Canada, with a wingspan of 83 to 113 mm (Layberry et al, 1998). The most distinct feature of the adult is the X-shaped pattern of yellow spots, crossing near the tip of the forewing, contrasted by the dark brown-black basal colouration of the wings (Figure 1) (Layberry et al., 1998). At the posterior end of each notched hindwing is a single tail, consisting of a large yellow dot bordered by dark brown-black. An orange eyespot is present on the inner edge of the upper hindwings. On the underside, the wings are a brilliant yellow, interspersed with black venation and blue and orange colour spots (Figure 1). The adult butterfly can be seen fluttering through the air, its large wings giving it a distinctly strong, high, and leisurely flight pattern (Klots, 1979). Flight in males is more “jerky” due to their smaller size (Klots, 1979). Besides overall size and flight pattern, sexes differ slightly in shape and coloration of the wings and body (Borror, 1976). In both sexes, the abdomen and head are bright yellow, with a dark brown-black stripe running along the top of the head and down the body.
Giant Swallowtail caterpillars, the larval stage of the butterfly, can range from around 2 mm after hatching to over 50 mm just before pupation (Bullock and Pelosi, 1991). Caterpillars collected on August 28th and 29th 2013 at QUBS ranged from 3.0 to 29.0 mm (Figure 2). Overall, the Giant Swallowtail caterpillar resembles bird droppings, having a smooth, dark brown body with creamy buff patches around the mid-body region and the posterior end (Figure 3) (Klots, 1979). The appearance of caterpillars changes as it progresses through 5 instar stages. Younger instars have more distinct bands of colour, and have setae (hairs) on prominent knobs (Leslie and Berenbaum, 1990). Over the 5 instar stages, caterpillars gradually increase in size, with the head becoming distinctly larger in comparison to the body (Leslie and Berenbaum, 1990). The body of the later instars becomes smoother in appearance, setae and knobs are reduced, and the white and brown coloration becomes more mottled (Leslie and Berenbaum, 1990). Giant Swallowtail caterpillars possess bright orange to red osmetaria, V-shaped scent organs that protrude from the back of the head when disturbed (Figure 4) (Layberry et al., 1998). Caterpillars observed on August 28th and 29th 2013 at the QUBS site showed variation in osmeteria colour among individuals and along the length of the osmeteria themselves. Colour appeared to range from orange at the base and clear at the ends of the osmeteria in the younger instars, to orange at the base and red at the ends in the later instar stages.
Figure 4: Video of a Giant Swallowtail caterpillar demonstrating its osmeteria defense upon disturbance.
The Giant Swallowtail has a broad distribution, spanning over 2.5 million km2 (Natureserve, 2009). It ranges from Mexico to California, up the eastern United States, and just passing over the Canadian border up to Point Pelee, Ontario and select surrounding areas where the host plant grows (Figure 5) (Layberry et al., 1998). Vagrants have been found in Winnipeg, Montreal, and Nova Scotia, but the species has yet to become established in these areas (Layberry et al., 1998). Indeed, other than Southwestern Ontario, the species is considered sporadic and rare in Canada (Layberry et al., 1998). A Giant Swallowtail butterfly was first spotted at QUBS in 2008, but its wings appeared very worn and the individuals was thought to have blown into the area (M. Conboy, pers. comm., 27 August 2013). By 2009, Giant Swallowtails were present, but rare (M. Conboy, pers. comm., 27 August 2013). In 2010, however, they were common, and by 2011 had become widespread in the area, as they remain today (M Conboy, pers. comm., 27 August 2013).
Predator defense in the caterpillar: Potential predators of the Giant Swallowtail caterpillar include birds, reptiles, amphibians, and invertebrate predators such as spiders and ants (Leslie and Berenbaum, 1990). The white and dark brown colouration of the larvae is thought to mimic the droppings of birds and lizards that might otherwise attack them (Layberry et al., 1998; Minno and Emmel, 1992). It has also been suggested that the light and dark banding colouration may be disruptive to predator vision (Minno and Emmel, 1992). In either case, the coloration appears to help protect the caterpillar from vertebrate predation. If the defensive coloration fails, and the caterpillar is provoked, it flings back the anterior end of its body, shooting out orange-red osmeteria from behind its head and releasing a foul-smelling odour (see Figure 4) (Layberry et al., 1998). Studies have demonstrated that the osmeteria and its odorous secretions help to deter both invertebrate and avian predators (Leslie and Berenbaum, 1990). However, it is unclear whether these osmeterial secretions are toxic, or simply pungent and unpalatable (Leslie and Berenbaum, 1990). Later instars have been suggested to resemble snakes, because of their large, mottled faces – complete with eyespots (Grossmueller and Lederhouse, 1985). This snake-mimic appearance may prevent predation on the larger and more conspicuous instar stages (Leslie and Berenbaum, 1990).
Food sources: In the caterpillar stage, the Giant Swallowtail feeds on the leaves of members of the Citrus family in the south, and the Hop Tree (Ptelea trifoliata) and Northern Prickly-Ash (Zanthoxylum americanum) in the north (Borror, 1976). The butterfly consumes the nectar of many flowers including lantana, thistle, and boneset (Oppler, 1998). Plant selection by “nectaring” butterflies is due to either visual cues or non-volatile contact chemicals of the plant (Scott, 1992).
Habitat and spatial ecology:
The preferred habitats of the adult butterfly are open woodlands and associated fields (Layberry et al., 1998). Giant Swallowtail butterflies are also found in other forest types (mixed, hardwood, or conifer forests), croplands, and even in suburban gardens (Natureserve, 2009). In Ontario, the caterpillar stage can be found on the Hop Tree and Northern Prickly-Ash (Layberry et al., 1998). The host plant selected and the placement of eggs onto that host plant are determined by the ovipositing female (Scott, 1992). Because the Giant Swallowtail generally pupates within 5 m of the original host plant, egg placement is important for caterpillar and chrysalis development (Grossmueller and Lederhouse, 1985) (Finkbeiner et al., 2011).
In the closely related Tiger Swallowtail butterfly, Papilio glaucus, a distinct preference was found in the location of oviposition, with females preferring sunny exposures (Grossmueller and Lederhouse, 1985). Indeed, caterpillars found in areas of sunny exposures were shown to develop 15-35% faster, and have higher survivorship, than those in low-sunlight areas (Grossmueller and Lederhouse, 1985). Once hatched, the young caterpillars were often found on the tops of leaves, allowing for greater sunlight capture (Grossmueller and Lederhouse, 1985). Available sunlight is limiting for the caterpillars, as they need sunlight to raise their body temperature (Grossmueller and Lederhouse, 1985). With high body temperature, the caterpillars have more energy for food consumption, allowing for faster growth (Grossmueller and Lederhouse, 1985).
In the Northern Prickly-Ash-lined fields at QUBS, I found a notable difference between the number of caterpillars evident on the south-facing versus the north-facing sides of fields in late August (Figure 6), with the former have greater numbers undoubtedly because this side receives the most sunlight. Also, during my surveys the number of caterpillars found per hour was higher in the field with more forest-cover (East Field 2, 3 and 4), compared to larger, more open fields situated closer to roads (East Field 1 and Chapman Field). Temperatures of south-facing sides of larger fields – measured by I-buttons attached to Northern Prickly-Ash stems – were greater than those of the north-facing sides, by up to 9 °C during the day (on average 2-3 °C). However smaller fields, which showed the greatest difference in number of caterpillars between the north and south-facing sides, showed minimal differences in temperature. This implies that a combination of sunlight, irrespective of temperature, in addition to forest cover, may influence caterpillar location (Fadamiro et al., 2010). Even with similar temperatures on both sides of the field, increased sunlight on the south-facing sides could result in higher host plant quality because of higher potential photosynthesis rates, and thus would be a better location for caterpillars to feed (Weiss et al., 1988). My visual observations on the sampling fields also suggest that the south-facing sides have more abundant and healthy plant growth, The observed preference of smaller, forest-sheltered fields could be due to protection provided from the wind, or because of lower levels of bird and insect predation than would be present in the more open fields (Grossmueller and Lederhouse, 1985).
Seasonal activity and metamorphosis:
All butterflies undergo complete metamorphosis, consisting of a series of life stages: the egg, larva (caterpillar), pupa (chrysalis), and finally, the adult butterfly. The transition between each stage is regulated by hormones (Scott, 1992). While three Giant Swallowtail generations (complete life cycles) per year occur in the south, two generations per year occur in the more northern parts of its range (Klots, 1979), including the QUBS site (M. Conboy, pers. comm, 27 August 2013). This is because the number of generations per year depends on climate (Scott, 1992). Climates with longer growing seasons and warmer temperatures promote increased growth and the completion of more generations (Scott, 1992). In Ontario, adult flight (the time when adults in a given generation are most active) occurs from May into July, and again from late July to later in September (Layberry et al., 1998). The winter is passed as a chrysalis (Borror et al., 1976).
The caterpillar stage occurs “instars” (Bullock and Pelosi, 1991). Between each instar, a molt occurs where the caterpillar sheds its exoskeleton to increase in size (Bullock and Pelosi, 1991). A new, larger exoskeleton forms underneath the old one, hardening soon after molting occurs (Bullock and Pelosi, 1991). Giant Swallowtail caterpillars undergo 5 instars, growing to a final length of about 5.1 cm before molting again to enter the chrysalis stage (Bullock and Pelosi, 1991). The chrysalis is connected to a twig or other surface by a structure called a cremaster, held upright and supported by a silken girdle around its midsection (Layberry et al., 1998). The chrysalis stage lasts about 10-12 days if formed in the summer, but those formed in the fall will remain until spring (Borror et al., 1976). The overwintering chrysalis undergoes winter diapause, a state of lowered metabolism where breathing is slowed and no feeding or growth occurs (Scott, 1992). To emerge as a butterfly, the Giant Swallowtail must breath in air to expand its body and break through the chrysalis (Scott, 1992). The wings and other adult organs are developed in the caterpillar and chrysalis stage in preparation for this event (Scott, 1992). Once freed, the adult butterfly hangs by its legs, pumping up its wings until they reach their full size (Scott, 1992). The adult butterfly once emerged, neither grows nor molts (Scott, 1992). After fertilization occurs, adult females will lay eggs singly, often on the tops of leaves (Layberry et al., 1998). Hatching from the egg is not considered a molt; the caterpillar grows inside the egg and chews its way out, consuming the remains of the egg soon thereafter (Scott, 1992, Bullock and Pelosi, 1991).
In the majority of swallowtail species, courtship and copulation occur in the afternoon (Scott, 1992). Adult males patrol through the woods, fields, or citrus groves in search of females (Scott, 1992). Mate selection can be aided by sight or smell, with butterflies using colour patterns or pheromones to locate or choose a mating partner (Scott, 1992). To begin copulation, the male holds onto the female in a pocket under her ovipositor (projections at the rear end of the female used for depositing eggs) (Scott, 1992). He then clasps her abdomen, injecting sperm into her mating tube (Scott, 1992). From the mating tube the sperm travels into the female spermatheca where it is stored for later fertilization (Scott, 1992). Eggs are produced in long tubes called ovarioles, resembling pearls on a string (Scott, 1992). Once each egg has grown large enough, it is released from the ovarioles, fertilized by sperm from the spermatheca, and then deposited using the two lobes of the ovipositor (Scott, 1992). The eggs are 1-1.5 mm, spherical, and appear orange due to an irregular coating of an orange-coloured secretion on the surface of the beige eggs (Figure 3A) (Layberry et al., 1998).
Climate and Range Expansion:
The Giant Swallowtail was typically considered a southern, tropical species (Finkbeiner et al., 2011). However, over the past decade, the species has expanded along the northeastern boundary of its range and into Southern Ontario including the QUBS region (Finkbeiner et al., 2011). The geographic range of a given butterfly species can relate to a variety of factors: the available preferred habitat, host plant abundance and condition, the abundance of parasitoids, and temperature (Finkbeiner et al., 2011). Temperature has been found to have a large effect on caterpillar and chrysalis development (Finkbeiner et al., 2011). In New York, Finkbeiner et al. (2011) found a strong correlation between lack of September frosts and an increase in Giant Swallowtail occurrences. In Kingston, Ontario approximately 50 kms south of QUBS, the average and minimum September temperatures have increased, on average, since the early 90s, with the last frost occurring in 2000 (Figure 7).
Even though the Giant Swallowtail caterpillar can survive multiple frosts, low temperatures are stressful physiologically (Finkbeiner et al., 2011; Steigenga and Fischer, 2009). Not only do low temperatures and frosts limit the activity of the caterpillars, they also lower the food quality by impeding growth and increasing senescence of the host plant (Weiss et al., 1988). With lower temperatures and food quality, the caterpillar is slower to reach the chrysalis stage, and is more vulnerable to parasites and predators in the process (Finkbeiner et al., 2011).
The increase in September temperatures and the absence of frosts prior to the arrival of the Giant Swallowtail at QUBS could potentially have lead to their colonization. With increased capacity for caterpillar development in early autumn, successful completion of the life cycle through to the next year could now occur, allowing for the production of multiple generations (Grossmueller and Lederhouse, 1985).
It has also been suggested that the more northern-ranging Giant Swallowtail individuals have adapted over the past century to endure cooler temperatures (Finkbeiner et al., 2011). Whether from warming of climate, adaptation, or a combination of both, the distribution of the Giant Swallowtail is predicted to expand north and east where host plant is available (Finkbeiner et al., 2011).
The following information was obtained from the Natural History Information Center (http://nhic.mnr.gov.on.ca/MNR/nhic).
GRANK (global rank across the entire range): G5 = globally secure demonstrably secure under present conditions (Aug 2009).
SRANK (provincial or sub-national rank): S3 (in Ontario) = Vulnerable in the nation or state/province due to a restricted range, relatively few populations (often 80 or fewer), recent and widespread declines, or other factors making it vulnerable to extirpation.
Ontario general status: May Be at Risk (12 January 2012) (NGSWG)
The Giant Swallowtail is not currently on the Ontario Ministry of Natural Resources Species at Risk list. The Giant Swallowtail is considered rare in Ontario; however its status overall is secure. At this point the range appears to be expanding, not contracting.
Although the distribution of the Giant Swallowtail in Ontario has been expanding, habitat loss could threaten continued inhabitancy and expansion. The Common Hoptree, one of the plant hosts of the Giant Swallowtail in Ontario, is a threatened species (MNR, 2013). With destruction of critical habitat in the Lake Eerie coastal area, these plant hosts could be lost (MNR, 2013). Further habitat destruction in other areas of Southern Ontario could lead to reductions of the Northern Prickly-Ash, the only other known Giant Swallowtail host plant in Ontario (Layberry et al., 1998).
The mechanisms underlying the range expansion of the Giant Swallowtail should be the continued focus of research. Although increases in temperatures over time appear to have contributed to the northeastern range expansion, the influences of climate change-induced differences in weather patterns, predation, and parasitism on range expansion should be studied further (Rodenhouse et al., 2009). Identifying potential genetic and physiological differences between the suggested southern and northern subspecies (Papilio cresphontes cresphontes and Papilio cresphontes pennsylvanicus, respectively) may also provide insight into possible adaptations obtained by the Giant Swallowtail that have allowed its recent colonization of more northern habitats.
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Reviewers: Grégory Bulté (Carleton Univ. ) and Stephen C. Lougheed (Queen’s Univ.)