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Diet breadth of Gynaephora groenlandica (Lepidoptera: Erebidae): is polyphagy greater in alpine versus Arctic populations? I.C. Barrio,1 D.S. Hik, J.Y. Liu Abstract—Gynaephora groenlandica (Wocke) (Lepidoptera: Erebidae) is a cold-adapted species, whose life history traits are dictated by cold and short Arctic summers. We used a recently discovered alpine tundra population in southwestern Yukon, Canada to investigate local adaptations to habitats with different environmental conditions (alpine versus Arctic). Using cafeteria-type experiments and field observations we examined the diet breadth of alpine populations of G. groenlandica beringiana Schmidt and Cannings, and compared these to published data on High Arctic populations of G. groenlandica groenlandica and to the closely related G. rossii Curtis. Gynaephora groenlandica beringiana appears to have a broader diet than High Arctic populations, but similar to that exhibited by alpine populations of G. rossii. Such trends could emerge from reduced synchrony between herbivores and their host plants in less extreme environments, and possibly from a reduced incidence of parasitoids in the life cycle of these populations. Our findings indicate the larval host plant plasticity of G. groenlandica in different environments, and are relevant to predictions regarding the fate of these populations under climate warming scenarios. Résumé—Gynaephora groenlandica (Wocke) (Lepidoptera: Erebidae) est une espèce adaptée au froid, dont les traits d’histoire de vie sont dictés par des étés Arctiques froids et courts. Nous avons utilisé une population de toundra alpine récemment découverte dans le sud-ouest du Yukon (Canada) pour étudier les adaptations locales à des environnements différents (alpins vs. arctiques). Utilisant des expériences de type « cafeteria » et des observations de terrain, nous avons examiné l'amplitude alimentaire de populations alpines de G. groenlandica beringiana Schmidt and Cannings, et nous les avons comparées aux données publiées sur des populations de G. groenlandica du Haut Arctique et des populations de G. rossii Curtis, espèce qui lui est étroitement apparentée. Gynaephora groenlandica beringiana semble avoir une amplitude alimentaire plus large que celle de populations du Haut Arctique, mais similaire à celle de populations alpines de G. rossii. De telles tendances pourraient résulter d'une synchronisation réduite entre herbivores et leurs plantes hôtes dans des environnements alpins, moins extrêmes, et éventuellement d'une incidence plus réduite des parasitoïdes dans le cycle de vie de ces populations. Nos résultats indiquent la plasticité du choix de la plante hôte par la chenille de G. groenlandica dans des environnements différents. Ces résultats ont une importance pour des prédictions quant à l’avenir de ces populations sous des scénarios de réchauffement climatique. Gynaephora groenlandica (Wocke) (Lepidoptera: Erebidae: Lymantriinae) is one of the best examples of cold-adapted insect species (Danks 2004). It has an extraordinarily extended developmental period, with up to seven overwintering larval instars (Danks 1992; Morewood and Ring 1998). Low temperatures restrict larval feeding activities and metabolism, and biotic factors, such as parasitism and phenology of their preferred plant
species (Kukal and Kevan 1987; Kukal and Dawson 1989), constrain larval activity to a threeweek period following snowmelt. After that, larvae build silky hibernacula amidst the tundra vegetation and become dormant until the next spring (Morewood and Ring 1998). Widely distributed in the Canadian Arctic and Greenland, G. groenlandica was, until recently, considered a High Arctic endemic species.
Received 1 November 2013. Accepted 24 February 2014. First published online 29 May 2014. I.C. Barrio,1 D.S. Hik, J.Y. Liu, Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E9 1 Corresponding author (e-mail:
[email protected]) Subject Editor: Chris Schmidt doi:10.4039/tce.2014.35
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However, the discovery of alpine populations of this species in the southwestern Yukon (Barrio et al. 2013b) led to the description of a new subspecies, G. groenlandica beringiana Schmidt and Cannings (as opposed to the nominal High Arctic subspecies, G. groenlandica groenlandica). The life history of these newly discovered alpine populations may differ from those in the High Arctic because they are exposed to different environmental conditions, including seasonal and daily patterns of temperature and photoperiod (Danks 1999). Such differences occur in a closely related species, G. rossii, which has a truly arcticalpine distribution. The two species share some ecological and morphological resemblance and their geographical distribution overlaps at many sites in the Canadian Arctic, but they are reproductively isolated (Morewood 1998). Gynaephora rossii shows a noticeable degree of geographical variation (Ferguson 1978) and there are several morphological and life history differences between alpine and Arctic populations (Schaefer and Castrovillo 1979). For instance, alpine populations of G. rossii have a shorter developmental cycle (estimated as 2–3 years versus 10 years in Arctic populations) and a longer annual activity period, females are more mobile than their flightless Arctic counterparts, and larvae have a broader diet spectrum (Schaefer and Castrovillo 1979). Thus, these differences between alpine and Arctic populations might also be expected for G. groenlandica. Gynaephora groenlandica can be one of the main invertebrate tundra herbivores (Mølgaard and Morewood 1996) and, similar to many insects, can have substantial impacts on the structure and diversity of tundra plant communities (Mulder et al. 1999; Roslin et al. 2013). For instance, the foraging activities of G. groenlandica caterpillars can affect the reproductive success and vegetative growth of their host plants (Mølgaard and Morewood 1996) and impact the foraging decisions of other herbivores (Barrio et al. 2013a). Their main host plant in the High Arctic is the Arctic willow, Salix arctica Pallas (Salicaceae); other food plants have been reported but consumption of these alternative hosts was very limited (< 10%; Kukal and Dawson 1989; Morewood and Lange 1997). Although the extended life-cycles in insect herbivores have been typically linked to a loose association with a
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host plant (Bale et al. 2002), seasonal phenology of Salix arctica seems to be a key driver of larval activity of G. groenlandica, with larvae ceasing to feed when leaf quality decreases in mid-summer (Kukal and Dawson 1989). Latitudinal changes in diet choice, with a progressive restriction of plant use towards higher latitudes, can be related to a tighter phenological bond between the herbivore and its host (Hodkinson 1997). In more severe environments foraging needs to maximise growth over a short period, which requires a closer synchrony between the herbivore and its host plant (Nylin et al. 2009). Food plant preferences of G. groenlandica beringiana were assessed by conducting cafeteria-type experiments, where caterpillars were presented with different plant species from which they could choose to eat. In addition, we recorded occasional observations of the plant species and part of the plant eaten by free-ranging larvae observed actively feeding on the tundra, following Morewood and Lange (1997). Twentynine late instar caterpillars (third instar or older; Kukal and Kevan 1987; Morewood and Lange 1997) were collected in early to mid-June on alpine meadows and adjacent talus at our study site in southwestern Yukon (Ruby Range; 61°12′ N, 138°16′ W). Until used in the experiment caterpillars were raised indoors in plastic pots (500 mL) on a varied diet of common tundra plants, to avoid affecting food preference due to prior experience (Pérez-Harguindeguy et al. 2003). Caterpillars were deprived of food for 24 hours before the experiment. Cafeteria experiments were set up in rectangular (34 × 20 × 10 cm) translucent plastic containers. The bottom of the container was lined with white open-cell foam, with a hole cut in each of the four corners to hold a 1.5 mL water-filled vial to support plant samples (MacLean and Jensen 1985). New plants were collected immediately prior to the start of cafeteria trials, with 1 cm of woody stem or taproot when possible; dry leaves were removed from plant samples and samples were arranged so that all leaves were above the water and available for grazing. The water inside the vials was replaced along with plants for every new trial. One caterpillar was placed in each cafeteria, where it was presented with four species of plants; each caterpillar was used only once. We used two combinations of four plants selected at random © 2014 Entomological Society of Canada
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Table 1. Diet of Gynaephora groenlandica at High Arctic sites on Ellesmere Island, Nunavut, Canada, as reported by (A) field observations of Morewood and Lange (1997), (B) cafeteria trials in Kukal and Kevan (1987), (C) field observations in Kukal and Kevan (1987), and (D) field observations of Kukal and Dawson (1989). Reference
Food plants
A
Salix actica (87%) Dryas integrifolia (7%) Saxifraga oppositifolia* (3%) Others: Arctagrostis latifolia, Festuca brachyphylla, Luzula confusa, Luzula arctica, Potentilla hyparctica, Vaccinium uliginosum Salix arctica Dryas integrifolia* Others (only reported for cafeteria trials): Oxyria digyna, Eriophorum angustifolium Salix arctica Dryas integrifolia Saxifraga oppositifolia* Salix arctica (97%) Others (
