Freedom to move: Arctic caterpillar (Lepidoptera) growth rate increases with access to new willows (Salicaceae)

Movement between host plants during the growing season is a common behaviour among insect herbivores, although the mechanisms promoting these movements are poorly understood for many systems. Two possible reasons why insect herbivores relocate include compensating for host plant quantity and/or quality changes and the avoidance of natural enemies. The Arctic caterpillar (Gynaephora groenlandica (Wocke); Lepidoptera: Lymantriidae) moves several metres each day, feeds on its patchily distributed host plant, Arctic willow (Salix arctica Pallas; Salicaceae), and has two main natural enemies, the parasitoids Exorista thula Wood (Diptera: Tachinidae) and Hyposoter diechmanni (Nielsen) (Hymenoptera: Ichneumonidae). We physically moved caterpillars between Arctic willows and restricted other caterpillar individuals each to a single willow throughout the active period of Arctic caterpillars. We found that growth rate, herbivory rate, and the proportion of available leaf fascicles eaten were higher for experimentally moved caterpillars. Parasitoid abundances were low and did not differ between experimentally moved and stationary caterpillars. Taken together, our study addresses the bottom–up and top–down controls on insect herbivore movement during the short duration of the growing season in the Arctic. Our results suggest that caterpillars are likely moving to new willow shrubs to access high quality resources.

Diet breadth of Gynaephora groenlandica (Lepidoptera: Erebidae): is polyphagy greater in alpine versus Arctic populations?

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.

Metabolic opportunists: feeding and temperature influence the rate and pattern of respiration in the high arctic woollybear caterpillar gynaephora groenlandica (Lymantriidae)

Arctic woollybear caterpillars, Gynaephora groenlandica, had the capacity to rapidly and dramatically increase respiration rates up to fourfold within 12–24 h of feeding and exhibited similar decreases in respiration of 60–85 % in as little as 12 h of starvation. At the peak of their feeding season, the respiration rates of caterpillars also increased significantly with temperature from 0.5 to 22 degreesC for both fed and starved caterpillars (Q10=1-5). Indicative of diapause, late season caterpillars had depressed respiration rates which were less sensitive to temperature changes (Q10 approximately 1.5), while respiration rates for caterpillars that had spun hibernacula were even lower. G. groenlandica did not appear to demonstrate metabolic cold adaptation compared with other temperate lepidopteran larvae. The seasonal capacity to adjust metabolic rate rapidly in response to food consumption and temperature (which can be elevated by basking) may promote the efficient acquisition of energy during the brief (1 month) summer growing and feeding season, while conserving energy by entering diapause when conditions are less favorable. These adaptations, along with their long 15–20 year life cycle and the retention of freeze tolerance year-round, promote the survival of G. groenlandica in this harsh polar environment.

REPRODUCTIVE ISOLATION IN ARCTIC SPECIES OF GYNAEPHORA HÜBNER (LEPIDOPTERA: LYMANTRIIDAE)

The genus Gynaephora Hübner (Lepidoptera: Lymantriidae) is represented in North America by two closely related species, Gynaephora groenlandica (Wocke) and Gynaephora rossii (Curtis), whose geographic distributions overlap broadly across the Canadian arctic archipelago (Ferguson 1978; Møllgaard and Morewood 1996). Like other lymantriid moths, females of these species fly little, if at all, whereas males are strong fliers and apparently rely on pheromones to locate mates. Cross-attraction has been observed but there is no confirmed evidence that the two species interbreed. This report describes experiments designed to document reproductive isolation of G. groenlandica and G. rossii at Alexandra Fiord, Ellesmere Island (78°53′N, 75°55′W).

Revision of the life history of the High Arctic moth Gynaephora groenlandica (Wocke) (Lepidoptera: Lymantriidae)

Many studies have explored the adaptations of arctic and alpine Gynaephora species (Lepidoptera: Lymantriidae) to their environment, and base-line life-history information is important for the interpretation of such studies. Data and observations on G. groenlandica (Wocke) collected in recent years at Alexandra Fiord, Ellesmere Island, Canada, contradict some of the life-history information previously published for this species from the same site. Detailed analysis of larval head capsule widths and consideration of growth ratios indicate that there are seven rather than six larval instars and that the pattern of development does not deviate significantly from that defined by the Brooks-Dyar rule. Field-rearing of larvae indicates that first-instar larvae overwinter, while field- and laboratory-rearing both indicate that larvae moult once per year, every year. These data and observations greatly shorten and simplify the life history from that previously published and suggest a life cycle of 7 rather than 14 years. This revised life cycle is not presented as an absolute, in recognition of the potential for individual variation, but rather as typical of the developmental pattern of most of the population. As such, it should provide a useful base line for further studies, especially those addressing the influence of predicted climate change in the Arctic.

Winter mortality and the function of larval hibernacula during the 14-year life cycle of an arctic moth, Gynaephora groenlandica

Larvae of the arctic moth Gynaephora groenlandica stop feeding and spin silk hibernacula before the peak of summer season in the Canadian High Arctic Archipelago. This study examines the function of these hibernacula in relation to the biotic and abiotic mortality factors of parasitism and temperature. Winter mortality of 10% among larvae in cages on the tundra was compared with previous results on parasitism (56% mortality). Prior to winter, the cages were used to record larval behaviour and the location of hibernacula. The majority of the larvae (81%) spun hibernacula, most of which were concealed between the stems of arctic heather, Cassiope tetragona. Fewer hibernacula were found on the primary host plant, arctic willow, Salix arctica, than on C. tetragona or Dryas integrifolia, which formed the dominant plant cover. Nearly one-half of all the larvae that spun hibernacula made joint hibernacula with other larvae. Frequency of larvae sharing hibernacula declined with increasing numbers of larvae per cage. At low population density about half of the larvae occupied communal hibernacula, whereas only one-quarter of the larvae at high density shared hibernacula. In most cases only 2 larvae spun a common hibernaculum, 3 larvae shared hibernacula less frequently, and greater numbers of larvae were rarely found in a single hibernaculum. Unlike the high excess body temperatures usually achieved through thermoregulation by feeding larvae and pupae, temperatures within hibernacula were nearly identical with those of the surrounding substrate over 18 h and rose < 5 °C during the afternoon. This study suggests that larval hibernacula lower summer and winter mortality of G. groenlandica larvae. Hibernacula are an effective barrier to parasitism, which is the primary mortality factor. Furthermore, the behavioural shift from feeding to spinning hibernacula may prevent energy depletion by inducing metabolic depression during mid to late summer, which may be essential for winter survival.