Temperature/Development relationships and life history strategies of arctic Gynaephora species (Lepidoptera: Lymantriidae) and their insect parasitoids (Hymenoptera: Ichneumonidae and Diptera: Tachinidae) : with reference to predicted global warming




Morewood, William Dean

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Increases in temperature and precipitation predicted under global warming are expected to be most pronounced and thus have their greatest impact on ecosystems at high latitudes. Insects constitute a major component of the foodwebs of terrestrial ecosystems and should be among the first organisms to show noticeable responses to predicted global warming, especially in the Arctic where climatic conditions are often limiting. However, interactions among species must also be taken into account. The genus Gynaephora Hübner (Lepidoptera: Lymantriidae) is represented in North America by two species, G. groenlandica (Wocke) and G. rossii (Curtis), and their geographic distributions overlap broadly across the Canadian Arctic. Previous studies have examined the biology, ecology, and physiology of these two species and have revealed many adaptations to the Arctic environment, but the immature stages of these insects have been misidentified even in recently published reports. Both species are found at Alexandra Fiord, Ellesmere Island, a High Arctic oasis largely isolated by expanses of ocean and icecap, and the population of G. groenlandica at this site is thought to be limited mainly by parasitoid-induced mortality rather than by climatic conditions. Field observations, surveys, and temperature-manipulation experiments were conducted at Alexandra Fiord during the spring and summer of 1994, 1995, and 1996; laboratory rearing was conducted under controlled conditions at the University of Victoria in the spring of 1996 and 1997. Immature stages of both species of Gynaephora were described and illustrated, and all species of insect parasitoids using Gynaephora species as hosts at Alexandra Fiord were identified. Life histories and seasonal phenologies for Gynaephora species and their insect parasitoids were elucidated from field studies, and temperature/development relationships for selected stages of most of these species were derived from laboratory rearing. The results of field studies and laboratory rearing were compared and used to formulate predictions about the responses of these insects to predicted global warming. Immature stages of the two species of Gynaephora are easily distinguished by differences in the colour patterns, form, and overall length of the larval hairs and by the structure of their cocoons. Both species of Gynaephora complete metamorphosis and reproduction within a single growing season but spread larval development over a number of years. In G. groenlandica, seven larval instars and annual moulting combine to produce a seven year life cycle whereas G. rossii develops through six larval instars at a rate of two or three moults per year, resulting in a three or four year life cycle. The parasitoid complex at Alexandra Fiord consists of three primary parasitoids, Hyposoter pectinatus (Thomson) (Hymenoptera: Ichneumonidae), Exorista n.sp. (Diptera: Tachinidae), and Chetogena gelida (Coquillett) (Diptera: Tachinidae), and one hyperparasitoid, Cryptus leechi Mason (Hymenoptera: Ichneumonidae). All of the parasitoids are univoltine, although H. pectinatus may undergo delayed development in some cases, and each of the primary parasitoids relies primarily on a single larval instar for hosts whereas the hyperparasitoid attacks the primary parasitoids during their metamorphosis. Seasonal phenologies of the parasitoids provide optimal access to new hosts but parasitoid-avoidance strategies of Gynaephora larvae ensure that a proportion of their populations escape parasitism. Laboratory rearing showed that the relative timing of host and parasitoid seasonal phenologies is maintained over a broad range of temperatures; therefore, temperature increases predicted under global warming are unlikely to have any great effect on host-parasitoid interactions. However, increased cloudiness associated with the predicted increase in precipitation might have profound effects resulting from lower ground-level temperatures caused by a lack of solar heating. The extent of this effect is uncertain but might lead to reproductive failure in Gynaephora species, with similar repercussions for the insect parasitoids.



Trees, Lepidoptera, Lymantriidae, Parasitic insects, Global warming