Seasonal adaptation, phenology and survival in gall-inducing sawflies (Tenthredinidae: Nematinae: Pontania)

Abstract

The phenological adaptation of herbivorous insects in the Arctic may be particularly prone to the negative impact of the current global climate change. Increasing temporal dissociation with their plant resources, and to their natural enemies such as parasitoids may be detrimental for their survival. The life history adaptations for gall-inducing sawflies in the Arctic (Tenthredinidae: Nematinae: Pontania nivalis, P. glabrifrons and P. arcticornis) followed the common outline for overwintering insects. During winter hibernation, diapause intensity prevented resumption of morphogenesis in early winter and turned into a post-diapause quiescence mid-winter, in which response to temperature synchronizes spring eclosion. Hindcasts of temperature conditions in spring for the last 21 years revealed a highly significant advancement in dates of eclosion, an evidence of global warming. There were, however, large inter-annual differences in eclosion timing, which probably is caused by timing of the spring snow melt. In early spring there is a short phenological window of opportunity for oviposition on suitable willow (Salix spp).host resources. Females showed a non-discriminatory oviposition preference pattern that did not enhanced larval performance, however larval survival was high. This pattern is probably caused by a brief and early spring eclosion when host plant cues are lacking. In autumn, diapause preparation was shown to occur in response to photoperiod during pre-diapause. Pontania spp. had a remarkable phenological synchronous behaviour in which larvae start to emerge from the galls in search for overwintering sites to pupate on 5 september. Although specialist insect species may be especially prone to the negative impacts of climatic change, this study does not provide any clear indications that this applies to Pontania spp..