Hibernation adaptation and eclosion synchrony in leaf-galling sawflies in subarctic Norway

Abstract

The winter dormancy adaptation in gallinducing sawflies is poorly known. Diapause termination and the following post-diapause quiescence enhance synchronous eclosion in spring. This is probably the most critical part in the life history in gall-inducing sawflies, as there is only a short phenological window of opportunity for mating and oviposition. In a 2 years' study, diapause duration, termination, survival and eclosion synchrony were experimentally investigated for three gall-inducing sawfly species (Symphyta: Tenthredinidae: Pontania nivalis, P. glabrifrons and P. arcticornis). Field-collected galls, sampled from willows (Salix spp.) in early autumn, were kept under natural temperatures outdoors until next spring. Subsamples were successively transferred to the laboratory at 14-day intervals from October to April in a test of development time to eclosion and survival ratio. The time to eclosion decreased throughout the experiment, whereas the proportion successfully eclosed and eclosion synchronicity increased, all indicative of prepupae entering a diapause in early autumn. The diapause terminates midwinter, and the prepupae enter a post-diapause quiescence until the temperature in spring allows a direct development and contributes to a nearly synchronous eclosion. In all three species, males eclosed 1-2 days prior to females (protandry). We hypothesize that synchronous eclosion as well as protandry enhance mating and oviposition success.Our finding indicates that gall-inducing sawflies are well adapted to its harsh subarctic and arctic environment.

Spring phenology and the response to global warming in gall-inducing sawflies

“…with a diapause until mid-winter and post-diapause quiescence contributing to a synchronous spring eclosion. To establish the developmental zero and the hour-degrees required, the eclosion experiment started in mid-March, at a time when diapause had ended (Barstad and Nilssen 2012), and the rate of development calculated therefore shows post-diapause morphogenesis (Fig. 2).…”

“…Synchronous eclosion has been evolutionarily fine-tuned to increase mating opportunity and success (i.e. protandry) (Nylin et al 1993;Morbey and Ydenberg 2001;Barstad and Nilssen 2012). However, for folivorous insects, there is a strong selection pressure to synchronize eclosion with host plant leaf flush.…”

“…The resumption of morphogenesis is postponed due to adverse climatic conditions (i.e. low temperature) until temperatures rise in spring (Hodek and Hodková 1988;Hodek 2002;Danks 2004;Kostál 2006; Barstad and Nilssen 2012). During post-diapause quiescence, time-degree accumulation above a certain temperature threshold dictates timing of eclosion and synchronizes spring eclosion (Danks 1987;Hodek and Hodková 1988;Kostál 2006).…”

“…Galls were placed individually in closed, ventilated containers (155 ml) with moistened peat moss (Sphagnum) and stored outdoors (in Tromsø, 69°N, 19°E) over the winter. In late March 2013, when most individuals should have terminated the diapause (Barstad and Nilssen 2012) and post-diapause development could occur, samples of each Pontania spp. were portioned and transferred to chambers held at four constant temperatures (9, 15, 23, 28°C).…”

“…The temperature threshold and the day-degrees needed for direct development [i.e. post-diapause (see Barstad and Nilssen 2012)] for Pontania spp. were established, and temperature-dependent models were developed to estimate spring phenology in one arctic and one subarctic site over 21 years .…”