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

Abstract

The phenological window of opportunity for mating, oviposition and larval development is an important part in the life cycle of gall-inducing sawflies. Spring temperatures play an essential role in the timing of eclosion and host plant leaf flush, which are evolutionarily synchronized. This study investigates important events in spring phenology and timing of eclosion for three sympatric gall-inducing sawfly species (Symphyta: Tenthredinidae: Pontania nivalis, P. glabrifrons and P. arcticornis) at two sites in northern Norway. The rate of development was established by monitoring the time to eclosion at constant temperatures of 9, 15, 23 and 28°C. There was no significant difference in the lower developmental threshold (2.36°C) and hour-degree requirement for eclosion (6909 hour-degrees) for the three Pontania species. These two values were used with field and historical temperature data to estimating eclosion over 21 years . The results showed later eclosion dates at the arctic than at subarctic site (mean difference 16 days, range 2-30). There was a highly significant advance of eclosion from 1993 to 2013 in both sites, an evidence of global warming. Adult presence in the subarctic site (field trapping, 2012 and 2013) supported the simulated model used. Measurements of gall size showed continuous gall growth until late August, well ahead of winter hibernation. To conclude, gall-inducing sawflies show large phenological plasticity in timing of eclosion, largely regulated by spring temperature, indicating that they are highly adapted to unpredictable subarctic and arctic climates.