Bottom-up factors contribute to large-scale synchrony in spruce budworm populations

Understanding the mechanisms that cause large-scale synchrony in insect population dynamics might yield key insights for predicting potential outbreak occurrence. Here, we evaluated which environmental factors best explain synchronous population fluctuations in the spruce budworm (Choristoneura fumiferana Clem.) (SBW), a major defoliator of coniferous forests in North America. SBW population levels were assessed with pheromone traps during the 1986-2014 period across a 625 000 [km.sup.2] territory located in the province of Quebec (Canada). The populations were characterized by abundance fluctuations that were often synchronized across the whole study area. Interannual population fluctuations were correlated with host tree cone production (a source of food) and high May temperatures, suggesting that synchrony was influenced by food availability and phenological mismatch during shoot development. Cone production was itself correlated with low precipitation during the previous summer. This study indicates that bottom-up trophic factors can drive spatiotemporal synchrony in insect populations and contribute to explain important and sustained population increases during some years. We also suggest that several biological processes, all influenced by weather, are likely to interact to explain population synchrony during the different phases of the approximately 35 year SBW outbreak cycle, complicating the prediction of climate change effects on this insect. Key words: spruce budworm, population synchrony, mast years, boreal forests, phenology. La comprehension des mecanismes qui causent le synchronisme a grande echelle dans la dynamique des populations d'insectes pourrait aider a mieux predire l'occurrence des epidemies. Dans cette etude, nous avons evalue quels facteurs environnementaux expliquent le mieux les fluctuations synchrones des populations de tordeuse des bourgeons de l'epinette (Choristoneura fumiferana Clem.) (TBE), un defoliateur important des forets coniferiennes d'Amerique du Nord. Les niveaux des populations de TBE ont ete evalues a l'aide de pieges a pheromones durant la periode 1986-2014 dans un territoire de 625 000 [km.sup.2] localise dans la province de Quebec (Canada). Les populations de TBE etaient caracterisees par des fluctuations d'abondance souvent synchronisees sur l'ensemble du territoire. La fluctuation interannuelle des populations etait correlee avec la production semenciere des arbres hotes (une source de nourriture) et des temperatures elevees lors du mois de mai, suggerant que le synchronisme est influence par la disponibilite de la nourriture et le decalage phenologique lors du debourrement. La production semenciere etait elle-meme correlee avec un faible niveau de precipitations durant l'ete precedent. Cette etude indique que la disponibilite en ressources peut expliquer le synchronisme spatio-temporel chez les populations d'insectes, et contribuer a expliquer les hausses de populations importantes et soutenues lors de certaines annees. Nous suggerons egalement que plusieurs processus biologiques, tous influences par les conditions meteorologiques, pourraient interagir pour expliquer la synchronisation lors de differentes phases du cycle epidemique de la TBE, compliquant la prediction des effets des changements climatiques sur cet insecte. Mots-cles: tordeuse des bourgeons de l'epinette, synchronisme des populations, annees semencieres, forets boreales, phenologie.
Introduction Large-scale synchrony in population fluctuations has been described for many animal species, including small mammals, ungulates, and particularly insects (Liebhold et al. 2012; Myers and Cory 2013). Synchronous population [...]