Phase-dependent outbreak dynamics of geometrid moth linked to host plant phenology

摘要

Climatically driven Moran effects have often been invoked as the most likely cause of regionally synchro-nized outbreaks of insect herbivores without identifying the exact mechanism. However, the degree ofmatch between host plant and larval phenology is crucial for the growth and survival of many spring-feeding pest insects, suggesting that a phenological match/mismatch-driven Moran effect may act as asynchronizing agent. We analyse the phase-dependent spatial dynamics of defoliation caused by cyclically outbreaking geo-metrid moths in northern boreal birch forest in Fennoscandia through the most recent massive outbreak(2000-2008). We use satellite-derived time series of the prevalence of moth defoliation and the onset ofthe growing season for the entire region to investigate the link between the patterns of defoliation and out-break spread. In addition, we examine whether a phase-dependent coherence in the pattern of spatialsynchrony exists between defoliation and onset of the growing season, in order to evaluate if thedegree of matching phenology between the moth and their host plant could be the mechanism behinda Moran effect. The strength of regional spatial synchrony in defoliation and the pattern of defoliation spread were bothhighly phase-dependent. The incipient phase of the outbreak was characterized by high regional synchronyin defoliation and long spread distances, compared with the epidemic and crash phase. Defoliation spreadwas best described using a two-scale stratified spread model, suggesting that defoliation spread is governedby two processes operating at different spatial scale. The pattern of phase-dependent spatial synchrony wascoherent in both defoliation and onset of the growing season. This suggests that the timing of springphenology plays a role in the large-scale synchronization of birch forest moth outbreaks.