Linking spatiotemporal and demographic patterns in modulating local population outbreak of invasive forest insect

Abstract

Understanding the spatial and demographic structure of local outbreaks is critical for identifying the ecological mechanisms underlying invasive forest insect dynamics. We investigated a population outbreak of Sirex noctilio within a pine plantation in Patagonia by integrating spatial analyses of infestation patterns with quantification of oviposition behaviour and adult performance. At the plot level, the distribution of infested trees shifted from a highly clustered pattern in previous seasons to a more spatially dispersed one of newly attacked trees Logs sampled from S. noctilio-infested trees located in the outbreak epicentre and periphery showed no significant differences in total oviposition drills or estimated egg numbers; however, single (non-reproductive) drills were more frequent in the periphery. Emergence of S. noctilio adults were higher in the periphery, though individuals were smaller, suggesting spatial variation in offspring performance within the outbreak area. These findings highlight spatial variation in oviposition behaviour and larval performance, suggesting the role of localized feedback and host availability in shaping outbreak trajectories. Our study emphasizes the importance of spatial structure in modulating demographic responses during a population outbreak, offering insights into context-dependent regulatory mechanisms of invasive forest insect populations in simplified plantation systems.