Spatial genetic structure at the leading edge of a spruce budworm outbreak: The role of dispersal in outbreak spread

Abstract Outbreaking insects often undergo rapid population growth synchronously in multiple locations separated by large distances. Dispersal may play an essential role in synchronizing outbreaks over large geographic areas but its role in outbreak spread remains unclear. In our study, we used population genetics to assess how much dispersal contributes to the spread of a major forest pest, the spruce budworm (Choristoneura fumiferana Clemens). Using spatial analyses of genotypic data, we determined the extent to which dispersing individuals from epidemic populations in Quebec leave their genetic signature in adjacent, rising populations in New Brunswick and Maine. A lack of genetic differentiation between endemic and epidemic populations would indicate that outbreak spread depends on effective dispersal from epidemic to endemic regions. In contrast, endemic populations that are distinct from epidemic population would suggest that dispersal plays a reduced role in fomenting outbreak rise. An intermediate possibility with differentiation between epidemic and endemic population as a continuous spatial gradient might suggest a temporal lag in genetic differentiation that may take the form of a genetic travelling wave. PCA, cluster analyses, isolation by distance, and sPCA were used to characterize spatial genomic variation using 300 SBW larvae sampled in 2015 and genotyped at thousands of genome-wide SNPs. We found support for a genetic travelling wave pattern, matching the SBW density pattern. These results indicate that dispersal may trigger the transition of populations from the endemic to the epidemic state and thereby drive outbreak spread. By supplementing persisting endemic populations, migrants may drive populations to beyond levels that can be controlled by local biotic constraints. These results support the underlying justification of ongoing outbreak containment efforts in Atlantic Canada (i.e., the Early Intervention Strategy), although continued sampling as the outbreak progresses is needed confirm the temporal stability of the observed patterns.