Assigning phenologically asynchronous spruce budworm moths to source populations using individual genotypes

Background/Question/Methods

Spatially synchronous outbreaks of forest insect pests have significant, large-scale impacts on Canadian forests. Outbreaks of forest pests such as the spruce budworm disrupt ecosystem dynamics, reduce timber supply, and can have significant socio-economic consequences. Novel Early Intervention Strategies (EIS) that aim to mitigate the negative effects of such outbreaks require extensive knowledge regarding their underlying mechanisms. A key question for EIS is whether outbreaks are synchronized by spatial autocorrelation in environmental conditions or dispersal from high- to low-density regions. In this study, we use genotyping-by-sequencing and population genetic analyses of SNP data to improve our understanding of the relative role of dispersal in driving spatial synchrony in budworm outbreak dynamics at the scale of the province of Quebec, Canada. Specifically, we investigated the changes in genetic diversity and spatial genetic structure among larvae and moths at different phenological timings (early-, mid-, and late-season) to distinguish residents from migrants and to characterize the inter-generational dispersal patterns of an ongoing outbreak. Then, we sought to assign putative migrant individuals to source populations based on their multi-locus genotypes using a machine learning algorithm implemented in the tool assignPOP.

Results/Conclusions

We identified similarly weak spatial genetic structure (i.e., Fst< 0.01) in all phenological groupings, indicating high genetic connectivity and high rates of dispersal between populations at the scale of our study area. Allelic richness and heterozygosity varied among phenological groups and geographic location. Putative residents (larvae and local moths) exhibited the highest levels of heterozygosity and allelic richness. While some evidence of effective dispersal was found in all phenological groupings, late-season moths appeared to contribute the most to overall genetic connectivity and spatial synchrony. Our results confirm the important role of dispersal in synchronizing outbreaks at the landscape scale and can be used to guide further early intervention strategies. Specifically, these findings further highlight the importance of targeting “hotspots” for spray (i.e., Bt) early in the outbreak and early in the season to prevent later season dispersal. Finally, this work demonstrates the utility of molecular markers in distinguishing migrants from residents and teasing apart complex population dynamics in outbreaking systems.