The Eurasian spruce bark beetle, Ips typographus, is a major pest, capable of killing spruce forests during large population outbreaks. Recorded dispersal distances of individual beetles are typically within hundreds of meters or a few kilometers. However, the connectivity between populations at larger distances and longer time spans and how this is affected by the habitat is less studied, despite its importance for understanding at which distances local outbreaks may spread. Previous population genetic studies in I. typographus typically used low resolution markers. Here, we use genome-wide data to assess population structure and connectivity of I. typographus in Sweden. We used 152 individuals from 19 population samples, distributed over 830 km from Strömsund (63° 46' 8″ N) in the north to Nyteboda (56° 8' 50″ N) in the south, to capture processes at a large regional scale, and a transect sampling design adjacent to a recent outbreak to capture processes at a smaller scale (76 km). Using restriction site-associated DNA sequencing (RADseq) markers capturing 1409-1997 SNPs throughout the genome, we document a weak genetic structure over the large scale, potentially indicative of high connectivity with extensive gene flow. No differentiation was detected at the smaller scale. We find indications of isolation-by-distance both for relative (F ST) and absolute divergence (Dxy). The two northernmost populations are most differentiated from the remaining populations, and diverge in parallel to the southern populations for a set of outlier loci. In conclusion, the population structure of I. typographus in Sweden is weak, suggesting a high capacity to disperse and establish outbreak populations in new territories.

Animal Welfare and Disease Control Copenhagen University Frederiksberg C Denmark

Department of Biology Lund University Lund Sweden

Department of Forest Genetics and Plant Physiology Umeå Plant Science Centre Swedish University of Agricultural Sciences Umeå Sweden

Department of Physical Geography and Ecosystem Science Lund University Lund Sweden

Department of Physics Lund University Lund Sweden

Department of Plant Protection Biology Swedish University of Agricultural Sciences Lomma Sweden

Excellent Team for Mitigation Faculty of Forestry and Wood Sciences Czech University of Life Sciences Prague Suchdol Czech Republic

Global Change Ecology Research Group University of the Sunshine Coast Sippy Downs Queensland Australia

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Complex Genomic Landscape of Inversion Polymorphism in Europe's Most Destructive Forest Pest