Genomic insights into the origin and invasion dynamics of the spruce-killing bark beetle <i>Ips typographus</i> in the UK.

<jats:p>The spruce bark beetle &lt;i&gt;Ips typographus&lt;/i&gt; (L.) (Coleoptera: Curculionidae) is a major tree-killing pest in Europe. Recent outbreaks in southern England raise urgent questions about its origin, invasion dynamics and establishment risk. We generated genome-wide SNP data from beetles collected in the UK across multiple years (2018–2022), plus comparative samples from across Europe, to infer the likely origins and population dynamics of the UK samples. Genotyping of 2,954 SNP loci across European samples recovered the distinct northern and southern genetic groups identified in previous whole-genome studies and refined understanding of their geographic extent. We identified highly differentiated outlier SNPs between regional groups that distorted clustering and assignment analyses, plausibly because such loci are partly decoupled from neutral demographic history and geographic provenance due to the inversion-rich genome of &lt;i&gt;I. typographus&lt;/i&gt;. After removing these outliers, all UK beetles clustered within the southern European group, excluding northern-group regions as likely sources. Temporal subdivision of UK samples revealed reduced numbers of polymorphic loci within annual cohorts relative to European samples, whereas reductions in observed and expected heterozygosity were modest. Pairwise FST among yearly UK cohorts was also relatively high given their close proximity and the otherwise weak structure within the southern European group, indicating temporal allele-frequency shifts. These patterns are consistent with repeated incursions by small founder groups rather than drift within a single homogeneous established population. Interpreted alongside independent surveillance evidence, the genomic results support recurrent wind-assisted dispersal into the UK from continental zones, making introduction via imported timber a less likely explanation. If introduced individuals persist despite eradication efforts, their southern European origin suggests they may be pre-adapted to milder climates and capable of two or more generations per year, heightening establishment risk. These genomic insights inform agile surveillance and biosecurity strategies.</jats:p>