Origin of non-native Xylosandrus germanus, an invasive pest ambrosia beetle in Europe and North America

Xyleborine ambrosia beetles (Coleoptera: Scolytinae: Xyleborini) are among the most important and destructive pests in forests, tree nurseries and plantations worldwide. Their cryptic lifestyle, fungal mutualism, inbreeding and broad host range have predisposed them to become remarkably successful invaders and colonize novel habitats across the world. The black timber bark beetle, Xylosandrus germanus (Blandford), is native to East Asia, but now established in North America and Europe. It is an economically significant invasive pest in North American nurseries and orchards, and European forests. Managing and preventing the spread of exotic species such as X. germanus requires an understanding of invasion pathways and mechanisms associated with their source populations. In this study, we sequenced the mitochondrial gene COI and nuclear gene ArgK from six native and 24 non-native X. germanus populations to identify their origins. The genetic structure of X. germanus in non-native habitats is highly uniform and points to introductions from Honshu and/or Hokkaido, Japan. However, different haplotypes of the inferred Japanese source lineage, which dominate in North America and Europe today, together with temporal incidence of records of X. germanus indicate that these continents were invaded independently. While European populations were probably introduced only once prior to 1951, the genetic pattern of North American populations suggests that X. germanus was introduced several times.