Host preference of a tree-killing bark beetle across a geographic boundary separating host species

The life cycles of bark beetles are intimately linked to their host species. Therefore, bark beetle species are expected to show traits that vary among different host species and across geographic ranges. The taxonomic proximity of host tree species can also influence host selection. Abies species native to Japan are genetically classified into three phylogenetic groups. Their natural distributions are separated by a distinct biogeographic boundary in Japan (i.e. the Tsugaru Strait, also referred to as Blakiston's Line). Especially, A. sachalinensis is the only species native to Hokkaido, north of the Tsugaru Strait. I, therefore, investigated the host preference of Polygraphus proximus Blandford within four allopatrically distributed Abies species across the north and south of the Tsugaru Strait through field surveys and field choice experiments. Field observations and field host-choice bioassays showed that A. sachalinensis was less severely attacked by P. proximus than was A. veitchii on both sides of the Tsugaru Strait. The beetles also preferred to attack A. firma (which belongs to a genetically distinct group (i.e., the section Momi) from that of A. veitchii and A. sachalinensis (i.e., the section Balsamea)), over A. sachalinensis in both Hokkaido and Honshu. Although A.homolepis also belongs to the section Momi, A. firma and A. veitchii had more attacks than A. homolepis. The results provide evidence that P. proximus does not show specialization across the different parts of their geographic range where the host species are different, and there is no positive correlation between taxonomic proximity and host preference for P. proximus.