Number of gallery arms of the tree-killing bark beetle Polygraphus proximus (Coleoptera: Curculionidae: Scolytinae)

•Gallery length increased as the number of gallery arms in a gallery system increased.•Polygraphus proximus laid more eggs in two-armed galleries than in one- and three-armed ones.•Galleries with more than two arms did not have more eggs compared to two-armed ones.•Gallery length with no oviposition increased as the number of gallery arms increased.•The optimal number of arms (in terms of the number of eggs oviposited) might be two.