Attack density negatively affects reproductive success but not fecundity or offspring body size in a bark beetle

We investigated the relationships between attack density and several different reproductive parameters—gallery length, number of eggs, reproductive success, and offspring body size—in the tree‐killing bark beetle Polygraphus proximus Blandford (Coleoptera: Curculionidae: Scolytinae) in the absence of host resistance. Neither the mean egg‐laying tunnel length nor the number of eggs per gallery system varied significantly with attack density. These results suggest that P . proximus adults disregard galleries constructed by conspecifics and continue to lay eggs irrespective of attack density. Conversely, the number of adult offspring per female decreased significantly as attack density increased, indicating that intraspecific competition among offspring within fallen trees limits reproductive success. The relationships between attack density and offspring density showed a clear carrying capacity. In contrast, offspring elytra length did not differ significantly among attack densities. Our findings suggest that P . proximus adults search for and locate already colonised hosts using conspecific pheromones, and as a result, intraspecific contest competition occurs among offspring and limits their reproductive success, but not their body size.