Host desiccation and interspecific competition limit reproductive performance of a bark beetle

Bark beetle outbreaks are often triggered by abiotic disturbances (e.g., storms and drought), but the mechanisms underlying outbreak initiation remain poorly understood. Moreover, the mechanisms that maintain populations at endemic levels remain poorly understood. During the endemic phase, secondary bark beetles compete with both conspecifics and other secondary species for weakened or dead trees, leading to reduced reproductive success. This interspecific competition may be influenced by host quality, such as phloem moisture. However, the combined effects of interspecific competition and phloem moisture remain unclear.

We conducted field experiments in Hokkaido, Japan, using experimental logs to evaluate how the reproductive success of Polygraphus proximus Blandford (Coleoptera: Curculionidae: Scolytinae) is affected by intraspecific competition, interspecific competition with the sympatric bark beetle species and host phloem moisture.

Our results showed that at high and mean phloem moisture levels, P. proximus reproductive success declined significantly with increasing presence of Cryphalus laricis . In contrast, when phloem moisture was low, C. laricis presence had no significant effect. Intraspecific competition had no significant effect. These findings indicate that the effects of interspecific competition are mediated by phloem moisture. Additionally, adult offspring density of P. proximus declined significantly with increasing presence of C. laricis across a range of phloem moisture.

Our study suggests that both interspecific competition and low phloem moisture can contribute to maintaining low population levels of P. proximus during the endemic phase. Cutting weakened or dead trees into shorter pieces may promote desiccation, reduce the quality of breeding material and suppress future outbreaks.