Effect of natural enemy exclusion on mortality of Ips typographus japonicus Niijima (Col., Scolytidae) in Hokkaido, Japan

Abstract

The impact of natural enemies on the reproduction and survival of Ips typographus japonicus Niijima in Hokkaido was tested by sequentially caging off cohorts of a single beetle generation so as to investigate the effects of natural enemies at different developmental stages. A consistent increase in mortality of between 17–18% in the first 2–3 weeks of exposure to natural enemies was observed, after which mortality remained relatively constant. Natural enemies which were found to occur in logs exposed during this time were the predators Medetera sp., Thanasimus substriatus, and the adult parasitoid Tomicobia watanabei. Of these, only the first two were found to be significantly associated with beetle brood mortality.

However, a confounding factor was the fact that beetle density also significantly increased in logs during the period when mortality was increasing. It was found that larval mine density, and hence intraspecific mortality, was the single most important factor correlated with beetle mortality during this period. It was thus concluded that increased intraspecific competition was responsible for most of the increase in mortality during the first 2–3 weeks exposure, with predators playing a more minor role. Parasitoids were observed to occur in logs late in the exposure period when mortality was relatively constant and so appeared to have little effect on beetle reproduction and survival.

The results of this study were compared to results from similar previous exclusion experiments with bark beetles and the fact that beetle density-dependent effects as a mortality factor was not considered in many of these studies noted. It is therefore suggested that some of these previous studies may have overestimated the importance of natural enemies and associates as mortality factors and subsequently underestimated the importance of intraspecific competition. Suggestions are made to improve bark beetle exclusion methodology so as to remove the confounding factor of beetle density from the analysis.