EFFECT OF NATURAL ENEMY EXCLUSION ON MORTALITY OF IPS-TYPOGRAPHUS JAPONICUS NIIJIMA (COL, SCOLYTIDAE) IN HOKKAIDO, JAPAN

The impact of natural enemies on tile reproduction and survival of Ips typographus japonicus Niijima in Hokkaido was tested by sequentially caging off cohorts of a single beetle generation sc as to investigate the effects of natural enemies at different developmental stages. A co nsistent increase in mortality of between 17-18% in the first 2-3 week s of exposure to natural enemies was observed, after which mortality r emained relatively constant. Natural enemies which were found to occur in logs exposed during this time were the predators Medetera sp., Tha nasimus substriatus, and the adult parasitoid Tomicobia watanabei. Of these, only the first two were found to be significantly associated wi th beetle brood mortality. However, a confounding factor was the fact that beetle density also significantly increased in logs during the pe riod when mortality was increasing. It was found that larval mine dens ity, and hence intraspecific mortality, was the single most important factor correlated with beetle mortality during this period. It was thu s concluded that increased intraspecific competition was responsible f or most of the increase in mortality during the first 2-3 weeks exposu re, with predators playing a more minor role. Parasitoids were observe d to occur in logs late in the exposure period when mortality was rela tively constant and so appeared to have little effect on beetle reprod uction and survival. The results of this study were compared to result s from similar previous exclusion experiments with bark beetles and th e fact that beetle density-dependent effects as a mortality factor was not considered in many of these studies noted. It is therefore sugges ted that some of these previous studies may have overestimated the imp ortance of natural enemies and associates as mortality factors and sub sequently underestimated the importance of intraspecific competition. Suggestions are made to improve bark beetle exclusion methodology so a s to remove the confounding factor of beetle density from the analysis .