EFFECT OF TREE HOST AND BEETLE DENSITY ON REPRODUCTION AND SURVIVAL OF IPS-TYPOGRAPHUS JAPONICUS NIIJIMA (COL, SCOLYTIDAE) IN HOKKAIDO, JAPAN

Reproduction and survival of I. typographus japonicus at non-outbreak densities in relation to two tree species, its natural host, Picea jezoensis, and an introduced host, Picea abies, was investigated in 1990 and 1991 in the natural mixed forest zone of Hokkaido, Japan. Beetles did not discriminate between logs of the two species during the attack phase, however, female beetles produced significantly longer egg galleries in P. jezoensis logs than in P. abies logs, particularly in 1990. Regression analysis showed that egg galleries were on average 2 cm longer in P. jezoensis in both years. A consistent trend toward higher larval survival in P. jezoensis logs was also observed. It is postulated that these differences were caused by a combination of differences in tree ages used in the studies and therefore their physiological status and inherent differences in the suitability of each tree species as breeding resource. Intraspecific density dependent effects on reproduction and survival were similar to those previously reported for outbreak or post-outbreak populations, with the strongest effects being on reductions in egg gallery length and larval survival as beetle density increased. These factors were important even at the relatively low densities found in this study. The relevance of these results to the management of I. typographus in the spruce forests of Japan and the importance of native insects exploiting non-native host plants as new resources is briefly discussed.