In order to elucidate the cause of the amount of emerging adult of the Japanese horntail
Urocerus japonicus varying on host trees, we investigated the relationship between the number of emerged adults from felled logs of Japanese cedar
Cryptomeria japonica and several factors on
U. japonicus and on its host trees. The number of emerged adult correlated with both of the number of oviposited pinholes and of hatched larvae. Furthermore, there was a strong correlation between the number of emerged adults and the diameter of host tree. There was not a correlation between the number of emerged adults and the percentage parasitism of
Megarhyssa praecellens. Also the moisture content of host trees correlated with the number of emerged adults, hatched ratio and emerged ratio of
U. japonicus, respectively. These relations were explained with an assumed model on the number of oviposited pinholes, the number of hatched larvae, the number of emerged adults, the diameter of host trees and the moisture content. We estimated that the size of host tree was one of the regulating factors on the number of emerging adults of
U. japonicus through the moisture content.
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