Difference in the Sex Ratio of the Pine Bark Weevil Parasite, Dolichomitus sp. (Hymenoptera : Ichneumonidae), Emerging from Different Host Species

Two experiments were conducted to study difference in the sex ratio of the pine bark weevil parasite, Dolichomitus sp., emerging from different host species. The first experiment was to ascertain whether the host weight influences the sex of the emerging adult. The adults emerging from Pissodes nitidus, which was lightest, were all males, those from Shirahoshizo spp., which were light, were 8 females to 82 males, those from Niphades variegatus, wich was heavier, were 54 females to 39 males. Obviously, the female adults of Dolichomitus sp. tended to emerge from heavier host species. Hence, the second experiment was conducted to clarify whether the parasite progenies changed sex in the course of their development, by transferring the eggs deposited on lighter host weevils to heavier ones and those on heavier ones to lighter ones. It was noted that the parasite sex could be determined at the time of oviposition. Hence, it is considered most possible that difference in the sex ratio of the parasite is due to difference in host species and that the females of Dolichomitus sp. control oviposition of fertilized eggs and deposit them selectively on heavier host weevils.