The actions of a natural enemy on the equilibrium population of a pest insect, especially on the height of the population level and the shape of the population fluctuation, were studied experimentally using the almond moth, Cadra cautella WALKER, and its parasitic wasp, Exidechthis canescens GRAVENHORST. A serrated fluctuation of the host population was observed with an interval of about 37 days, which corresponds to an approximate length of a host generation, both in the single host system and the host-parasite interacting system. An average level of population density was stable in the single host system, while the population fluctuated with an interval of about 3 generations of the host in the interacting system. Food, rice bran, was less consumed by the host larvae in the interacting system than the single host system. Increased host finding ability of the wasp resulted in the increased amplitude of population fluctuation of the host and parasite and in the decreased food consumption by the host larvae. It was clarified, however, that the population in the host-parasite interacting system was regulated through the density effect of the host itself as well as through the effect of the interaction between the host and parasite. Through an analysis of UTIDA'S data on the system involving the azuki bean weevil, Callosobruchus chinensis, and its parasite, Anisopteromalus calandrae, it was demonstrated that the S-shaped functional response of the parasite to host density (B-type of HOLLING, 1961) was undetectable in his host-parasite interacting system. However, the possibility of the operation of the S-shaped functional response was noticed in the system of the almond moth and its parasite dealt with by the present writer. From this point of view the population regulation in the experimental system was considered as a model of natural insect populations.
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