ダイコン

Structure of host-parasite population and mechanism of parasitization in the population which occurred between the green peach aphid, Myzus persicae (Sulzer), and its parasite, Aphidius gifuensis Ashmead, were studied in a radish field in Fukuoka City by analyzing the spatial distribution of aphids and their mummies formed by the parasite. 1) Frequency distributions both of aphid counts and of mummy counts per plant fitted to the negative binomial distribution. 2) Some analyses were attempted by using Morisita's Iδ-index. The spatial distribution pattern of mummies closely resembled to that of the latter in unit of a plant. The aphid population had a structure: Colony―aggregation of individuals on one or two plants (small group)―larger aggregation on six to ten successive plants (large group)―total population in a field. Parasitization behavior of A. gifuensis was also considered. 3) Concerning both colony and aggregation on a plant, there was certain limit in the number of parasitized aphids. This relationship may cause inversely density-dependent parasitism. However, it was sometimes observed that percentage parasitism in the aggregation was nearly constant regardless of variation in host abundance. This fact was considered to be caused by a coaction of the occurrence of upper limit in attacks of parasites and increase in probability of parasite visits to a host group. 4) Concerning the large group of the aphid, any relations were not found between abundance of host and the number of mummies.