This paper deals with the results of investigation on the faunistic composition and its seasonal change in a Japanese radish field. All insects on Japanese radish field were collected weekly from May to June and October to November, in 1963 in the Shimane Agricultural College farm.
1. The composition of the major species in Japanese radish field are shown in Table 1 and 2.
2. The succession of the structure of insect community in Japanese radish field was summarized as follws :
Pieris rapae crucivora > Aphididae > Phytomyza atricorinis-association in the early May →Aphididae > phytomyza atricornis > Piers rapae crucivora > Phaedon brassicae-association in the middle May →Aphididae > Phytomyza atoicoris > Parasites of the aphid-association in the end of May→Pieris rapae crucivora > Aphididae > Plutella maculipennis-association in the middle to the end of June.
Aphididae > Phaedon brassicae > Parasites of the aphid > Pieris rapae crucivora-association in the middle to the end of October→Phaedon brassicae > Aphididae > Parasites of the aphid > Athalis rasae japonensis-association in the beginning to the end of November.
3. The values of a and b obtained in each time in the law of geometrical progression, Log y+ax=b, offered by MOTOMURA (1932) are shown in Table 3 and Figure 2～3. The number of individuals was observed to increase from middle to the latter part of May, and the faunistic composition was most complex in the early of May. From middle to the end of October, the simplest fauna were shown in the composition thougth the population was high, but from middle to the latter part of November it became complex.
4. The important injurious insects in Japanese radish field were Aphididae, Pieris rapae crucivora Atholia rosae Japonensis, Ahoedon brassicae, Plutella maculipennis.