Seasonal fluctuation of spider population in paddy fields

Abstract

For the purpose of understanding of spider's role in paddy fields, this study was carried out from 1966 to 1968 at Utsunomiya, Tochigi Prefecture, Japan. Popu-lation density of spiders was examined in two paddy fields of Mine and Ishii Town. Seasonal fluctuations of all spiders in two paddy fields are shown in Fig. 2 and 3. In every cases, just after the rice-planting season, numbers of spiders found in these paddy fields, were few. Then they gradually increased up to autumn. Maximum number of spiders was about 90/m2 at the Mine field (Fig. 3). By the insecticidal applications the spider population was scarcely influenced in early stage, however the number of spiders decreased in late stage of rice plants. Seasonal fluctuations of each main species are shown in Fig. 4-12. In the late stage of rice plants, the population density of main species in Ishii field where insecticides were sprayed by helicopter was low compared with that in Mine field where those were not done, but only the population density of Pirata subpiraticus kept exceptionaly high in the Ishii field as well as that in the Mine field. Many species reached maximum density in October, that is, harvesting time of rice plants. On the other hand, leaf hoppers and plant hoppers reached the maximum density in September. Therefore the spiders in late season, do not serve enough as the natural enemy of these insect pests. But the maximum den-sity of Pirata subpiraticus emerged in August, in cases in which the density around June was high to some extent. From these results, Pirata subpiraticus seems to be the most useful spider as predator to insect pests in paddy fields.