Abstract
ITO, Yosiaki, Kazuyoshi MIYASHITA and Katsui SEKIGUCHI (Div. Ent. Nat. Inst. Agri. Sci., Tokyo and Chiba Pref. Agri. Exp. Sta., Chiba) Studies on the predators of the rice crop insect pests, using the insecticidal check method. Jap. J. Ecol. 12,1-11 (1962). To elucidate roles of polyphagous predators, especially of spiders, in natural control of insect pests of the paddy field, experiments with the insecticidal check method were conducted at two localities near Tokyo. In all the insecticide-treated plots, the green rice leafhopper and the planthoppers remarkably increased several weeks after the treatment. The rates of increase in the treated plots were higher than in the untreated ones. The primary cause of this increase was estimated to be the destruction of the spider populations by the insecticidal spray. Mortality of the rice stem borer larvae during their dispersal was lower in the treated plot than in the untreated one, suggesting an important role of the predation by the spiders and by other arthropod predators on the dispersing larvae. Life tables for the rice stem borer showed three mortality peaks, of which two were suggested to be mainly due to the predation. Within low density levels of the prey, the action of the spiders was dependent with the prey density, while it was density-independent or inversely density-dependent at high prey densities. This indicates a characteristic role of the polyphagous predators on insect populations. A schematic food-chain around the rice plant was presented according to the results of the present study.
