THE ANALYSES OF STRUCTURE AND FUNCTION OF THE BIOTIC COMMUNITY IN RICE PADDY FIELDS, FOR DEVELOPING AGRICULTURAL TECHNIQUES EMPLOYING NATURAL MECHANISMS.

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The biotic communities in rice paddy fields were investigated especially on the population dynamics of plant hoppers, comparing the natural and organic farming systems to the conventional farming system at several rice fields in Hiroshima Prefecture.Nearly equal number of plant hoppers imigrated into the fields of both kinds, and the hopper population increased in the conventional farming fields to induce considerable damages on rice plants in spite of insecticidal sprays. On the other hand, the population did not increase in the natural farming fields managed for more than 10 years.The structure of biotic community changed depending on the age of natural farming management after a big modification of field structure with the disturbance of a balance in the biotic community to suppress the plant hopper increase.Among several kinds of natural enemies a parasitic nematode, Agamermis unka was the most efficient biological control agent to suppress plant hopper population. However, because of it sedentary habit with a low migratory ability the recovery of its population was greatly delayed to induce hopper outbreaks after a strong habitat disturbance, intermittent rice cultivation, or temporal disappearance of plant hoppers with chemical control.The conventional management system of rice plant cultivation characterized the plant physiology with a dense color of rice leaves, more hill numbers per hill, and rapid growth at early stage of development. These plant characters produced brachypterous females to increase the hopper population. On the other hand the rice plants of natural farming had characters of pale leaf color, reduced stem number per hill, and slow development, which decreased the percentage of brachypterous female to reduce the rate of population growth of plant hoppers.