Abstract
The infective larvae entered the roots of rice plant in the middle of May. Adultfemale deposited eggs in the geratinous matrix after five weeks from penetration. Cyst formation was completed after six to eight weeks from entry. In larval population fluctuation four peaks were observed; it is considered that the first peak observed in the middle of June was that of larvae which emerged from cysts surviving the winter, the second one in July to August was derived from the first generation, the third one in the middle of August was from the second generation and the last one in the beginning of September was from the third generation. The first and the second peaks were higher than the later two ones. Cyst density gradually decreased from early April to late August, then it increased to the initial density at September except in fallow plot. The rates of empty eggs in cyst were very low in spring but it increased after sowing the rice. Developmental zero was 10.6°C and sum of effective temperature was 455 day-degrees centigrade for a completion of one generation. The adult female produced a large quantity of eggs in the egg mass. The time required to larval hatch was shorter in egg mass than in cyst and hatching rate was higher in egg mass than in cyst. Therefore, it is considered that larvae from egg masses chiefly infested during rice growing season, on the other hand, cysts surviving the winter played an important part in the propagation in the next spring. In generation-isolating experiments, three generations were observed in a rice-growing season. Vertical and horizontal distribution patterns of this nematode in soil were closely related to the root distribution. The population density of cysts was highest in the soil layer from surface to depth of 10 cm, and in the widths of 12.5 cm toward the right angle with row from the center of the hill.
