The influence of different rice growing seasons and rice cultivation phases on aquatic organisms was investigated in the Manik Rambung rice field (MRRF) ecosystem in North Sumatera. Composite collections of core samples and aquatic net samples from four rice growing seasons and five cultivation phases (fallow, plough, transplanting-young, tiller, mature-preharvest) were analysed. There were marked seasonal variations in abundances of these organisms, but the abundance pattern in various rice cultivation phases was comparable among rice growing seasons. The rice field was dominated by tubificids, baetids, chironomids and ceratopogonids. High populations of these organisms were observed during the plough, transplanting-young, and tiller phases, but lower in the other rice cultivation phases. The proliferation of larval Agriocnemis femina (Odonata: Zygoptera), one of the rice pest predators in the adult stage, was dependent on its interactions with 14 aquatic taxa in different phases; nine taxa in the plough phase and three to five taxa in other phases. Hemipterans, coleopterans, other odonates, chironomids, baetids and tubificids were important for successful emergence of Agriocnemis femina to a predatory adult. Rice cultivation managements that focus on enhancing the population of Agriocnemis femina would contribute to more effective biological control of rice pests in MRRF.
