Rice white stemborer Scirpophaga innotata (Walker) in southern Mindanao, Philippines. II. Synchrony of planting and natural enemies

Although the rice white stemborer ( WSB) Scirpophaga innotata ( Walker) has been an epidemic pest in other locations in the Philippines and Indonesia, its population has remained at chronic pest levels in Koronadal, southern Mindanao, Philippines. Field studies were undertaken to determine the role of egg parasitoids and general predators in suppressing WSB numbers. The results revealed greater mortality levels on WSB than in any other location where beneficials have been assessed. Four hymenopteran parasitoids ( Tetrasticus schoenobii Ferriere [ Eulophidae], Telenomus rowani ( Gahan) [ Scelionidae], Telenomus dignus ( Gahan), and Trichogramma japonicum Ashmead [ Trichogrammatidae]) combined to parasitise 65% of eggs in the second 1990 - 1991 rice crop. Multiple parasitism occurred in 61% of parasitised egg masses. Egg predation averaged 44% principally by the tettigoniids Conocephalus longipennis ( de Haan) and C. maculatus ( Leth.). Generalist predators steadily built up over the rice crop. A contributing factor to the high success of natural enemies could be related to the special situation of two irrigation systems interspersed among each other. Communal systems fed by artesian wells resulted in asynchronous planting areas where farmers can plant five crops in 2 years juxtaposed to a larger river diversion system typified by highly synchronous double rice cropping. The heterogeneous habitat created by the two irrigation systems could favour the density-dependent T. schoenobii, the keystone parasitoid in the rice ecosystem, which acts as both a parasitoid and predator.