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

Abstract

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.

Keywords:

• Rice insect pest
• ecology
• planting synchrony
• damage
• Scripophaga innotata
• S. incertulas
• biocontrol
• egg parasitoids
• predators
• multiple parasitism
• ecological diversity

Acknowledgements

The able assistance of the Koronadal site staff Beatriz Datijan, Anita Labarinto, Hector Corpuz, and Joseph Siazon is gratefully acknowledged. The work of J.H. Lourens in translating van der Goot's 1925 treatise from Old Dutch to English was a key to understanding WSB ecology. Thanks also go to the Philippine National Irrigation Administration staff in Koronadal for providing the rainfall data as well as the staff in the Philippine Department of Agriculture Region XI and XII offices in Davao and Koronadal for supporting the project. The kind assistance of Dale Bottrell of the University of Maryland in providing literature sources is much appreciated.

Notes

¹Note the distinction between egg mass parasitism and egg parasitism. Also parasitism includes the contribution of predation from T. schoenobii which plays both roles but is considered a parasitoid.