Rice white stemborer Scirpophaga innotata (Walker) in southern Mindanao, Philippines. I. Supplantation of yellow stemborer S. incertulas (Walker) and pest status.

The rice white stemborer (WSB) Scirpophaga innotata (Walker) supplanted the closely related rice yellow stemborer (YSB) S. incertulas (Walker) in irrigated, double-rice culture in both Koronadal, South Cotabato, Philippines and Java, Indonesia. In Java, WSB became a serious pest but not in Koronadal where damage levels remained unchanged. The species shift and subsequent outbreaks are seen as independent events. The former occurred just after the El Niño droughts in 1982 – 1983 which suggests that climatic factors triggered the demise of YSB which does not aestivate. A contributing factor may have also been the crop failures from pest outbreaks that occurred during the same period creating long fallows which would have allowed WSB larvae selectively to survive. WSB larvae have been known to diapause for up to 10 – 12 months and consequently is able to survive climatically driven long fallows. Diapause is terminated by rainfall >10 mm after larvae have rested for a minimum of 4 months. Supportive evidence in favour of climate as the causal agent of change is that the distribution of WSB in Asia and Oceania overlaps that of the Intertropical Convergence Zone of the Southern Oscillation. YSB on the other hand is more adapted to a monsoon climate. It is also hypothesised that WSB, normally associated with single-crop, rainfed-rice culture, was able to adapt to double-cropped, irrigated culture due to the introduction of early maturing rices. From what is known about rice stemborer ecology, five cultural practices and natural events were postulated as the reasons keeping WSB densities in check in Koronadal: (1) medium maturing varieties to reduce the dry season fallow, (2) low inorganic N levels, (3) natural enemies, (4) destruction of crop residue harbouring larvae and pupae during land preparation, and (5) rainfall in the dry season causing suicide flights. Control strategies for areas where WSB is a pest should be based on: (1) reducing the fallow period by growing longer maturing varieties and (2) undertaking early ploughing for the wet season crop. [ABSTRACT FROM AUTHOR]