NOXIOUSNESS OF THE AFRICAN WHITE STEM BORER, MALIARPHA-SEPARATELLA RAG (PYRALIDAE, PHYCITINAE) IN IRRIGATED PADDY FIELDS AT LAKE ALAOTRA (MADAGASCAR)

A study of the noxiousness of the African white rice stem borer, Malia rpha separatella Rag carried out under controlled field conditions has revealed that the impact of this pest on the yield and its components is assessable. For mean infestation rates up to 68% during the period of highest sensitivity of a paddy rice culture (second half of tiller ing to panicle initiation), no significant decrease could be observed in the number of panicles, nor any significant increase in the number of white heads and the number of empty grains per hill. However, with increasing infestation rates, the number of immature panicles increase d and the number and weight of filled grains per hill decreased. An ex plorative analysis of the infestation rate-yield loss relationship, sh owed that a sigmoid model describes this interdependence appropriately . The proposed economic injury level (EIL), is here considered more th e result of an approach, which had as its goal the re-definition of th e status of the pest than as an instrument of a chemical control progr amme with treatments based on an action threshold.