Rice crop stage susceptibility to the rice yellow stemborer Scirpophaga incertulas (Walker) (Lepidoptera : Pyralidae)

From field trials with artificial infestation of rice yellow stemborer Scirpophaga incertulas (Walker) (YSB) egg masses over the post-transplanted crop cycle and reports in the literature, it was concluded that rice passes through three stages of general morphological resistance interceded by two stages of susceptibility. Resistance occurs on very young rice, at mid-growth, and after panicle exsertion. The resistant period at mid-growth varied by plant maturity. On an early maturing rice variety (IR72), resistance occurred from panicle initiation (PI) to pre-booting, while on the medium maturing IR70, resistance was extended from the late vegetative stage to booting. High deadheart (severed tiller) densities occurred post transplanting because, being small, up to three tillers are damaged by a single larva. Whiteheads (severed panicles) were formed from infestations from booting onwards and were relatively higher than deadhearts on the early maturing variety but relatively less on the medium maturing variety. Zinc deficiency delayed maturity of IR72, and the period of naturally high resistance of seedlings was extended throughout most of the vegetative stage as well as after PI through pre-booting. Resistance at mid-growth and after panicle exsertion could be from: ( 1) lignin and cellulose deposits on the cell walls, (2) tight wrapping of the leaf sheath, and (3) deposition of silica. All three trials produced similar quadratic models of the yield response to crop age when infested. Deadheart damage was compensated to a large degree, but lowest yields occurred from whitehead damage when infested at or just after the pre-booting stage.