The types and levels of damage affecting rice yields at different crop stages were evaluated by simulating pest damage. Many plant species have the ability to decrease the negative effects of injury on yield, a process known as compensation (Pedigo 1991). Rice can compensate for stem borer damage at the tillering stage (Rubia 1994). Rice plants, however, do not compensate for cut tillers to the same degree that they compensate for stem borer damage (Rubia 1996).
Four damage simulation trials were conducted from February 1998 to January 1999 in Phnom Penh, Cambodia, using a randomized complete block design with four replications and four treatments per replicate. Each replicate consisted of a concrete basin (2.8 m x 2.4 m surface, 0.6 m deep), filled with soil to a depth of 0.4 m. The NPK rate was the same in all experiments–60-60-30 kg ha-1, applied as 30-30-30 NPK basally and 30-30 NP at panicle initiation (PI). There were 30 seedlings treatment-1 (1 seedling hill-1) at 20-cm spacing. IR66 seedlings (25 d old) and medium-duration CAR11 seedlings (33 d old) were transplanted. In these experiments, "50% damage" refers to complete removal of half of the leaves or stems, not cutting the leaves or stems in half. The weight and moisture content of each plot's yield were recorded. Weights were adjusted to 14% moisture and converted to t ha-1. Significant differences in mean treatment yields were detected by ANOVA and LSD with IRRISTAT 4.0.