Abstract: Optimum time and concentration of insecticide application depend on many factors including pest population dynamics, crop susceptibility to damage, crop yield in unit area, unit price of production, degradation rate of insectioide and its efficiency, and weather、The marginal analysis method is applied to develop a system model for determining the optimum time and concentration of insecticide application. The system model consists of four submodels: 1) population dynamic model; 2) yield loss forecasting model; 3) insecticide degradation model; and 4) insecticide efficiency model. Moreover, a weighted factor reflecting rice susceptibility to damage of paddy stem borer is introduced. By maximizing a function, the optimum time and concentration of insecticide application can be obtained.

Key words: Paddy stem borer, Insecticide, Susceptibility, Yield loss, Economic thieshold