By means of the systems approach,a simulation model was developed in this paper,which deecribas the effects of the population dynamics of the brown plant hopper (BPT) in paddy fields,the loam in rice yield caused by BPT,and the functional reaponse of the dwarf ipider ((?) (?)) to BPT on the (?) threchold of BPT.Data obtained from indoor or outdoor experiments were used to run the daily aimulations,and the results abowed that the economic threahold varied with the population levels of BPT and its age-structure,the growing stage of the rice plant,the rice yield,and the density of the dwarf spider.