A tri-trophic ecosystem model was developed, combining a functional-structural plant growth model GreenLab and a discrete dynamic population model. Two issues of modeling are investigated: (1) analysis of sensitivity of model out-puts to model parameters and (2) parameter identification. Results revealed that parameters which contribute greatly to model output can be identified. Two applications of insect pest management techniques are optimized based on the proposed tri-trophic ecosystem model. The optimal results indicate that to protect plants, biological control and pesticides should be applied as early as possible.