Within the Interreg project ‘De Gezonde Kas’ one of the aims is to develop a decision support system (DSS) for the use of natural enemies against greenhouse pests. Part of the DSS is a model which simulates population dynamics of the pest and its beneficial. The first pest we model is greenhouse whitefly (Trialeurodes vaporariorum), one of the major pests in Dutch greenhouses, and its natural enemy Encarsia formosa. In the presentation, I will give you an overview of the modelling principles, like the boxcar train method for development of both species, and how Encarsia searches for its host and decides whether to eat it or parasitize in it. Simulations results are compared to experimental greenhouse data, and scaling issues that arise are addressed.