Evolutionary Ecology of parasitoid wasps focuses on how and why to do in the real world to increase fitness. How parasitoid wasps adjust their reproductive strategy according to the surrounded environment to increase fitness. My work concentrated on different environmental factors producing the patterns of variations of life history and behavioral traits in the parasitoid wasps. I investigated how the environmental factors affect the ecology of parasitoid wasps and as a result their behaviors. Animals are considered to make decisions between alternative courses of actions (Charnov et al. 1981). Such decisions can be analysed in terms of their costs and benefits. A powerful tool in this approach is optimality theory, which allows us to test the hypotheses about the costs and benefits of alternative behaviours by predicting their effects on the animal’s fitness (Krebs and Davis 1993).A model (Iwasa et al., 1984) which describes optimal host utilisation of C. rubecula in relation to the host densities of P. rape used to analysis the optimum decision of C. rubecula in relation to the host densities of Pieris rape.