The pear tree is a main economical orchard crops under temperate climate with more than twenty-five million tons of fruit produced each year. The psylla Cacopsylla pyri is the most important pest, as it infests all commercial pear tree varieties, it sucks the phloem sap and it cause damages to the trees by nutrient subtraction and disease transmission (sooty molds and phytoplasm). The management of this insect is mainly based on integrated pest management with a mix of natural and chemical controls. However, with the ban of a growing number of plant protection products and the increasing public interest in an “organic” mode of consumption, it is important to develop innovative ways of pest managements respectful of the environment. Trechnites insidiosus is the most abundant parasitoid of C. pyri but it has been poorly studied. The aim of this study is to evaluate the parasitism behaviour of T. insidiosus toward the different larval stages of C. pyri, and to evaluate the quality of the next generation individuals. We observed that stage 3 and 4 larvae are the most interesting hosts for T. insidiosus in order to produce individuals in quantity and in quality. This provides a basis for further studies and fulfill the lack of data concerning this insect in the literature, particularly with regard to its biology, its behaviour and its use in biological management.