Trichogramma dendrolimi Matsumura (Hymenoptera: Trichogrammatidae) is an economically important egg parasitoid with a wide host range and is mass produced for biological control programs. With the development of in vitro rearing techniques, an artificial medium has been developed that is believed to hold potential for the large-scale production of the parasitoid. We sought to evaluate the effectiveness of an artificial rearing medium by including biochemical analyses and not just by comparing the biological parameters of reared wasps. In particular, this study was conducted to determine differences in proportions of nutrients between the artificial medium and a natural host used for mass rearing T. dendrolimi by biochemical analyses of carcasses of the latter. Biochemical parameters including protein concentration, lipid content, trehalose content, and trehalase activity in T. dendrolimi continuously reared in an optimized artificial medium versus on Antheraea pernyi Guérin-Méneville (Lepidoptera: Saturniidae) eggs were compared. The adults produced in vitro showed lower protein concentrations and higher lipid and trehalose contents and trehalase activity compared with those reared in vivo. Different—and even contrary—evolution of biochemical parameters was found when comparing adults reared in the artificial medium with those reared in natural host eggs over 10 generations. The results suggested that the artificial medium had a non-optimal composition for the in vitro rearing of T. dendrolimi, and that it could be improved by altering the composition of the medium, especially its protein concentration and lipid content. Biochemical analysis may be a useful method for optimizing artificial rearing media for other entomophagous insect species.