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The Asian citrus psyllid, Diaphorina citri, is the primary vector for the bacterium Candidatus Liberibacter asiaticus, the causal agent of Citrus Huanglongbing. In this study, we investigated the effects of temperature and psyllid host age on the biology of Tamarixia radiata, the species-specific dominant nymphal parasitoid of D. citri in South China. Both temperature and host age significantly affected the fecundity, parasitism, and development of T. radiata. For the temperature effects, the averaged fecundity and parasitism of T. radiata was significantly increased with temperatures between 18 and 26 °C. The highest averaged fecundity (15.6 eggs/female) and parasitism (78%) were recorded at 26 °C. The developmental time of T. radiata on the 4th and 5th instars nymphs averaged 19.6 days at 18 °C, 13.4 days at 26 °C, and 10.2 days at 34 °C. The optimal temperatures for the development and survival of T. radiata was in the range of 26–30 °C. For the age effects, the development time from egg to adult parasitoid of T. radiata increased with the age of host nymph, 11.5 days in the 3rd instar, 12.3 days in the 4th instar, and 13.4 days in the 5th instar at 26 °C. Moreover, citrus psyllid host age also affected the sex ratio of T. radiata, with 42.78%, 60.66%, and 76.56% females developing from 3rd, 4th and 5th instar nymphs, respectively. These findings may prove to be helpful toward the development of biological control strategies targeting D. citri.