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Simple Summary: Diapause is a critical state of an insect's life cycle when it undergoes arrestment of growth and/or reproduction to survive adverse environmental conditions and/or food shortage. Aprostocetus fukutai is a specialist egg parasitoid of the citrus longhorned beetle, Anoplophora chinensis, a high-risk invasive woodboring pest. The parasitoid overwinters as diapausing mature larva in the host egg and emerges in early summer in synchrony with the egg-laying of A. chinensis. Here, we determined the optimal conditions for diapause survival of this parasitoid. We showed that the parasitoid had a low (36.7%) diapause survival rate inside host eggs laid on potted plants due to desiccation or tree wound defense response under semi-natural conditions. Under laboratory rearing conditions, when parasitized host eggs were extracted from wood, the parasitoid did not survive at low humidity (44% RH). Survival rate increased with humidity, reaching the highest at 100% RH. Survival rate also increased with increasing chilling period temperature from 2 to 12.5 °C. Post-diapause developmental time decreased with increased humidity or temperature, but the reproductive fitness of the parasitoid was not significantly affected by the temperature regimes. Overall, high humidity (100% RH) and mild temperatures (12.5 °C) are the most suitable survival conditions for the diapausing parasitoid and thus should be used in laboratory rearing. Aprostocetus fukutai is a specialist egg parasitoid of the citrus longhorned beetle Anoplophora chinensis, a high-risk invasive pest of hardwood trees. The parasitoid overwinters as diapausing mature larvae within the host egg and emerges in early summer in synchrony with the egg-laying peak of A. chinensis. This study investigated the parasitoid's diapause survival in parasitized host eggs that either remained in potted trees under semi-natural conditions in southern France or were removed from the wood and held at four different humidities (44, 75, 85–93 and 100% RH) at 11 °C or four different temperature regimes (2, 5, 10 and 12.5 °C) at 100% RH in the laboratory. The temperature regimes reflect overwintering temperatures across the parasitoid's geographical distribution in its native range. Results show that the parasitoid resumed its development to the adult stage at normal rearing conditions (22 °C, 100% RH, 14L:10D) after 6- or 7-months cold chilling at both the semi-natural and laboratory conditions. It had a low survival rate (36.7%) on potted plants due to desiccation or tree wound defense response. No parasitoids survived at 44% RH, but survival rate increased with humidity, reaching the highest (93.7%) at 100% RH. Survival rate also increased from 21.0% at 2 °C to 82.8% at 12.5 °C. Post-diapause developmental time decreased with increased humidity or temperature. There was no difference in the lifetime fecundity of emerged females from 2 and 12.5 °C. These results suggest that 100% RH and 12.5 °C are the most suitable diapause conditions for laboratory rearing of this parasitoid. [ABSTRACT FROM AUTHOR]