Simple SummaryInsects regulate their physiology and behavior through their circadian clock in response to daily changes in the photoperiod. Parasitoid wasps are well-known biocontrol agents. Therefore, understanding the circadian activities of parasitoid adults may help improve biological control strategies. In the present study, we characterized the circadian patterns of emergence, mating, and oviposition of an ectoparasitoid wasp,Pachycrepoideus vindemmiae. We also identified eight clock candidate genes, most of which showed significant rhythmic expressions. These results serve as a starting point for further functional studies of the clock genes inP. vindemmiaeas well as in other parasitoid wasps. The findings of this study also provide information that could contribute to improving biological control strategies using parasitoid wasps.AbstractDespite the importance of circadian rhythms in insect behavior, our understanding of circadian activity and the molecular oscillatory mechanism in parasitoid wasp circadian clocks is limited. In this study, behavioral activities expected to be under the control of the endogenous circadian system were characterized in an ectoparasitoid wasp,Pachycrepoideus vindemmiae. Most adults exhibited emergence between late night and early morning, while mating only occurred during the daytime, with a peak at midday. Oviposition had three peaks in the early morning, late day, or early night and late night. Additionally, we identified eight putative clock genes fromP. vindemmiae. The quantitative PCR (qPCR) results indicate that most clock genes showed significant rhythmic expressions. Our comparative analysis of clock genes inP. vindemmiaeand 43 other parasitoid wasps revealed that none of the wasps possessed thetimelessandcry1genes commonly found in some other insect species, suggesting that the circadian clock system in parasitoid wasps is distinct from that in other non-Hymenoptera insects such asDrosophila. Thus, this study attempted to build the first hypothetical circadian clock model for a parasitoid wasp, thus generating hypotheses and providing a platform for the future functional characterization ofP. vindemmiaeclock genes as well as those of other parasitoid wasps. Finally, these findings onP. vindemmiaecircadian activity will aid the development of effective field release programs for biological control, which can be tested under field conditions.