Abstract Dastarcus kurosawai Sasaji (Bothrideridae: Coleoptera), endemic to the Nansei Islands of Japan, is considered a promising natural enemy for the augmentative biological control of Monochamus alternatus Hope (Coleoptera: Cerambycidae). However, mass-rearing methods for this parasitoid have yet to be fully established. In this study, we evaluated once-frozen Zophobas atratus Fabricius (Coleoptera: Tenebrionidae) pupae as an alternative host and estimated the optimal inoculation density for mass rearing. Parasitism rates were significantly higher using once-frozen Z. atratus pupae than fresh Z. atratus pupae and fresh Psacothea hilaris Pascoe (Coleoptera: Cerambycidae) pupae. Adults reared on once-frozen pupae had slightly shorter forewings than those from P. hilaris , but other developmental traits were comparable. Pupation success and pupal size declined at a 40-larval density, while no significant adverse effects on body size were observed at 10–20 larvae per host. However, at a 20-larval density, the parasitism success rate tended to decrease. Based on these results, we conclude that once-frozen Z. atratus pupae are suitable as a substitute host for mass rearing of D. kurosawai , and that approximately 10 larvae per host represents the optimal inoculation density for balancing production efficiency, parasitism success, and parasitoid quality.