Pine wilt disease in Korea can be managed by reducing vector density through chemical application. To reduce the side effects from pesticides, effective natural enemies must be identified and used to reduce the vectors’ natural density. Sentinel logs were used to investigate a parasitoid wasp parasitic to Monochamus alternatus and Monochamus saltuarius, the vectors of Bursaphelenchus xylophilus, which causes this disease. During 2016–2017, the parasitoid wasps distributed in the Pinus densiflora and Pinus koraiensis forests were surveyed using sentinel logs at six different areas. An ectoparasitoid wasp Spathius verustus Chao of M. alternatus and M. saltuarius was identified. We showed for the first time that S. verustus was parasitic to M. alternatus in South Korea and to M. saltuarius worldwide. The parasitism rates were affected by region and session but not by the sentinel log height in the tree and the distances between the trap and forest edge locations. Studies also showed that S. verustus appeared to prefer M. alternatus to M. saltuarius as a host. This study unveiled the ecological details of S. verustus. Further research on various environmental factors such as regional differences, host density differences, and the degree of damage from the pine wilt disease is required to understand the effects of environmental or ecological factors on parasitism rates.

Pine wilt disease, caused by the pinewood nematode, affects Pinus densiflora, P. koraiensis Siebold and Zucc., and P. thunbergii Parl. in South Korea. It is primarily spread by vectors such as Monochamus alternatus Hope and M. saltuarius (Gebler). However, biological control research in this context remains limited in South Korea. We identified 15 parasitoids of these vectors, namely, two ectoparasitic larval parasitoids, Cyanopterus flavator (Fabricius) and Spathius verustus Chao, both in Braconidae, Hymenoptera. A field survey was conducted in Pohang from 2018 to 2020 to elucidate the ecological and biological characteristics of these parasitoids. The average parasitism rates of C. flavator were 6.3% and 1.0%, whereas those of S. verustus were 0.7% and 0.5% on M. alternatus and M. saltuarius, respectively. Both C. flavator and S. verustus appeared to prefer M. alternatus to M. saltuarius, but a statistically significant difference was observed only for C. flavator (p < 0.001). The peak season and tree height with the highest parasitism rate of C. flavator coincided with the period when M. alternatus larvae lived beneath the bark and the highest larval density of M. alternatus and M. saltuarius occurred in pine trees. These findings provide valuable information for determining the optimal release time and height for C. flavator to achieve effective biological control.