Ecological and Biological Studies of Two Larval Parasitoids on Two Monochamus Vectors of the Pinewood Nematode in South Korea

Pine wilt disease, caused by the pinewood nematode, affects <i>Pinus densiflora</i>, <i>P. koraiensis</i> Siebold and Zucc., and <i>P. thunbergii</i> Parl. in South Korea. It is primarily spread by vectors such as <i>Monochamus alternatus</i> Hope and <i>M. saltuarius</i> (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, <i>Cyanopterus flavator</i> (Fabricius) and <i>Spathius verustus</i> 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 <i>C. flavator</i> were 6.3% and 1.0%, whereas those of <i>S. verustus</i> were 0.7% and 0.5% on <i>M. alternatus</i> and <i>M. saltuarius</i>, respectively. Both <i>C. flavator</i> and <i>S. verustus</i> appeared to prefer <i>M. alternatus</i> to <i>M. saltuarius</i>, but a statistically significant difference was observed only for <i>C. flavator</i> (<i>p</i> < 0.001). The peak season and tree height with the highest parasitism rate of <i>C. flavator</i> coincided with the period when <i>M. alternatus</i> larvae lived beneath the bark and the highest larval density of <i>M. alternatus</i> and <i>M. saltuarius</i> occurred in pine trees. These findings provide valuable information for determining the optimal release time and height for <i>C. flavator</i> to achieve effective biological control.