The ectoparasitoid Scleroderma guani (Hymenoptera: Bethylidae) uses innate and learned chemical cues to locate its host, larvae of the pine sawyer Monochamus alternatus (Coleoptera: Cerambycidae)

In Asia, the pine sawyer beetle Monochamus alternatus Hope (Coleoptera: Cerambycidae) is the most important vector of the pinewood nematode Bursaphelenchus xylophilus Steiner and Buhrer (Aphelenchida: Parasitaphelenchidae), the causal agent of pine wilt disease, and the ectoparasitoid Scleroderma guani Xiao et Wu (Hymenoptera: Bethylidae) is the most important natural enemy of this pest. Efficient host location is critical to parasitoid fitness, and chemical cues are key factors guiding the host searching process. This study was conducted to elucidate the odor cues guiding host location in S. guani and the impact of previous host experience on this behavior. Tests were conducted in which S. guani oriented to different odor resources associated with M. alternatus and its habitat, and components of these volatiles were analyzed with gas chromatography/mass spectrometry. Orientation to various odors also was investigated using adult S. guani subjected to different adult experiences. Female S. guani could easily distinguish M. alternatus larvae from other odor resources. Hydrocarbons were the main components of volatiles derived from M. alternatus larvae and were absent in volatiles from wood and frass associated with M. alternatus larvae. Female S. guani oriented most strongly to 1st and 3rd instars of M. alternatus. Prior experience of host odor without oviposition decreased orientation of S. guani towards the host, and the host used for rearing had no impact on subsequent odor orientation by adult S. guani. Volatile hydrocarbons emanating from host larvae might be the key to host location by adult S. guani, and experience with hosts appears to reinforce behavioral responses. The present results may be useful references for improving augmentative biological control using S. guani against M. alternatus and other forest pests.