Abstract: Successfully locating a host is one of the crucial steps in parasitoid reproduction, which is regulated mainly by volatile compounds. Firstly, female parasitoids often use volatiles from host plants, especially herbivore-induced plant volatiles (HIPVs), to locate the habitat of their hosts at long distances, which helps female parasitoids narrow their host searching range according to the cues provided by HIPVs. The blends of HIPVs are extremely complicated, and their components and contents can be modulated by a wide range of factors. Nevertheless, terpenoids are common compounds in HIPVs and have been confirmed by most behavioral studies to play a role in host localization of parasitoids. Subsequently, when the parasitoids find a plant related to hosts and land on them, they exploit host-derived cues at a close range to locate hosts. Parasitoids are usually attracted to volatile compounds released from the host body, cocoon, feces, etc. The odor of some host feces can be used as the main cues for parasitoids to locate hosts. Moreover, the components of volatiles in host feces may vary among plant species which insects feed. In addition, semiochemicals from the parasitoid itself are also beneficial for other individuals of the same species to locate their hosts. Sensory perception of odor molecules by parasitoids relies on olfactory sensilla, primarily located on the antennae, but the research on the molecular mechanism of their olfactory recognition is still limited. Because odorant-binding proteins (OBPs) are the crucial proteins in the olfactory system that mediate odor recognition, most studies only focus on the analysis of antennal OBPs. According to the current research, the number of genes encoding OBPs is highly variable among parasitiod species, and most studies have verified that OBPs play an important role in host location of parasitoids by means of gene silencing, fluorescence competitive binding assay, molecular docking and so on. Research on host localization and olfactory mechanism of parasitoids has important ecological significance, and also has a good application perspective in the integrated management of agricultural insect pests. Applying volatile compounds, or growing companion plants and transgenic plants that emit useful volatiles can enhance the host location of parasitoids in the field, so as to achieve a better biological control efficacy against insect pests.

Key words: Parasitoid, host location, olfactory cues, olfactory mechanism, biological control