The ethological significance and olfactory detection of herbivore-induced plant volatiles in interactions of plants, herbivorous insects, and parasitoids

Tritrophic interactions play a pivotal role in maintaining a functional agroecosystem. After damaged by phytophagous insects, host plants release a blend of odorants called herbivore-induced plant volatiles (HIPVs) that are attractive to natural enemies including arthropod predators and, in particular, parasitoids. In the last three decades, the identities of HIPVs have been meticulously characterized in a variety of tritrophic systems by gas chromatography-mass spectrometry (GC-MS) analysis. A plethora of HIPV components have been physiologically screened by gas chromatography-electroantennogram detection (GC-EAD) and single sensillum recording (SSR). The effects of induced odorants on behavior of herbivores and parasitoids have been investigated using Y-tube olfactometer assays and wind tunnels in the laboratory and bait trap tests in the field. Given the potential utility of parasitic wasps for pest control, the understanding of olfactory mechanisms of how HIPVs are detected by herbivores and parasitic wasps could facilitate the exploitation of parasitoids as bio-control agents. As the advent of the genome sequencing and transcriptome analysis, a large repertoire of chemosensory protein genes including odorant receptors and odorant binding proteins has been identified in herbivores and parasitic wasps, providing an unprecedented opportunity to debunk the molecular basis of olfaction-based interactions. In this review, we will summarize the recent progresses in characterization of HIPVs, the studies of olfactory mechanisms underlying tritrophic interactions with a focus on parasitoids, Lepidopteran pests, and related host plants.