Olfactory function of tarsal sense organs in forelegs of phytoseiid mites: a case study of <i>Neoseiulus barkeri</i> (Acari: Phytoseiidae) and herbivore induced plant volatiles

Neoseiulus barkeri Hughes, a widely used biological control agent for small pests, relies on its sophisticated olfactory system for key behaviors such as foraging, prey location, and mating. While herbivore-induced plant volatiles (HIPV) are well-studied in other insects and large natural enemies, the olfactory mechanisms of phytoseiid mites like N. barkeri remain largely uncharacterized, hindering advances in biological control. This study investigated the potential function of foreleg tarsal sensory organs of the phytoseiid mites in HIPV recognition. The results showed N. barkeri females and males were both attracted to the full blend and six individual volatile originated from cowpea leaves infested by Tetranychus urticae Koch. N. barkeri females no longer showed odor preferences once tarsi of leg I were excised, but attraction was not affected when tarsi of leg IV were excised. SEM analysis characterized the setae types in the distal part of tarsi in leg I as stout peg-shaped structures without sockets, identified as olfactory sensilla. Excising tarsi of legs did not affect predation or copulation in N. barkeri, but reduced its walking speed and mate-searching efficiency. Expression of three Niemann-Pick type C2 genes in excised tarsi of leg I was significantly down regulated when induced by ocimene. These results suggest that tarsi of forelegs of phytoseiids are involved in the perception of HIPV, as well as in discrimination of odor signals. Our study provides a foundation for further elucidation of effective control strategies against small insect and mite pests with predators and HIPV-based components as attractants or repellents.