# Combinatorial Olfactory Signaling in Short-Distance Determines Host Plant Recognition in Locust
## 1 Introduction  
One critical driver for insect herbivores to search for and select appropriate plants to eat is to acquire sufficient nutrients for survival. More nutritious compounds and less hazardous substances mostly define the host plants that fuel the processes of development and reproduction. Plants containing all the nutrients required by insects are selectively eaten, whereas nonhost plants are actively avoided [1, 2, 3]. Insets are capable of determining their food spectrum when given a choice because they are equipped with sensitive sensory organs and efficient metabolic processes to distinguish and adapt to host and nonhost plants [3, 4]. However, a critical question is, therefore, how does an insect herbivore localize the nutritious host plants among the external complexity, especially before they eat? Specialist lepidopteran insects localize a food source based on attractive host odor components [5, 6, 7]; for example, checkerspot butterflies detect iridoid glycosides [8] and cabbage butterflies detect glucosinolates [9]. However, studies with bark beetles show that avoidance of nonhost volatiles is important in their host location process [10]. In addition, whether the foraging strategy differs between poly and oligophagous insects in choosing host plants remains largely enigmatic [11, 12, 13]. One ideal candidate would be the various plant-produced chemical cues that provide the relevant traits to facilitate herbivores to locate and assess the quality of the potential foods [14, 15, 16]. The molecular mechanisms of insect olfaction have been extensively elucidated [17]. Insects detect external signals using several families of receptor proteins, including odorantreceptors (ORs) and ionotropic receptors [18]. Previous studies have found that insect ORs are involved in the recognition of plant volatiles. For example, AlinOR59 is involved in chemoreception of floral scent in _Adelphocoris lineolatus_[19]. In addition, larval antenna-specific OR, LstiOR5, of the meadow moth, _Loxostege sticticalis_, is narrowly tuned to a foraging-induced volatile of the host plant's leaves [20]. However, these studies did not demonstrate the possibility of the combinational recognition of plant volatiles by multiple ORs. Considering the extremely diverse odorant composition derived from the host/nonhost plants, it is reasonable to speculate on the involvement of multiple ORs in food selection as a molecular strategy for combinatorial coding to odorants [21]. The migratory locust, _Locusta migratoria_, is a notorious oligophagous herbivore insect that feeds on a small number of gramineous species, such as corn and wheat, as well as _Cyperus_ spp. Once the plant is touched, the locust vibrates the palps frequently, in close proximity to potential food, to rapidly collect chemical cues called palpation [22]. Many reports have focused on the gustatory roles of palpation in judging food palatability, for example, the active physiological responses to taste and nutrients [23; 24; 25; 26]. Previously, other authors and we reported that locust palps react actively to various odors in a broad response intensity, both physiologically and behaviorally [27; 28; 29]. In addition, we identified complex encoding patterns between olfactory receptors and volatile ligands, as well as established spatial compartmentalization between olfactory sensilla and volatile ligands on the palps [30]. Fewer ORs may be expressed on the palps than on the antennae, which provides a favorable system for studying the combinational function of multiple ORs. Thus, in this study, we examined the olfactory roles of palpation in distinguishing host plants from nonhost plants in a novel free-moving olfactory assay. We observed that, within a short distance, olfactory coding on the palps was associated with opposite behavioral effects between host and nonhost plants.