# Combinatorial Olfactory Signaling in Short-Distance Determines Host Plant Recognition in Locust
## 4 Discussion
The present results supported the conclusion that characteristic individual volatiles from host and nonhost plants have opposite behavioral effects, and this \"double-check\" would greatly enhance the accuracy of final decision-making. This delicate \"pull-push\" strategy in locusts complements the host-plant selection theory, especially in understanding how specialist insects select their food source. During the foraging behavior of insects, they will encounter, detect, and behaviorally respond to plant volatiles from long range to short range. As the distance changes, the content and concentration of the compound changes over long to short distances. Generally, insects initially trace the common chemicals that are emitted by host and nonhost plants over long distances. While approaching a host, they need to accurately judge the plant species based on the characteristic volatiles released by host and nonhost plants. This study showed that locusts were attracted by two host-specific volatiles, (_E_)-2-pentenal and 2-methyl-2-pentenal but were repulsed by the non-host-specific heptanol and heptanal volatiles at short-range in the DER assay. A previous study [28] reported that the host-specific volatiles (_E_)-2-pentenal and 2-methyl-2-pentenal had significant effects on the biting response in locusts, but the non-host-plant volatiles heptanol and heptanal showed no effect on biting behavior. In addition, the common volatile hexanal resulted in locust biting. These observations are consistent with the present findings. In addition, it suggests that olfaction and taste together determine the locust feeding selection in the short range,and olfaction occurs before taste. The chemical cues are used to decide whether to eat, and then the quality of the food is further evaluated through taste. Gustatory and tactile factors have been shown to guide the short-range evaluation of food plants, whereas volatiles are believed to act in long-range attraction via antennal olfactory signaling [25; 28]. For orthopteran locusts, the domes of the maxillary and labial palps among the mouthparts are sparsely affiliated with olfactory basiconica sensilla and canonical ORco-based olfactory signaling was demonstrated to tune common green-leaf volatiles of host plants [30; 48]. In this study, suppression of olfactory signaling, putatively in the palps, disrupts host-plant selection in the short range. This demonstrated the direct roles of olfactory coding in the close differentiation of plants. An interesting question is how are the opposite behavioral effects for host and nonhost plant volatiles encoded in the olfactory receptor neurons (ORNs) and then relayed and presented in the olfactory center in the brain? First, the cellular expression modes between ORs involved in sensing wheat and cotton odors should be determined in the palps. The compartmentalization of ORNs for opposite plant odors would be expected. In addition, whether this spatial separation of ORNs is associated with a physiological reaction to volatiles with opposite valence needs to be addressed. Second, how do the plant odors of wheat and cotton trigger opposite behavioral consequences? One possibility is that different ORNs that receive host and nonhost odors project into spatially separated areas in the brain. An early report supported the hypothesis that odor-induced vomiting is combinatorially triggered by palp ORNs that project to the lobus glomerulatus (LG) in the locust brain [30]. It would be of particular interest to check the projection patterns in the LG between olfactory inputs from single odors of wheat and cotton. Genetically labeling of specific ORs identified in this study could clarify this question in the future.