Behavioral Response Of Locusts Driven By Plant Volatile Sensing At Different Range

Plant volatiles influence herbivorous insect host choice during their foraging procedures.Several hypotheses have been proposed to explain how herbivorous insects approach plants by sensing plant volatiles.Insect antennae and maxillary palps are believed to have crucial roles in the detection of host plant volatiles.However,few studies have assessed the roles of these olfactory organs in food selection in terms of the effects of individual volatile compounds from plants at various distances.Therefore,we compared the difference of electroantennogram(EAG)and electropalpogram(EPG)of locust(Locusta migratoria)nymphs to plant volatiles from host and non-host plants,and assessed locust palp-opening response(POR),biting response,and their selection behavior in response to volatile compounds at various distances.?.Differences between Antennae and Palps in Electrophysiological Responses to Plant Odors.Electrophysiological experiments showed that locust antennae and palps perceive odorants differently to some extent.Both the antennae and maxillary and labial palps showed responses to most of the odorants at 1%v/v.However,the relative values of electrophysiological responses of the antennae were stronger than those of the palps to odorants.Contrasted to the antennae could be elicited strong responses by a plenty of odorants,only two odorants,(E,E)-2,4-hexadienal and(E,E)-2,4-heptadienal,induced strong erelectrophysiological responses to both the maxillary and labial palps than to the antennae.As plant aldehydes are sensed by mouthpart palps significantly higher than chemicals in other functional groups,it is possible that they are playing great roles in locust host detection at short range.?.Locust Mouthpart Palps Opening Response and Biting Response to Plant Volatiles.POR is associated with their feeding choice and can be valuable for evaluating the sensitivities of locusts to plant volatiles.We tested locust PORs with 85 plant volatiles.The results indicated selective and dose-dependent PORs to various plant volatiles,and when odorant concentrations were decreased to 1%v/v,the POR index also decreased.Active and non-active compounds can be distinguished based on the POR index.In this study,we identified 27 active chemicals with POR index values>0.5 at a concentration of 5%v/v,including 9 aldehydes,9 alcohols,5 ketones,2 esters and 2 compounds from other functional groups.Aldehydes and alcohols appeared to particularly influence PORs.Moreover,half of these chemicals were common volatiles,whereas the other active volatiles were specific to host or non-host plants.The active compounds selected based on the POR index were used to evaluate the preference of insects to volatile odorants by estimating the BR index.Some chemicals(hexanal,(E)-2-pentenal,(E,E)-2,4-hexedienal,(Z)-3-hexenol,and 2-heptanone)had a BR index similar to the positive control.However,other chemicals(e.g.,hexenol,4-keto-isophorone,heptanal,and(E)-2-hexenol)elicited negative responses.These results suggest that the chemicals can act as either attractants or repellents in locusts.Among the active volatiles,16 chemicals from host plants,non-host plants,and common volatiles were selected for subsequent T-maze tests.And statistics analysis indicated that 12 of these chemicals had significant effects on the biting response at the short range,and the three with no effects were common volatiles.?.Plant Odors Play Various Roles in Locust Choice Responses at Different Ranges.The locust behavioral choice assay was performed with the T-maze olfactometer at a concentration of 1%v/v chemicals.The results indicate that only the antennae of locusts support long-range olfaction detection.Only four common volatiles and one non-host odorant influenced choice at a distance of 3 m.When locusts were closer(1 m)to the odor,ten chemicals from various plant sources had either attractive or repellent effects on the insects.Besides,the ratios of no choice response for each odorant were also changed with distances.The results also indicated that the concentration of some common chemicals significantly influenced choice behavior.Furthermore,most of the tested aldehydes had no influence on behavior at a distance of 3 m but significantly affected biting behavior at a short range.All volatiles from host and non-host plant attracted fifth-instar nymphs at long distances(wheat,P=0.029;maize,P=0.0115;broad bean,P=0.0294;mulberry,P=0.001).Locusts may respond positively to the common volatiles of green leafy plants but cannot distinguish between host and non-host plants specifically over long distances.Furthermore,the locusts showed significantly different responses when the quantities of plant tissues and distances from source were changed,where the behavior changed from being attracted to being repelled with an increase in the quantity of non-host leaves.The response of locust at different distances suggests that insects might obtain more accurate olfactory information from plants the closer they get to them,first with their antennae and subsequently their palps,enabling them to perform specific responses.Results suggested that plant volatiles can generally attract locust nymphs in long distances,but the effects of odorants to locust are influenced by their contexts and concentrations.Antennae and palps on mouthpart have significant functional differences on detecting volatiles in distances.Locusts receive the plant odours that transported over long distances by antennae and make continuous decisions on approaching direction;when arriving very near to their host plants mouthpart palps would identify the odorants existed on foliage surface and make final decision for their food selection.Thus,the mechanism of locust food selection by olfaction can be divided into several continuous sections according to the sensibilities of two chemosensory organs and the characters of plant odorants.