Gustatory Characteristics Of Sensilla Chaetica In Antennae And Sensilla Styloconica In Proboscis Of Female Helicoverpa Armigera (H(u|..)bner) (Lepidoptera:Noctuidae)

Helicoverpa armigera(Hubner) (Lepidoptera:Noctuidae) is a polyphagous insect species and it has almost 200 host plant species. In nature, H. armigera is exposed to a complicated environment filled with the chemosensory information of plants. During the process of host-plant selection, gustatory chemoreception plays a crucial role especially after insects has landed on the plant surface. Historic evidences show that gustatory chemosensilla of insects mainly locate on the legs, antennae, mouthparts and ovipositor. H. armigera use their forelegs, antennae and proboscis to taste the plants when they land, then distinguish and estimate them, and ultimately make a decision:accept or reject. The characteristics of the chemosensilla on forelegs have been reported previously, but those on antenna and proboscis have not been studied systematically. Using morphological observation, tip-recording technique and behavioral bioassays, we studied the electrophysiological responses of antennal and proboscis chemosensilla to plant metabolites (sugars, amino acids and deterrents) and the effects of these compounds on feeding behavior in adult female H. armigera.1. Gustatory characteristics of sensilla chaetica in antennaeThe adult female of H. armigera has a pair of filiform antennae, and each is composed of a scape, a pedicel and a flagellum. The flagellum has about 80 segments sparsely packed with sensilla chaetica distributed on the ventral surface, lateral edges and dorsal surface of the flagellum. Total number of sensilla chaetica in each antennal segment varied among different antennal portions. Proximal segments showed significantly more sensilla chaetica than distal ones, while distal segments have the longer sensilla chaetica. Last antennal segment carries 7-8sensilla chaetica while each of other segments has 4-6 sensilla chaetica. Also, the sensilla on the distal part, especially the last segment, are more sensitive to sugars than those on other parts.The antennal sensilla chaetica were characterized through electrophysiological experiments by stimulating with sucrose, glucose, fructose, maltose, trehalose, myo-inositol, nine common amino acids, and four plant secondary substances including salicine, nicotine, sinigrin and strychnine. In the electrophysiological recordings, responses of sensilla chaetica to the compounds were first measured at 100 mmol/L, except for nicotine and strychnine which are tested at 10 mmol/L. Only sucrose, fructose and nicotine elicited significant electrophysiological responses. The lowest concentrations of sucrose, fructose and nicotine causing a reaction are all 1 mmol/L. The further recordings from sensilla chaetica in antennal tip part with mixtures of sucrose, fructose and nicotine show that the electrophysiological responses of the sensillum to sucrose and fructose are elicited by the same gustatory sensory neuron (GSN), while the responses to sucrose and nicotine are elicited by different GSNs. Meanwhile, the activities of sugar-sensitive GSNs to sucrose and fructose is inhibited by nicotine.In behavior experiments, the results show that both sucrose and fructose elicit strong proboscis extension responses (PER) and then induce feeding behavior. The PER responses are concentration-dependent. Sucrose causes the strongest behavioral responses. The electrophysiological and behavioral responses caused by a range of sucrose and fructose concentrations were positively correlated. The PER responses which are induced by sugar is inhibited by nicotine as well as the electrophysiological response. We conclude that nicotine is an important deterrent to the feeding of H. armigera and the antennal sensilla chaetica play an indispensable role in the process of nicotine perception.2. Gustatory characteristics of sensilla styloconica in proboscisThere are about 90 sensilla styloconica on proboscis. The sensilla distribute on one-third part of the proboscis and get shorter and shorter from the proximal to the distal gradually. Sucrose, fructose and glucose elicit significant responses of these sensilla, and the responses are concentration-dependent. Sucrose is the strongest. There is no significant difference in the sensitivity to sugars among sensilla on different parts. We also prove that the electrophysiological responses of the sensillum styloconica in proboscis to sucrose and glucose are elicited by the same GSN.The proboscis feeding rates of H. armigera to eight sugar solutions show that sucrose, glucose and fructose are feeding stimulants, and sucrose is the strongest. The results further prove the indispensable significance of these sugars for insects.