Studies On The Plasticity Of Gustatory Perceptions And Feeding Behaviors In Larvae Of Helicoverpa Armigera (Hübner)

Helicoverpa armigera (Hubner) (Lepidoptera:Noctuidae) distributes widely in our country and is an important typical polyphagous pest, the main host plants of H. armigera are maize, cotton, tomato, tobacco etc. Sugar is one of the most important source of energy in insects'life, and also is an important signal substances of inducing feeding behaviors. Insects distinguish sugars by sugar neurons distributed on proboscis, maxillae, hypopharynx, epipharnx, pharynx, tarsus and ovipositor. What functions of gustatory receptors neurons in the process of insrcts food selection and hosts evolution? In the present study, the plasticity of feeding behaviors and gustatory perception of H. armigera caterpillars with different experienced diets were studied. Furthermore, the pathways and projection positions of taste neurons from gustatory sensillum in maxillae to the central nervous system were indentified, for the aim to explainning the neural basis of gustatory perception and feeding behavior. The main results are as follows:Gustatory electrophysiological responses of the medial sensellum to sweet substances:The gustatory electrophysiological responses of larval medial sensellum of H. armigera to different kinds of sugars and inositol using tip-recording method. The results showed that:(1) obvious responses were observed in the processes of medial sensillum to D-inositol, D-fructose, xylose, D-arabinose, D-glucose, maltose, D-trehalose and L-rhamnose. But the responsing patterns between to sugars and to inositol were different with a concentration-dependent responses in inositol and non-concentration-dependent responseses in sugars; (2) the differences of responsing patterns between the pure sugar compound and the mixture of fructose, arabinose and trehalose were not significant, indicating the same cell were induced by the 3 sugars and mixtures; (3) responses of the mixtures of fructose and inositol indicated that inositol-sensitive cell responsed to fructose, too. The firing rate of mixture of fructose and inositol were higher than that of the two pure compounds, suggesting the synergistic interaction of responses of the two stimulus. All the results suggested that the inisitol-sensitive cell in medial sensillum of the polyphagous species of H. armigera may be one of "broad-spectrum" taste cell.Diet-induced plasticity in gustatory electrophysiological responses and feeding behaviors of H. armigera: 1. In the behavioural assay by using dual-choice leafdisc tests, caterpillars preferred to feed on sucrose-treated discs whatever the experienced diets are, The results showed that:(1) caterpillars with sucrose-diet feeding experience or not preferred to feed on sucrose-treated discs; but caterpillars with sucrose-diet experience were of not significant differences in the area consumed between sucrose-treated discs and control discs, while caterpillars with non-sucrose diet experience include wild polulation of larvae preferred to feed on sucrose-treated discs more significantly than to feed on control discs, indicating that caterpillars reared on sucrose-diet were significantly less sensitive to sucrose in behaviours; (2) after several generations exposured to sucrose diet, then short elimination sucrose-diet couldn't restore the positive feeding on sucrose leaf-disc significantly; (3) after one generation of feeding experience with sucrose-diet, wild population of larvae didn't feed significanly on sucrose leaf-disc any more. The above results suggested sucrose-diet experiences can affect significantly the subsequently feeding bahaviours of caterpillars on sucrose-treated discs.2. Results of gustatory electrophysiological responses of the lateral sensellum showed that:(1) sucrose cell was in the lateral sensellum of maxillae. Sucrose-diet chronic experiences induced the gradual desensitization of the sucrose cell in the lateral sensellum to sucrose. Sucrose cell in caterpillars with sucrose diet experiences were gradual less sensitive to sucrose with non response to sucrose after 8 generations any more. (2) sucrose cell in caterpillars which had desensitized to sucrose can rapidly recover the sensitivities to sucrose after feeding on non-sucrose diets for only one generation. (3) results above suggested, caterpillars with exposured to sucrose diet chronically for at least 4 several generations desensitized the sucrose cell to sucrose, but the sensitivity of sucrose cell could recover as long as caterpillars isolate from sucrose diet for several days. The mutiplex results show sucrose palyed an important role in feeding processes of H. armigera. From the opinion of evolution, the results suggests the sensitivity of sugar cell to sucrose was stable and important in the processes of host evolution.Neuronal projectional pathways and projections positions of H. armigera gustatory neurons:The neuronal pathways and projections positions of gustatory neurons of sensilla styloconica of the H. armigera were identified using biotin tracer analysis. Results showed that:(1) the neuronal pathways shows that the gustatory neurons axons in the peripheral sensellum entered the anterior and lateral suboesophageal ganglion via the maxillary nerves, and then entered the tritocerebrum via circumoesophageal connective; (2) the projections position of the periphery taste neurons in CNS shows that suboesophageal ganglion and tritocerebrum are the most important projectonary positons. Investigations in neuron projections can contribute to the understanding of the neural basis of regulation feeding behavior in insects. In the other hand, the plasticity in taste cell and feeding bahaviour may be the results from the combined influences of physiological changes in central nervous systems and peripheral chemosensory desensitization of neural cells.