THE ROLES OF PLANT VOLATILES AND THEIR POTENTIAL USE AS FLY ATTRACTANTS

The essential oils composition of the skin, pulp and seeds from fruits of two Sicilian cultivars of Opuntia ficus-indica (cv. Sanguigna and cv. Surfarina has been obtainedby hydrodistillation and the possible semiochemical roles have been investigated comparing the data with those reported in the literature. Several compounds identified in this study have been reported to influence the behaviour of Ceratitis capitata, a phytophagous which causes severe damages to severalcrops including O. ficus-indica and the kairomonal activity of the odour of the fruits seems provided by a blend of compounds found in the various matrices analysed. In sapromyiophilous plants, up to date, long range attraction of fly pollinators has been thoroughly investigated and attributed to \u2018\u2018fetid\u2019\u2019 floral compounds, while the \u2018\u2018sweet\u2019\u2019 floral scent fraction has not been specifically investigated and its role has received little attention. The aim of the present study was to verify if terpenoids, which are the main compounds of the floral bouquet of Caralluma europaea, play a role in the attraction of its pollinator Musca domestica. Terpinolene, \u3b1-terpinene and linalool, described as the three main volatiles of the flowers of C. europaea, were evaluated in electrophysiological investigations and blends of these compounds as well as the whole fresh flowers were used in behavioural assays. Antennae of housefly adults showed positive dose-dependent responses to all the chemicals tested. Houseflies were attracted by the odour of the fresh flowers and by the reconstructed terpenoid blend at the dose of 100 \ub5g. At the dose of 10 \ub5g, the blend did not produce any attraction. The results of the present study support the hypothesis that terpinolene, \u3b1-terpinene and linalool emitted by C. europaea flowers are involved in pollinator attraction and demonstrate the importance of the \u2018\u2018sweet\u2019\u2019 scent in this sapromyiophilous species. Floral scent in sapromyiophilous plants often consists of complex blends with not only fetid (e.g., sulfides) but also sweet (e.g., terpenoids) volatile organic compounds. However, little is known about the number and identity of compounds involved in pollinator attraction in these deceptive plants that mimic breeding sites of the fly pollinators. In the present paper, we studied flower volatiles of sapromyiophilous Periploca laevigata and their capability to elicit biological responses in Musca domestica pollinators, to improve our understanding of the chemical ecology of this pollination system and generally the sapromyiophilous pollination syndrome. Floral volatiles were collected by dynamic headspace and analysed by gas chromatography-mass spectrometry (GC-MS), and electrophysiological (GC-EAD) and behavioural assays (two choice olfactometer) were conducted. In the floral scent of P. laevigata, we detected overall 29 compounds of which indole, \u3b2-caryophyllene, and germacrene D dominated the scent. These three compounds together with dimethyl trisulfide were not only electrophysiologically active in the antennae of the flies, but a synthetic blend of these compounds also was behaviourally active in M. domestica. Dimethyl trisulfide was present only in trace amounts in P. laevigata, but it elicited the most prominent response in the antennae of the housefly. Our results thus highlight the sensitiveness of the olfactory system of houseflies towards dimethyl trisulfide and suggest that this compound is very important in sapromyiophilous pollination systems even if it is often present only in trace amounts in floral scents. Based on our electroantennographic measurements we also conclude that only a limited number of the compounds emitted in sapromyiophilous flowers is biologically active in specific pollinators