Electrophysiological and olfactory activity of orange VOCs on Ceratitis capitata (Wiedemann) (Diptera: Tephritidae)

The Mediterranean fruit fly, Ceratitis capitata (Wiedemann) (Diptera: Tephritidae), is a pest of prominent economic importance. Control strategies, previously based on larval chemical control, are currently directed to adults (poisoned baits, mass trapping). Adult control methods need of a deeper knowledge of the factors involved in host finding behaviour. The aim of this work was to analyze volatile compounds (VOCs) emitted by orange fruits (cv. Valencia late) and to study their activity on the olfactory system and behaviour of C. capitata adults. Headspace odours were extracted by a continuous air stream flowing on ripe fruits. Volatiles were desorbed by eluting an activated-charcoal cartridge with 10ml hexane, then identified with a Gas Chromatograph-Mass Spectrometer. GC-MS analysis identified 26 volatile compounds in the orange headspace and the most abundant were valencene (66.6%), limonene (9.3%) and trans-caryophyllene (7.1%). Electrophysiological recordings (EAG) were conducted on C. capitata adults. Males and virgin and mated females antennae were stimulated by the orange extract and by the most abundant volatile compounds (1:10 v/V hexane solutions of R(+) limonene, purity 97%; S(-) limonene, purity 96%; valencene, purity 70%). EAG responses to orange extract were lower than to the pure volatile compounds. The higher EAG responses were obtained after stimulation with valencene. There was a significant difference in the responses of mated females to valencene, higher than the ones recorded by virgin females and males. EAG dose-response curves were also calculated using the same compounds at increasing concentration (from 0.01μg/μl to 100μg/μl). Recordings showed a clear dose-dependent EAG response. Preliminary behavioural studies were carried out in a Y-shaped wind-tunnel. After insect introduction, observations were made at 15 and 45 min. Air was blown into the tunnel at 0.1m/sec. The room was kept at 26.5°C, 47% RH and 1.700 lux. Virgin females were strongly oriented to the headspace extract after 15 min (p≤0.01) while mated females and males were not attracted. After 45 min, also virgin females became indifferent to the tested stimulus. R(+) limonene, valencene and a mixture of valencene/limonene (7.17:1) (from 0.1μg/μl to 100μg/μl) were not attractive to mated females, both at 15 and 45 min after the start of the test. Results evidence a different influence of the orange headspace extract on the behaviour of C. capitata adults, since it was attractive only to virgin females. Probably mated females need of crucial visual cues (i.e. fruit colour, shape and size). However, further behavioural studies based on the detected compounds of headspace extract and their mixtures are necessary to identify more active cues.