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

Abstract: The Mediterranean fruit fly, Ceratitis capitata (Wiedemann) (Diptera: Tephritidae), is apest of prominent economic importance. Control strategies, previously based on larval chemicalcontrol, are currently directed to adults (poisoned baits, mass trapping). Adult control methodsneed of a deeper knowledge of the factors involved in host finding behaviour. The aim of thiswork was to analyze volatile compounds (VOCs) emitted by orange fruits (cv. Valencia late) andto 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. Volatileswere desorbed by eluting an activated-charcoal cartridge with 10ml hexane, then identified with aGas Chromatograph-Mass Spectrometer. GC-MS analysis identified 26 volatile compounds inthe orange headspace and the most abundant were valencene (66.6%), limonene (9.3%) andtrans-caryophyllene (7.1%).Electrophysiological recordings (EAG) were conducted on C. capitata adults. Males andvirgin and mated females antennae were stimulated by the orange extract and by the mostabundant 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 thanto the pure volatile compounds. The higher EAG responses were obtained after stimulation withvalencene. 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 alsocalculated 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 insectintroduction, observations were made at 15 and 45 min. Air was blown into the tunnel at0.1m/sec. The room was kept at 26.5°C, 47% RH and 1.700 lux. Virgin females were stronglyoriented to the headspace extract after 15 min (p ≤ 0.01) while mated females and males were notattracted. 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 ofC. capitata adults, since it was attractive only to virgin females. Probably mated females need ofcrucial visual cues (i.e. fruit colour, shape and size). However, further behavioural studies basedon the detected compounds of headspace extract and their mixtures are necessary to identify moreactive cues.