## 4 Discussion  
This report investigated the capability of an aphelinid parasitoid _A. melinus_ to respond to citrus fruit chemical cues associated with damage induced by a phytophagous pest. The results indicated that _A. melinus_ was able to discriminate between citrus fruit infested with _A. aurantii_ and non-infested fruit by using olfactory cues. Among 34 VOCs extracted and identified by SPME and GC-MS techniques, only two VOCs, b-limonene, and \(\beta\)-ocimene were increased in all infested three citrus species, which were already known as a constituent of the odours of citrus fruit and citrus oils [16; 20; 23]. However, since b-limonene and \(\beta\)-ocimene are attractive to many other parasitoid species [19; 20; 24], these two compounds can be considered as putative VOCs as attractants of _A. melinus_. PCA analysis has highlighted that VOC emissions are particular for each species and chemical. Fruits differentially emitted the VOCs after insect infestation and the VOCs change depending on the citrus species. Indeed, several plants react to herbivore damage by producing blends of metabolites that change in number or proportion [25]. The increasing in VOCs follow the infestation could be explained for all species, but we also identified some particular compounds emitted under the condition of _A. aurantii_ infestation. Besides b-limonene and \(\beta\)-ocimene, infested mandarin fruits displayed increased emissions of another three compounds (\(\gamma\)-terpinene, alloocimene and alloaromodendrene). Infested lecom fruits increased the emission of just b-limonene and \(\beta\)-ocimene and produced \(n\)-hexadecanoic acid. When attacked by _A. aurantii_, orange fruits similar to other species increased the production of b-limonene, \(\beta\)-ocimene, (\(3E\))-4,8-dimethyl-,1,3,7-nonatriene, hexyl caproate and alloaromodendrene, and exclusively produced five compounds (acetic acid, hexyl ester, alloocimene, \(\alpha\)-terpineol, nerolidol and (3E,7E)-4,8,12-trimethyltrideca-1,3,7,11-tetraene). Most of these compounds are known as common floral compounds, but interestingly some of them are also recognised as pheromones for several hypenopteran species [26]. Since the variance in VOCs depends mainly on the fruit species, we cannot exclude the possibility that other VOCs produced specifically by one species or increased in all species may act as an attractant toward _A. melinus_. Indeed, non-infested fruit also produce VOCs that are attractive to parasitoids [25], and aphelinid parasitoids, such as _A. melinus_ could be able to perceive cues from non-infested plants to locate a host [19; 20]. Therefore, short-range VOCs produced by the plants or as feeding larvae secretion could be useful for successful localisation [27]. Overall, since VOCs are known to act as attractive chemicals for several parasitoids [25; 28; 29], further research is needed to assess the activity of these compounds on parasitoid behaviour. Indeed, understanding tritrophic system communications has potential implications for biological control programs [30]. Olfactory cues from host-infested fruit are known to be essential for parasitoids during foraging for hosts, food and mates in complex environments [13; 31]. For _A. melinus_, the source of volatile cues that the wasp uses to locate the host species were determined when those hosts occurred on more than one host plant species [19]. The evidence that olfactory cues from infested fruit evoke behavioural responses from mated _A. melinus_ females, and the presence of chemicals that were produced exclusively or in higher amounts by infested citrus fruit, supported our hypothesis that VOCs could act as short-range