## 1 Introduction  
The citrus industry is considered one of the largest fresh fruit industries in Australia, as well as the largest fresh fruit reporter, with an annual average export volume of 170,000 tonnes and a value of AUS$190 million [1]. _Aonidiella aurantii_ ([PERSON]) (Hemiptera: Diuspididae), is a major insect pest of citrus crops, causing significant production losses [2]. _A. aurantii_ prefers to attack citrus, and it is one of the most important citrus pests worldwide [3]. _A. aurantii_ attacks all citrus species, but there are varying levels of susceptibility, so the order of vulnerability in descending is lemon trees (_Citrus limon_ ([PERSON]), graprefruit (_C. xparadis_ [PERSON]), orange (_C. sinensis_ ([PERSON] [PERSON]) and two different species of mandarin (_C. reticulata_ [PERSON] and _C. unshiu_ [PERSON]) [4]. California red scale _A. aurantii_ attacks all aerial parts of the citrus tree including fruits, leaves and branches and twigs [5]. However, the fruit is the preferred plant substrate for _A. aurantii_, followed by leaves, while branches and twigs are the least preferred substrate [6]. This is economically important as the presence of this scale on fruit considerably reduces their market value, causing huge economic losses [7]. Chemical control of _A. aurantii_ is difficult and is frequently followed by reinfestation within a short period, resistance to different products used for its control, and the risk of eliminating natural enemies present in the field [8]. Plant VOCs can serve as semiochemicals to protect plants from insect and pathogen attack, attract beneficial animals, and as communication signals within and among plants [9]. Terpenoids and other VOCs emitted from plants in response to insect attack allow parasitoids and predators to distinguish between infested and non-infested plants and thus aid in locating hosts or prey [10]. In addition, the herbivore-induced plant volatiles (HIPVs) vary quantitatively and qualitatively related to various biotic and abiotic factors (e.g., number of infesting pests, period and duration of infestation and previous infestation) and the VOCs are specific to each plant-pest association [11]. It is well-known that insect feeding activity leads to biochemical changes in plants. Plants respond to the presence of pests by activating their defence system [12], but they can also trigger indirect defences, such as the emission of HIPVs [13]. The role of VOCs on host location by natural enemies has been widely investigated [14; 15]. Many parasitoids of phytophagous insects orient to VOCs, using them as VOCs to guide their search for hosts [11; 13] and differential emissions have been reported for _A. aurantii_-infested and non-infested fruits [16]. The aphelinid _A. melinus_ is a primary ectoparasitoid of armoured scale insects. It is the most commonly used biocontrol agent for California red scale _A. aurantii_ across the world, through augmentative releases [17; 18]. _A. melinus_ females rely on stimuli to successfully locate their host. _Aphytis_ use learned volatile cues from host plants as long-range attractants to potential habitats of their hosts [19; 20]. Previous work with _A. melinus_ and California red scale showed that wasps were attracted to the odours of scale-infested lemon fruit [21]. In this study, we investigated the olfactory cues used by _A. melinus_ females to locate their host microhabitat. We hypothesise that the VOCs from _A. aurantii_ infested citrus fruit may play a pivotal role in affecting _A. melinus_ host location. Three different citrus fruit, lemon fruit, orange fruit and mandarin fruit, were tested to determine parasitoid attractiveness and VOC emissions. First, the VOCs emitted from non-infested and infested citrus fruits were extracted with SPME fibre and analysed by gas chromatography-mass spectrometry to determine whether that VOCs are attributable to herbivores' activity and determine possible VOCs. Subsequently, we evaluated female preferences for infested or non-infested fruit in olfactometer bioassays.