## 1 Introduction  
The olive (_Olea europaea_ L.) is today an international agricultural crop that has been cultivated for thousands of years for its drupes and the production of oil. Although this tree is known for its hardness and ability to tolerate sub-optimal growing conditions (e.g., poor soils, drought, and high temperatures), its yield is frequently reduced by the olive fruit fly _Bactrocera oleae_ ([PERSON], 1790) (Diptera: Tephritidae), which is considered the key pest of olive worldwide [1]. The olive fruit fly is a small insect (approximately 4-6 mm in length) that lays its eggs in the olive fruit. The feeding tunnel excavated by the hatched larva causes fruit spoilage and premature drop due to direct (mechanical damage, subtraction of material) and indirect (opportunistic pathogens) effects [1; 2; 3]. The life cycle of the insect is completed in approximately 4-6 weeks, strongly depending on the climate conditions and fruit availability [4; 5]. The interaction between the olive fruit fly and the olive tree is complex [1; 6]. The insect is a specialist pest, with a very narrow host range. It feeds largely on olives and, to a lesser extent, wild olives and some other species of the genus _Olea_[1]. _B. oleae_ has coevolved with the olive tree, developing specific adaptations that allow it to use the fruit resources and overcome most of the plant defense, as it is one of the very few insects that is able to feed on the olive pulp [7; 8]. The olive has developed a range of direct and indirect defense mechanisms that help both to reduce pest damage on the fruit and deter the adult fly. One of the most important defense mechanisms is the production of secondary metabolites such as phenolic compounds (e.g., oleuropein, hydroxytrosol, and tyrosol), terpen (e.g., alpha- and beta-pinene, limonene, and linalool), and a range of volatile organic compounds (VCCs) [9; 10; 11]. These chemicalshave been shown to significantly impact plant-pest interaction and are likely to control the behavior of the olive fruit fly by deterring the females from laying eggs on the fruit, hampering larval feeding and reducing the attractiveness of the drupe [11; 12; 13]. In addition to chemical defenses, the olive tree also has evolved physical defenses [14]. These include, for instance, the thickness and elasticity of the fruit skin, which can hamper the sting of the fruit and subsequent oviposition [14]. Moreover, the olive fruit fly is also influenced by the skin color and pulp texture [15; 16; 17]. It is generally thought that Tephritidae are particularly attracted to mature fruits that have a softer texture and, according to the variety and species, also a darker color [18; 19]. The olive germplasm has ample diversity because of the long history of cultivation and selection, the ability of the plant to survive without cultural practices, the diffusion of local germplasm, and a slow varietal renewal in the widespread traditional, low-input farming systems [20]. Olive varieties can differ in the level of susceptibility to _B. oleae_[6; 21; 22; 23]. The molecular response of the drupe to the fruit fly involves a number of functions and biological processes [9; 21; 24; 25], and it is likely that various chemical and physical factors, along with their interactions with the environment, are the base of the variation in resistance in the olive germplasm. For instance, different susceptibility levels in olive varieties have been associated with the physical and/or chemical characteristics of the fruits [16; 17; 23; 26; 27]. The intimate relation between _O. europaea_ and its specialized pest _B. oleae_ implies a coevolutionary process that should be characterized by reciprocal changes in morphological characteristics over a long period of time. This process should have generated an intimate adaptation and counter-adaptation that allow both parties to better exploit or defend against each other [28].