## 1 Introduction  
Dehesas (_Quercus_-dominated open woodlands) are characterised by ecological and economic complexity, reflecting the biological and environmental diversity of Mediterranean agrobioosystems [1], and these systems have permitted the social development of certain historically impoverished areas. Cork oak, _Quercus suber_ L., is the dominant tree species in dehesas in western Spain [2]. Defoliating insects are an important component of forest ecosystems: they influence tree growth and have an impact on the nutrient cycle, providing food for upper trophic levels, while some species contribute to pollination [3]. Among the defoliators of the genus _Quercus_, there are 60 species of Lepidoptera (in 14 families, the majority Nocutiidae), one Hymenoptera (_Perclista andrei_ [PERSON]), and the genus _Cooliodes_ [PERSON] ([PERSON]) [4]. The effects of defoliators range from damage to tree flowering and foliation, diminishing fruit and wood production, to physiological changes [5]. Numerous defoliator species, such as _Catocala mymhagga_ ESEP, tend to persist at relatively low population densities, only causing marked damage occasionally in the event of population \"explosions\" [6, 7, 8]. On the other hand, some species tend to frequently cause substantial damage, and these include _Lymantria dispar_ L., _Tortrix viridana_ L., and the Hymenoptera species _P. [PERSON]_ in the western Mediterranean region [4; 8; 9; 10]. For example, sustained defoliation by _T. viridana_ over 2 years can reduce tree growth by as much as 70% [11], with an estimated yearly acorn loss due to this tortricid species of more than 500 kg ha\({}^{-1}\)[12]. Severe continuous defoliation is considered a trigger of tree decline, predisposing trees to attack by other organisms, such as pathogens or xylophagous insects [8; 13].  
Terpenes and other volatile organic compounds play a key role both in plant physiology and the interactions between plants and their environment; for example, protecting plants against stressors, acting as a mechanism for both intra- and inter-specific plant communication (e.g., releasing chemical signals after herbivore damage), and even attracting predators and herbivore parasitoids [14; 15]. Trees of the _Quercus_ genus are among the highest emitters of volatile organic compounds [16], with _Q. suber_ being considered a strong emitter of monoterpenes [17; 18]. Foliar compounds have various effects on defoliating insects. For example, they act as signals for intraspecific host selection or as repellents if emitted specifically by a plant affected by grazing [19]. It has been suggested that volatile compounds such as \(\alpha\)-pinene, \(\beta\)-pinene, and 1-hexanol may attract _L. dispar_ towards _Larix gmelini_ Rupr. [20] and that terpenid influence patterns of _L. dispar_ grazing on _Quercus_\(\acute{\text{e}}\)x L. [21].  
This study aimed to help improve our understanding of factors that determine the species composition of defoliating insects (Lepidoptera and/or Hymenoptera) on _Q. suber_, especially regarding the relationship between leaf-eating larvae (their presence as well as the leaf damage they cause) and foliar monoterpene chemotypes. The objective of this study was to investigate how the profile of foliar monoterpene emission from trees affects (i) the species composition and (ii) the field grazing activity of soft-leaf defoliating insects.