## 1 Introduction  
The Eurasian Spruce Bark Beetle _Ips typographus_ (L. 1758) (Coleoptera: Scolytinae) is the main pest of Norway spruce, _Pica abies_ (L.) Karst. forests in the Central European region. Over the past decade, the combination of ongoing climate change, economically driven silvicultural practices, and the presence of spruce stands in areas beyond their natural range have weakened the natural defense mechanisms of trees and resulted in the occurrence of severe bark beetle spreading [1]. In the Czech Republic, outbreaks started after severe drought events in 2015 and 2018 [2] and led to an exponential increase in salvage logging volume from 2017 to 2020, with the volume rising from approximately 5.9 million m\({}^{3}\) in 2017 to 26.2 million m\({}^{3}\) in 2020 [3,4] (Figure S1). The initial step in managing a bark beetle outbreak is early detection of newly infested trees to enable timely salvage before the emergence of offspring [4]. Forest keepers typically rely on traditional methods, which involve personally observing the boring dust produced by infesting beetles [5]. However, during bark beetle outbreaks in large, forested areas, this approach has severe limitations, often resulting in the exponential spread of the beetles. Hence, an alternative method for early detection on a large scale is needed. Remote sensing techniques have been extensively investigated, using the detection of different indicators from spectral features to temperature [6] and recently also involving chemical substances (Sentinel SP) [7]. Recent research by [8] has proposed measuring the emission of volatile organic compounds (VOCs) from infested spruce as an indicator of bark beetle attacks. Furthermore, various methods for detecting these VOCs at different developmental stages have been introduced. Current research is investigating the utilization of an electronic nose equipped with nonspecific sensors for VOC detection [9]. Likewise, nonspecific metal oxide sensors have been mounted on UAVs to assess the concentration of \(\alpha\)-pinene in forest environments [10]. Notably, natural olfaction systems of dogs trained to detect bark beetle pheromones have proven more effective in finding infested trees in cooperation with their dog handler compared to human experts only [11; 12]. The VOC emitted by conifer mainly consists of hemi- and monoterpenes, which are produced as defense secondary metabolites. The conifer's immediate defense mechanism against wood-boring insects is the exudation of constitutive resin, which has a toxic and immobilizing effect on beetles [13]. In the later stages of a bark beetle attack, the production of resin is induced in the newly formed resin ducts in the phloem, xylem, and bark [14]. The resin is a mixture of terpenic compounds. The monoterpenes are volatile and form the main content of VOCs emitted by conifers. In spruce, \(\alpha\)-pinene, \(\beta\)-pinene, \(\Delta\)-carene, limonene, \(\beta\)-phellandrene, camphene, and myrcene dominate [15] but resin also contains sesquiterpen and a high content of highly viscous diterpenes [16; 17]. Oxidized forms of all terpen are also present. In addition to resin emissions from the stem, volatile terpen are also emitted from the needles in the canopy of conifers [18; 19]. The emission rate of volatile terpen from healthy trees is influenced by various macro- and microclimatic conditions, such as temperature and humidity [17; 20]. Different temperatures, and consequently varying VOC emissions, are observed in clearings and forest edges within fragmented forests [17; 21]. Furthermore, VOC emissions in conifer forests exhibit vertical variations [22] and follow a diurnal rhythm dependent on tree physiological processes [19].