Peeking under the bark: Within-tree densities of the spruce bark beetle (<em>Ips typographus</em>) and its enemies in forest conservation areas

The European spruce bark beetle Ips typographus is a major forest pest in Europe, with outbreaks intensifying due to climate change-induced weather events. In this study, we evaluated within-tree densities of the spruce bark beetle and its natural enemies during an outbreak in northern Europe. Densities were compared across different forest management types: managed forests and two conservation areas (woodland key habitats and nature reserves). We assessed beetle colonization density, production, reproductive success, predator abundance, and forest characteristics through bark sample analysis during four years. Our results indicate that spruce bark beetle densities were positively associated with surrounding tree mortality, with no significant differences in within-tree densities across forest types. Instead, variations in density were more pronounced across different outbreak phases and forest conditions, derived from remote sensing data. Enemy abundance, particularly Plegaderus spp. and two fungivorous insect groups, were generally higher in nature reserves compared to managed forests. We conclude that complex interactions between forest conditions, outbreak phases, and the occurrence of some specific predators play more crucial roles in bark beetle dynamics than whether a forest is a conservation area or not. For example, in wetter forests, more beetles are needed to overcome defences, while in deciduous forests, non-host volatiles may inhibit pheromones. Specifically, we highlight the less known clown beetle Plegaderus spp. as an important predator, negatively affecting bark beetle reproductive success. These findings emphasize the importance of local forest characteristics in managing bark beetle infestations, suggesting that outbreak risk assessments should focus on these variables rather than on forest conservation status, though potential variations at larger scales are beyond the scope of our study. Methods The study was conducted by annual surveys during the autumns (Oct-Nov) between 2020 and 2023 in Southern Sweden (Fig.1). We collected 222 bark samples from M (N=71), WKH (N=72) and NR (N=79), typically in spatial triplets (blocks). In total we used 49 sites in this study (M=16, WKH=15, NR=18). For the existing forest cover around our sampled locations, we utilized data from the National Land Cover Database (Swedish Environmental Protection Agency 2020). The database categorizes Sweden into various land cover classes with a 10-meter resolution. Mean soil moisture was extracted from a national-scale raster mapping (ranging from 0 to 100, dry to wet, 10-meter resolution), based on digital terrain indices and ancillary environmental information (Ågren et al. 2021). However, values exceeding 98 were excluded as they commonly indicate open fens or bogs and, thus, are non-forest areas.