Peeking under the bark: Within-tree densities of the spruce bark beetle (Ips typographus) 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 infestation 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. • The role of conservation areas in bark beetle dynamics and its enemies is unclear. • Within-tree densities did not differ between conservation areas and managed forests. • Enemies and some specific predators had higher abundances in nature reserves. • One example is the clown beetle that potentially can reduce bark-beetle populations.