Specific Responses of Natural Enemies to Ips acuminatus Density in Endemic and Post-Outbreak Populations in Scots Pine Stands: A Case Study

Outbreaks of the sharp-toothed bark beetle, Ips acuminatus, have become increasingly frequent across Europe, yet little is known about how natural enemy communities vary between bark beetle population phases. This study compared the composition, diversity, emergence patterns, and relationships of natural enemies associated with I. acuminatus in two Scots pine (Pinus sylvestris) forest areas representing endemic and post-outbreak population conditions in Latvia. Bark beetle colonized stem sections were collected and maintained under controlled laboratory conditions to quantify emerging insects. Community composition and diversity were analysed, and linear mixed-effects models were used to evaluate relationships between bark beetle and natural enemy densities. A total of 5468 insects representing 21 taxa emerged, including 12 natural enemy taxa. Natural enemy community composition and Shannon diversity were similar between areas/populations. However, the relationship between densities of I. acuminatus and natural enemies differed significantly between the two areas (p = 0.002), with a substantially stronger positive relationship detected in the one with an endemic population. Under laboratory conditions, parasitoids emerged synchronously with the I. acuminatus, whereas predator emergence peaked later. These findings indicate that compositionally similar natural enemy assemblages can exhibit different density relationships between systems representing contrasting bark beetle population conditions, emphasizing the importance of considering abundance relationships alongside community composition.