The bacterial community of the European spruce bark beetle in space and time

The European spruce bark beetle Ips typographus is a pest causing severe damage in forests dominated by Norway spruce in Europe. Microorganisms play essential roles in bark beetles, including nutrition, fitness as well as in overcoming host defenses. Here, we performed high-throughput 16S rRNA metabarcoding of I. typographus across different populations in Europe, to assess its bacterial community. We investigated four postglacial refugial areas in Europe and focused specifically on a current bark beetle hot spot in the Dolomites where we compared populations with different epidemiological phases (outbreaking vs. non-outbreaking) and across different seasons (pre-overwintering vs. overwintering). Our results show that the bacterial community structure varied among populations from the refugial areas and between different regions within the Dolomites. We found a significant difference in the bacterial community between pre-overwintering and overwintering individuals, but we did not find differences between epidemic and endemic populations. The prevalence of the genera Erwinia (which was present in all individuals) and Pseudoxanthomonas (present in almost all individuals) across all populations suggests that these taxa form the core bacterial community of I. typographus. Furthermore, several additional bacterial taxa occurred in all populations, but with variable frequencies. This study highlights a complex interaction of I. typographus and various bacterial taxa across different regions and ecological phases and provides new insights into the role of microorganisms in the biology of this important pest species.