Characterization of fungal communities associated with the bark beetle Ips typographus varies depending on detection method, location, and beetle population levels

The Eurasian spruce bark beetle Ips typographus and their fungal associates can cause severe damage to Norway spruce forests. In this paper, by using both molecular and cultural methods, we compared fungal assemblages on bark beetles from different locations, characterized by different beetle population levels. Ips typographus was trapped in the western Alps in two outbreak and in two control areas. Sequencing of clone libraries of Internal Transcribed Spacer (ITS) identified 31 fungal Operational Taxonomic Units (OTUs), while fungal isolations yielded 55 OTUs. Only three OTUs were detected by both molecular and cultural methods indicating that both methods are necessary to adequately describe fungal richness. Fungal assemblages on insects from these four and from an additional 12 study sites differed among stands in response to varying ecological conditions and to the limited spreading ability of I. typographus. Ophiostomatoid fungi showed higher diversity in outbreak areas; the pathogenic Ophiostoma polonicum was relatively uncommon, while O. bicolor was the most abundant species. This result was not unexpected, as insects were trapped not at the peak but at the end of the outbreaks and supports the hypothesis of a temporal succession among Ophiostoma species. Ubiquitous endophytes of trees or common airborne fungi were present both in outbreak and in control areas. Wood decaying basidiomycetes were almost never detected on beetles. Yeasts were detected only by molecular analysis, and the OTUs detected matched those reported elsewhere in Europe and in the world, suggesting a very long association between some yeasts and bark beetles.