Bacteria in oral secretions of an endophytic insect inhibit antagonistic fungi

1. Colonisation of host trees by an endophytic herbivore, the spruce beetle, Dendroctonus rufipennis, is accompanied by invasion of its galleries by a number of fungal species. Four of these associated species were identified as Leptographium abietinum, Aspergillus fumigatus, Aspergillus nomius, and Trichoderma harzianum. 2. Trichoderma and Aspergillus significantly reduced spruce beetle survival and reproduction in controlled assays. 3. A previously undescribed behaviour was observed, in which spruce beetle adults exuded oral secretions, especially within fungus-pervaded galleries. 4. These oral secretions inhibited the growth of fungi except A. nomius, and disrupted the morphology of the latter. Administration of these secretions indicated a dose-dependent inhibitory effect. 5. Oral secretions cultured on microbiological media yielded substantial bacterial growth. 6. Filter-sterilised secretions failed to inhibit fungal growth, evidence that the bacteria are responsible for the antifungal activity. 7. Nine bacterial isolates belonging to the Actinobacteria, Firmicutes, Gammaproteobacteria, and Betaproteobacteria taxa were obtained from the secretions. 8. Bacterial isolates showed species-specific inhibitory activity against the four fungi antagonistic to spruce beetle. The bacterium with the strongest fungal inhibition activity was the actinomycete Micrococcus luteus. 9. The production of bark beetle secretions containing bacteria that inhibit fungal growth is a novel finding. This suggests an additional level of complexity to ecological associations among bark beetles, conifers, and microorganisms, and an important adaptation for colonising subcortical tissue.