We describe invasion of the galleries of an endophytic herbivore, spruce beetle, Dendroctonus rufipennis, by antagonistic fungi, and an ensuing behavioral response consisting of oral secretions detrimental to these fungi. The antagonistic fungi were identified as Aspergillus fumigatus, A. nomius and Trichoderma harzianum. They significantly reduced spruce beetle survival and reproduction. We also identified antagonistic interactions between spruce beetles and their predominant ophiostomatoid fungal associate, Leptographium abietinum. Spruce beetle oral secretions inhibited the growth of all fungi, except A. nomius, in a dose-dependent fashion. Isolations from these secretions yielded substantial bacterial growth, and filter-sterilized secretions failed to inhibit fungal growth, evidence that the bacteria are responsible for the antifungal activity. The production of bark beetle secretions containing bacteria inhibiting 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 colonizing subcortical tissue.