Many bark beetles are closely associated with fungi, but what drives bark beetle-fungal interactions? While the fungi gain transport to an otherwise inaccessible host tree, the benefit of the interaction to the beetle has been debated. One paradigm holds that the fungi help the beetle kill the tree; however, many fungal associates are nonpathogenic and many species of bark beetles only attack dead trees, suggesting that other factors may be involved in the relationship. Using the mountain pine beetle (Dendroctonus ponderosae) and its two filamentous fungal associates (Ophiostoma clavigerum and O. montium) as a model system, we tested the hypothesis that bark beetles gain nutritional benefits from some of their fungal associates. Surface sterilized eggs were reared to adulthood in phloem sandwiches with O. clavigerum, O. montium, both fungi, or neither fungus. O. montium may act as a dietary supplement, because beetles developing with O. montium or both fungi consumed significantly less phloem, but were not significantly smaller, than beetles developing with O. clavigerum or neither fungus. In addition, newly eclosed beetles were switched between treatments as well as within treatments (controls). Results indicate that O. montium conidia (asexual spores) may be an important dietary supplement for young adults during maturation feeding. The implications of a dietary supplement to this aggressive bark beetle that kills trees by attacking en masse are two-fold: a reduction in intraspecific competition at high population levels (outbreaks); and improved nutrition when beetles are in marginal host trees (endemic population levels, during or after sustained outbreaks).