Sap beetles (Coleoptera: Nitidulidae) are commonly associated with ophiostomatoid fungi, especially those belonging to the family Ceratocystidaceae. This coexistence of insects and fungi, usually on tree wounds, offers the ophiostomatoid fungi an effective means of dispersal. The selective advantage of this association to sap beetles is, however, confounded by the versatile life history strategies of these insects. In this study, we complemented field observations with rearing and feeding behavior experiments in the laboratory, to investigate the symbiology of interactions between sap beetles and co-occurring fungi, from the insect perspective. We determined that all predominant sap beetle vectors of the Ceratocystidaceae in a natural woodland feed on and use the mycelial mats of these fungi to nurse their offspring in tree wounds. When reared on fungal cultures in the laboratory, several of these insects successfully completed their life cycle. We were able to maintain Carpophilus hemipterus on this exclusive fungal diet over several generations. The feeding preference of this insect was generally consistent with the patterns of its fungal associations in the field as previously reported. There also appeared to be a correlation between the attractiveness of Ca. hemipterus to various fungi and its fitness benefits from feeding on these fungi. Overall, our results suggest that, from their partnership with ophiostomatoid fungi, sap beetles benefit from essential nutritional supplementation, enabling them to survive in saproxyly in woodland ecosystems.