We evaluated whether Dendroctonus frontalis Zimmermann populations were influenced by nontrophic interactions involving commensal mites, their mutualistic bluestain fungus Ophiostoma minus (Hedgc.) H. and P. Sydow, and beetle-mutualistic mycangial fungi. We tested for effects of delayed, nonlinear, or positive feedback from O. minus and mites on D. frontalis population growth. We predicted that (1) high mite densities have demographic consequences for beetles by influencing the prevalence of O. minus and antagonistic interactions between O. minus and mycangial fungi, and (2) inter-relations and abundances of mites and fungi differentially vary throughout the year in a seasonally variable climate. Surveys of D. frontalis populations revealed that temporal and spatial patterns in abundance of mites and their mutualistic fungus, O. minus were inversely related with beetle population growth. Negative demographic effects of O. minus on D. frontalis were nonlinear, only affecting beetle per capita reproduction when fungi colonized >35% of phloem habitat. Mite abundance was strongly correlated with O. minus and was an important driving force in promoting bluestain prevalence within trees. Spring abundances of mites and the prevalence of O. minus during D. frontalis infestation formation were strong predictors of beetle population decline later that year. The two mutualistic fungi associated with D. frontalis cycled seasonally but did not seem to influence beetle population dynamics.