The pollen provisions of Osmia sp. host diverse microbes that provide life-sustaining benefits for their larvae. We examined the effects of altering, reducing, and removing these external symbionts on larval health using diet-manipulation experiments. Our study using O. ribifloris, an Ericaceae specialist, indicated that larvae obtained substantial nutrition from eating pollen-associated microbes, and the incremental removal of these exosymbionts caused a predictable decline in larval fitness. Next, we reared the specialist larvae either on host pollen or non-host pollen, with or without the respective pollen-associated microbiota. For larvae feeding on host pollen diets, the relative effect of microbes was substantially greater than that of pollen type. Despite having similar nutrient concentration, microbe-rich host pollen produced the fittest larvae, while microbe-deficient host pollen, the weakest. This implied that the ability of specialist larvae to exploit poorer quality host pollen likely arose from a partnership with their exosymbionts. To investigate if the magnitude of such microbe-derived benefits varied based on the identity of the microbes themselves, we reared generalist larvae only on microbe-rich pollen provisions, where the sources (conspecific versus heterospecific) of both the microbes and the pollen were simultaneously manipulated. The results indicated that both microbial and pollen sourcing significantly influenced larval performance, and the effect size of each was similar. Our findings imply that the presence of exosymbionts was vital for both specialists and generalists, but more notably, that the composition of the specific microbial community within provisions may be as critical as the composition of the pollen itself.