Floral nectar is ubiquitously colonized by a variety of microorganisms among which yeasts and bacteria are the most common. Microorganisms inhabiting floral nectar can alter several nectar traits, including nectar odor by producing microbial volatile organic compounds (mVOCs). Evidence showing that mVOCs can affect the foraging behavior of insect pollinators is increasing in the literature, whereas the role of mVOCs in altering the foraging behavior of third-trophic level organisms such as insect parasitoids is largely overlooked. Parasitoids are frequent visitors of flowers and are well known to feed on nectar. In this study, we isolated bacteria inhabiting floral nectar of buckwheat,Fagopyrum esculentum(Polygonales: Polygonaceae), to test the hypothesis that nectar bacteria affect the foraging behavior of the egg parasitoidTrissolcus basalis(Hymenoptera: Scelionidae) via changes in odors of nectar. In behavioral assays, we found thatT.basaliswasps are attracted toward nectar fermented by 4 out of the 14 bacterial strains isolated, which belong toStaphylococcus epidermidis,Terrabacillus saccharophilus(both Firmicutes),Pantoeasp. (Proteobacteria), andCurtobacteriumsp. (Actinobacteria). Results of chemical investigations revealed significant differences in the volatile blend composition of nectars fermented by the bacterial isolates. Our results indicate that nectar-inhabiting bacteria play an important role in the interactions between flowering plants and foraging parasitoids. These results are also relevant from an applied perspective as flowering resources, such as buckwheat, are largely used in agriculture to promote conservation biological control of insect pests.Supplementary InformationThe online version contains supplementary material available at 10.1007/s00248-022-02078-6.