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Flowering plants can be introduced in modern agroecosystems to support resident natural enemies in the context of Conservation Biological Control (CBC). Buckwheat (Fagopyrum esculentum) (Polygonales: Polygonaceae) has been shown to enhance the longevity of several parasitoids through the provision of high quality and easily accessible floral nectar. Yet floral nectar is ubiquitously colonized by microbes which can change nectar chemistry with consequences for parasitoids. Nonetheless, how bacteria associated with buckwheat floral nectar affect parasitoid performance is not known. In this study, adult females of Trissolcus basalis (Hymenoptera: Scelionidae) and Ooencyrtus telenomicida (Hymenoptera: Encyrtidae), two parasitoids of Nezara viridula (Hemiptera: Pentatomidae), were provided with synthetic nectar fermented by 14 bacterial isolates originating from buckwheat nectar. We recorded the effect of bacterial fermentation on female longevity and nectar chemistry. In the case of T. basalis, females consuming nectar fermented by Bacillus sp., Brevibacillus sp., Brevibacterium frigoritolerans, Saccharibacillus endophyticus, and Terribacillus saccharophilus significantly enhanced their longevity compared with females fed with non‐fermented nectar. For O. telenomicida, enhanced longevity was recorded only in the case of B. frigoritolerans and Pantoea dispersa. For both parasitoids, no negative effects due to bacterial fermentation of nectar were recorded. Chemical investigations of bacteria‐fermented nectars revealed an increased diversity in the composition of sugars and sugar alcohols, whereas non‐fermented nectar only contained sucrose. Our findings show that nectar‐inhabiting bacteria are important “hidden players” in the interactions between flowers and parasitoids, an indication that a better understanding of plant–microbe–insect interactions could improve CBC programmes. [ABSTRACT FROM AUTHOR]