Neglected Microbes in Floral Nectar: Influence of Filamentous Fungi on Nectar Scent and Parasitoid Olfactory Responses.

Floral nectar is a sugar-rich resource which is ubiquitously inhabited by a wide array of microorganisms. Fermentation by nectar-inhabiting microbes can alter several nectar traits, including nectar scent, via changes in the blend of volatile organic compounds (VOCs). Although there is growing evidence on how yeasts and bacteria influence the foraging behavior of flower-visiting insects, the potential role of other microbial taxa that can colonize nectar has been largely neglected. In this study, we investigated how filamentous fungi isolated from the floral nectar of buckwheat, Fagopyrum esculentum, affect nectar scent and the olfactory responses of two co-occurring egg parasitoid species, Trissolcus basalis and Ooencyrtus telenomicida. Among nectar-feeding insects, adult parasitoids are common visitors of flowers as they depend on sugar-rich resources to satisfy their energetic and nutritional needs. In olfactometer assays, we found that nectar fermentation by two out of six fungal strains, namely Cladosporium sp. SAAF 22.2.11 and Cladosporium sp. SAAF 22.3.29, elicited a behavioral response in the egg parasitoid species when tested against non-fermented nectar. In particular, O. telenomicida displayed positive olfactory responses to both Cladosporium-fermented nectars, while T. basalis only responded positively to nectar fermented by Cladosporium sp. SAAF 22.2.11. Chemical investigations revealed significant differences in the VOC blends across all six fungus-fermented nectars, despite a partial overlap between the blends emitted by attractive versus non-attractive nectars. Altogether, these findings highlight previously unexplored interactions between flower-visiting insects and flower-associated microbes, broadening our understanding beyond the well-studied influences of yeasts and bacteria.