Host and gut microbiome modulate the antiparasitic activity of nectar metabolites in a bumblebee pollinator.

Antimicrobial nectar secondary metabolites can support pollinator health by preventing or reducing parasite infections. To better understand the outcome of nectar metabolite-parasite interactions in pollinators, we determined whether the antiparasitic activity was altered through chemical modification by the host or resident microbiome during gut passage. We investigated this interaction with linden ( spp.) and strawberry tree () nectar compounds Unedone from nectar inhibited the common bumblebee gut parasite and in gynes. A compound in nectar, 1-[4-(1-hydroxy-1-methylethyl)-1,3-cyclohexadiene-1-carboxylate]-6--β-d-glucopyranosyl-β-d-glucopyranose (tiliaside), showed no inhibition at naturally occurring concentrations but reduced infections of workers. Independent of microbiome status, tiliaside was deglycosylated during gut passage, thereby increasing its antiparasitic activity in the hindgut, the site of infections Conversely, unedone was first glycosylated in the midgut without influence of the microbiome to unedone-8--β-d-glucoside, rendering it inactive against , but subsequently deglycosylated by the microbiome in the hindgut, restoring its activity. We therefore show that conversion of nectar metabolites by either the host or the microbiome modulates antiparasitic activity of nectar metabolites. This article is part of the theme issue 'Natural processes influencing pollinator health: from chemistry to landscapes'.