Environment and Pollen Diversity Differentially Affect the Gut Microbiomes of Introduced Honeybees and Bumblebees

Invasive species may exhibit shifts in their gut microbiome in response to novel environments and diet, but this may differ across host species and their time since colonisation. We investigate if site environmental variables and foraged pollen resources differentially shape the gut microbiomes of two bee species with contrasting introduction histories: The European honeybee, Apis mellifera (introduced 1831), and the recently invasive bumblebee, Bombus terrestris (invaded 1992). Using landscape-scale metabarcoding across the island state of Tasmania in Australia, we characterised gut bacteria (16S rRNA) and corbicular pollen diversity (ITS2) for each species. Gut bacterial composition was significantly associated with mean annual temperature for A. mellifera and with mean annual precipitation and percentage of pasture for B. terrestris. In B. terrestris, the core and facultative gut microbial diversity and richness showed associations with precipitation, foraged pollen diversity, wind velocity and temperature. Foraged pollen diversity of native plants more strongly predicted the facultative gut microbiome across species. Overall, the gut microbiome of B. terrestris showed a stronger response to abiotic and biotic predictors compared to A. mellifera. Our findings advance understanding of how environmental and dietary factors shape pollinator gut microbiomes at landscape scales, with implications for pollinator health and survival.