Beyond pesticides: Evaluating the role of botanical origin and nutritional composition in shaping honey bee stress responses

The western honey bee (Apis mellifera L.) is increasingly affected by chronic dietary exposure to pesticide-contaminated pollen. This study investigates the long-term effects of Malus domestica, Phacelia tanacetifolia, and Taraxacum sp. pollen collected from orchard and alpine habitats alongside a commercial feed additive (Promotor-L Apis) on honey bee survival, physiology, and gut microbiota. Multiresidue analysis revealed distinct pesticide and heavy metal profiles across pollens, while compositional analyses showed variation in amino acids, flavonoids, and phenolamides. Despite high contamination, Malus pollen with elevated flavonoid content promoted the highest vitellogenin accumulation and did not totally inhibit survival. Phacelia pollen from organic vineyards, though low in pesticides, had high copper levels and showed high mortality. Taraxacum pollen from apple orchards, though moderately contaminated, supported high survival. Unexpectedly, alpine Taraxacum pollen with elevated histidine content caused the highest mortality and microbial disruption, despite no pesticide residues. Promotor-L improved survival but did not increase vitellogenin. Pollen-fed bees generally exhibited higher gut microbiota abundance, while pathogen levels (including Nosema ceranae and Serratia marcescens) were specifically elevated under Taraxacum-based diets. These findings highlight that the impact of pollen nutrition on bee health is multifactorial, governed not only by pesticide exposure but also by botanical origin, nutritional traits, and secondary metabolites.