Pollen diversity activity on bee health
Résumé
Agricultural landscapes do not provide to pollinators a regular pollen resource. Pollen is essential for the honey bees development and is the main source of lipids. Some fatty acids are known for their antibacterial activity, and major lipid components of pollen contain some of them. We hypothesize that the pollen diversity could contribute to bee health when a balanced supply in fatty acids is supplied to bees. Our study used bee pollens harvested in a farming landscape at different times of a year. We performed palynological and physico-chemical analysis of these multi-floral samples. Some pollen foraged by the honey bees on crops, crop weeds or woody species naturally contained high rate of C18:2n-6 and C18:3n-3. The extract preparation had to be adjusted to solubilization properties of pollen components. We carried out antibiograms on Paenibacillus larvae cultures using total lipid extract or lipid fraction of these pollens in order to screen candidates for further trials. The antibacterial activity of total or fraction lipid extracts has been assessed by inhibition area in dishes. The effect was improved with the single saponifiable fraction, and some of the unsaponifiable components could limit it. Extracts were tested with Nosema in caged adult workers which were lipid-enhanced fed, compared with some synthetic fatty acids. C12:0 was efficient to protect adult bees against Nosema spores. Experiments on larvae reared in vitro exposed to Paenibacillus larvae were further conducted in the laboratory, including lipids in the artificial food. We modelled a method for testing pollen extracts using in vitro larvae rearing during pathogen exposure. Toxicity and protection activity of the samples was assessed. In conclusion, we suggest that some bee-harvested pollen can be particularly active in bee health and that the floral environment has to be considered in order to benefit from antimicrobial properties of some species.