Ratios rather than concentrations of nutritionally important elements may shape honey bee preferences for ‘dirty water’

1. Honey bees require minerals for a complete diet. However, minerals from flowers can be inadequate in concentration and composition. Therefore, honey bees may drink ‘dirty water’ from natural sources such as puddles. Some research has attempted to simulate this through honey bee bioassays, but to date, these have tested minerals individually, not as mixtures as would occur in nature. Here, for the first time, we use honey bees in bioassays in which a range of mineral mixtures are presented together in choice experiments. 2. Six minerals (NaCl, KCl, CaCl2, MgCl2, NH4Cl, and KH2PO4) were used in mixtures to simulate different mineral stoichiometries, which may occur in ‘dirty water’, such as puddles, from which honey bees often drink. Based on the honey bee mineral tolerance ranges from the literature, these mixtures were offered in aqueous solutions at low, medium, high, and mixed molar concentrations. Deionised water and sucrose were neutral and positive controls, respectively. Petri dishes were set up in containers in a laboratory. Twenty worker honey bees (Apis mellifera L.) were placed into each container and observed for drinking behaviour for 1 h. 3. Honey bees preferred the mixed molar treatment comprising a high Na:K ratio, a medium molarity of NaCl and a low molarity of the other minerals. This novel finding suggests that mixed mineral ‘dirty water’ should be investigated on a larger scale with multiple hives in the field and highlights the importance of stoichiometrically balanced honey bee diets. Honey bees (Apis mellifera L.) exhibited a significant preference for a particular mixed molarity of minerals in an aqueous stoichiometric solution compared to water and sucrose. This honey bee foraging preference for ‘dirty water’ was predominantly driven by the Na:K ratio rather than specific concentrations of any element. This may help explain honeybees' preference for ‘dirty water’. These results can inform the design of ‘dirty water’ delivery systems to enhance honeybee fitness.