Drinking dirty water: Why do honey bees (Apis mellifera) collect agricultural water and urban runoff?

Honey bees reportedly have a preference to drink brackish water, presumably because they prefer the higher salt concentrations found in such water. Such water may also contain pesticides or other harmful chemicals. Thus, determining how salt concentrations influence bee water foraging has implications for understanding their basic biology and how they may be exposed to harmful compounds. We used the Proboscis Extension Response (PER) to determine how bees responded to a wide range of salt concentrations (ranging from 0%-10%). Mean bee PER was significantly higher for 1.5% NaCl, 1.5% MgCl₂, and 0.4% and 0.75% Na2HPO₄ (phosphate). For KCl, bee PER was significantly higher for 0-1.5%. For MgCl₂, KCl, and Na₂HPO₄, bee PER was significantly lower for a 10% solution. Surprisingly, bees would tolerate a 10% NaCl solution, but exhibited significantly lower PER for the lowest concentration of 0.05%, similar to pure water. In addition, we analyzed water collected by bees from 36 sites in San Diego and Los Angeles Counties. There is a significant effect of water type on the concentration of Na, Mg, and K, but not on the concentration of phosphate. For Na, seawater (0.93%) and swimming pool water (0.6%) contained higher sodium concentrations than all other water sources. For Mg and K, only seawater had significantly elevated ion levels compared to all other water sources. This study provides a new technique, PER, to determine honey bee salt preferences and suggests future methods for applying salts at specific concentrations to deter bees from collecting agricultural runoff.