Bee swimming is adaptive but disrupted by insecticide

A unique type of locomotion was recently discovered that honey bees (Apis mellifera L.) could propel their own body on water surface by keeping their upper wing surface dry while moving their wings. However, it was not clear whether such locomotive behavior was ecologically meaningful. Here we show that honey bees preferred a dark region (skototaxis) while hydrofoiling on the water surface in trying to reach the edge. However, this skototaxis was disrupted by a neonicotinoid insecticide thiamethoxam, via reducing the honey bees' motor control. Finally, we show that mason bees (Osmia excavata) also displayed skototaxis, showing a significantly stronger preference for dark than honey bees. The female mason bees exhibited higher efficiency than males in swimming, as indicated by their shorter durations and distances. These findings suggest that swimming behavior in bees evolved before sociality and serves important adaptive and ecological functions. However, environmental pollution from excessive pesticide use may negatively impact this behavior.