Seasonality, alarm pheromone and serotonin: insights on the neurobiology of honeybee defence from winter bees.

Honeybees maintain their colony throughout the cold winters, a strategy that enables them to make the most of early spring flowers. During this period, their activity is mostly limited to thermoregulation, while foraging and brood rearing are stopped. Less is known about seasonal changes to the essential task of defending the colony against intruders, which is regulated by the sting alarm pheromone. We studied the stinging responsiveness of winter bees exposed to this scent or a control (solvent). Surprisingly, winter bees, while maintaining their responsiveness in control conditions, did not increase stinging frequency in response to the alarm pheromone. This was not owing to the bees not perceiving the pheromone, as shown by calcium imaging of the antennal lobes. As the alarm pheromone is thought to act through an increase in brain serotonin levels, ultimately causing heightened defensiveness, we checked if serotonin treatments would affect the stinging behaviour of winter bees. Indeed, treated winter bees became more inclined to sting. Thus, we postulate that loss of responsiveness to the sting alarm pheromone is based on a partial or total disruption of the mechanism converting alarm pheromone perception into high serotonin levels in winter bees.