Photoperiod and Temperature as Seasonal Cues for the Initiation of Brood Rearing in Honeybees.

The timing of seasonal life cycle events in many organisms is regulated by environmental cues, and understanding these relationships is essential for predicting species' responses to climate change. In honeybee colonies, brood rearing must align with floral resource availability to ensure colony growth and survival. However, the cues that initiate brood rearing remain unclear. While temperature is hypothesized to be influential, the role of photoperiod is not well understood. We used field experiment data in 2021 and 2022 from 95 colonies across eight European countries to examine how temperature and photoperiod interact to regulate the initiation of brood rearing in early spring. We found that both cues are important: longer day lengths and higher temperatures were associated with a higher probability of brood rearing, with temperature having a stronger effect under shorter day lengths and diminishing as day length increased. Notably, the strong positive effect of photoperiod suggests it may play a more prominent role than previously recognized. This interaction underscores the complexity of phenological regulation in honeybees. As global temperatures rise and seasonal patterns shift, colonies may struggle to synchronize brood rearing with floral resources. Our findings suggest that incorporating multiple environmental cues and species-specific sensitivities may improve predictions of climate-driven shifts in plant-pollinator interactions.