The carry-over effects of pollen shortage decrease the survival of honeybee colonies in farmlands.
Many studies have reported honeybee colony losses in human-dominated landscapes. While bee floral food resources have been drastically reduced over past decades in human-dominated landscapes, no field study has yet been undertaken to determine whether there is a carry-over effect between seasonal disruption in floral resource availability and high colony losses. We investigated if a decline in the harvest of pollen by honeybees in spring affected managed honeybee colony dynamics (brood size, adult population and honey reserves) and health (Varroa mite loads and colony survival) throughout the beekeeping season. A decline in pollen harvest was associated with a direct reduction in brood production, leading to a negative effect on the adult population size later in the season, and lower honey reserves before the onset of winter. Furthermore, the decline in pollen harvest negatively impacted the health of the colony, resulting in higher Varroa mite loads and higher seasonal and winter colony losses. Early-warning signs of these carry-over effects were identified, showing that preferential investment in honey reserves instead of brood production early in the season increased the decline in pollen harvest and its associated carry-over effects. Synthesis and applications. The results suggest that the decline in pollen harvest may have been overlooked as a cause of pollen shortage and associated bee colony losses. Strategies to avoid such losses in intensive farmland systems include (i) limiting or avoiding honey harvests in spring, (ii) monitoring colonies for early-warning signals of colony failure and (iii) increasing the amount of floral resources available through wise land-use management.