The carry-over effects of pollen shortage decrease the survival of honeybee colonies in farmlands.

International audience1. Many studies have reported honeybee colony losses in human-dominated landscapes.While bee floral food resources have been drastically reduced over past decades in humandominatedlandscapes, no field study has yet been undertaken to determine whether there is acarry-over effect between seasonal disruption in floral resource availability and high colonylosses.2. We investigated if a decline in the harvest of pollen by honeybees in spring affected managedhoneybee colony dynamics (brood size, adult population and honey reserves) and health(Varroa mite loads and colony survival) throughout the beekeeping season.3. 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 honeyreserves before the onset of winter. Furthermore, the decline in pollen harvest negativelyimpacted the health of the colony, resulting in higher Varroa mite loads and higher seasonaland winter colony losses.4. Early-warning signs of these carry-over effects were identified, showing that preferentialinvestment in honey reserves instead of brood production early in the season increased thedecline in pollen harvest and its associated carry-over effects.5. Synthesis and applications. The results suggest that the decline in pollen harvest may havebeen overlooked as a cause of pollen shortage and associated bee colony losses. Strategies toavoid such losses in intensive farmland systems include (i) limiting or avoiding honey harvestsin spring, (ii) monitoring colonies for early-warning signals of colony failure and (iii) increasingthe amount of floral resources available through wise land-use management