Rapeseed ( Brassica napus L.) bloom enhances colony growth but increases Varroa destructor infestation in Apis mellifera L

Visits of honey bees (Apis mellifera L.) to large-scale monocultures may pose risks to colony survival due to nutritional stress and increased pathogen exposure. In this study, we evaluated the effects of honey bee foraging on rapeseed (Brassica napus L.) during bloom on colony population growth, food stores, and health status. We monitored 20 colonies assigned to two treatments: 10 colonies exposed to rapeseed bloom (R) and 10 non-exposed colonies (NR). Colonies were assessed on five occasions during the flowering period across four consecutive years. By the end of the bloom period, R colonies exhibited significantly greater brood area, honey and bee bread stores, and adult bee population (measured as frames covered with bees) compared to NR colonies. R colonies also showed higher infestation levels of Varroa destructor but lower infection levels of Nosema spp. than NR colonies. Virus prevalence did not differ between treatments, although it varied across sampling times. Overall, our results indicate that visits of honey bees to rapeseed bloom enhance colony growth, particularly in early spring, by promoting brood production and adult bee development, thereby facilitating colony multiplication. However, this benefit is accompanied by a trade-off: although rapeseed foraging promotes early-season colony growth, it requires heightened post-bloom management of V. destructor to maintain colony health and productivity.