Pilot Study of Grape Pomace Extract Impact on Nosema ceranae-Infected Honey Bees

Nosema ceranae infection represents one of the major health threats to honey bees (Apis mellifera), contributing to colony weakening, oxidative stress, immune suppression, and increased mortality. Due to restrictions on the use of fumagillin and the growing demand for natural alternatives, this pilot study evaluated the potential of grape pomace (GP) extract (at a concentration of 200 μg/g) as a dietary supplement for improving honey bee health and controlling Nosema ceranae infection. Newly emerged worker bees were experimentally infected with N. ceranae and maintained under laboratory cage conditions for 15 days. Bees were supplemented with GP extract either preventively, simultaneously with infection, or after infection. Survival, spore loads, and expression of immune-, antioxidant-, and detoxification-related genes were evaluated. Results showed that N. ceranae infection significantly reduced bee survival and induced strong oxidative and immune stress responses. Supplementation with GP extract significantly improved survival and reduced spore loads in infected bees, particularly when administered preventively or at the onset of infection. The extract also modulated the expression of antioxidant-related genes, including CAT, Cu/ZnSOD, GST, and MnSOD, indicating attenuation of infection-associated oxidative stress. In non-infected bees, GP supplementation stimulated the expression of immune-related genes, especially defensin and hymenoptaecin, while such immunostimulatory effects were not fully maintained in infected groups. The findings suggest that grape pomace extract acts primarily through antioxidant and gut health-associated mechanisms rather than direct immune protection during infection. Overall, grape pomace extract demonstrated promising potential as a natural dietary supplement for improving honey bee resilience and reducing the impact of Nosema ceranae infection. Further studies with more concentrations and temporal dynamics need to be evaluated.