Immune priming of honey bees protects against a major microsporidian pathogen

BACKGROUND Honey bees face significant threats from pathogens like Nosema ceranae , a microsporidian parasite that contributes to global colony declines. Immune priming, exposure to pathogen antigens to stimulate protective responses, could mitigate infection risks. We tested whether priming honey bee larvae and adults with heat‐killed N. ceranae spores reduces susceptibility to subsequent live infections in laboratory and field conditions. RESULTS Priming consistently lowered infection levels and rates. The strongest effect occurred in laboratory‐reared larvae, with a 97% reduction in mean spore counts relative to unprimed controls. Newly emerged adults primed and maintained in field colonies showed a 56% decrease, larvae primed by nurse bees in colonies had a 52% reduction, and newly emerged adults primed in laboratory cages showed a 34% reduction. Primed adults in field colonies also exhibited increased survival following live pathogen exposure as compared to positive controls. However, immune priming alone sometimes reduced bee lifespan. Priming altered expression of Toll pathway immune genes associated with Nosema resistance, particularly defensin‐1. CONCLUSION Immune priming significantly protects honey bees from N. ceranae infection, reducing pathogen loads and enhancing survival across diverse conditions. Despite potential longevity trade‐offs, these findings support immune priming as a viable strategy to improve honey bee health and to reduce microsporidian‐induced colony losses.