Feeding the disease: The impact of nutritional supplementation on Nosema (Vairimorpha) infection in honey bees (Apis mellifera).
Honey bees (Apis mellifera) experience variable colony losses across regions and years, with infectious diseases representing a key component of colony health challenges. Among the most prevalent pathogens are the microsporidian parasites Nosema apis and Nosema ceranae, whose impacts on host survival and transmission vary widely depending on context. While nutritional supplementation is commonly used to support honey bee health, its effects on Nosema infection outcomes remain unclear. Here, we experimentally tested whether dietary enrichment alters survival and infection intensity following exposure to a mixed Nosema inoculum. Newly emerged worker bees were challenged with Nosema spores and maintained on either a basic sucrose diet or the same diet supplemented with a commercial pollen substitute. Dietary enrichment significantly increased both mortality risk and infection intensity in Nosema-infected bees, while having no detectable effect on survival in uninfected controls. These results indicate that supplementation can, counter intuitively, exacerbate nosemosis by promoting parasite replication rather than enhancing host resistance. Our findings highlight the importance of distinguishing nutritional effects on host tolerance versus resistance, and caution that interventions intended to improve bee nutrition may inadvertently increase pathogen production and transmission potential under certain conditions.