Glucosinolate-Rich Floral Extracts Hinder Nosema (= Vairimorpha) ceranae Infection in Caged Apis mellifera Workers

Colony losses in Apis mellifera are caused by multiple stressors, among which the parasitic microsporidian Nosema (= Vairimorpha) ceranae. As an alternative to the controversial fumagillin, sustainable control strategies based on natural bioactive compounds are increasingly being explored. This study evaluated the efficacy of glucosinolate (GSL)-based formulations derived from flowers of Eruca sativa and Reseda lutea in controlling N. ceranae infection in worker honey bees under laboratory conditions. Freeze-dried flowers were characterized for their GSL profile and myrosinase activity, and formulation studies were conducted to choose the best material for bioassays. Then, water-soluble GSL-enriched extracts were produced. Cage feeding assays were performed on uninfected newly emerged bees using sugar syrup supplemented with 2 and 4 µmol g⁻¹ of total GSL of either formulation. Extract administration reduced food consumption and bee survival in a concentration-dependent manner compared to the control group. The lower concentration was further tested on 6-day-old bees artificially infected with N. ceranae spores, showing consumption comparable to control, but still a slight decrease in survival. At this concentration, both E. sativa- and R. lutea-based extracts significantly inhibited N. ceranae abundance, by 23.8 and 44.6%, respectively, with no significant difference between plant species. Isothiocyanates-the GSL hydrolysis products-were detected in bee intestines, accumulating primarily in the hindgut and at higher levels in bees fed with R. lutea extracts. These findings highlight the potential of floral GSL-rich formulations as natural, environmentally sustainable tools to support colony health and reduce pathogen load.