Melatonin Enhances Thermal Resilience and Extends Worker Lifespan in Apis cerana via Redox-Metabolic Reprogramming

Apis cerana is widely managed in apiculture in Southern China but experiences substantial colony losses during prolonged summer heat. Developing effective strategies to support colony over-summering is therefore critical. This study demonstrates that dietary supplementation with melatonin significantly enhances thermal tolerance and extends worker lifespan in A. cerana under heat stress. Laboratory bioassays revealed that melatonin supplementation (20 µg/mL) markedly improved worker survival at both 35 °C and 37 °C, with the most pronounced effect at 37 °C, where mortality was significantly reduced. Consistently, field trials demonstrated that melatonin supplemented colonies gained significantly more weight during summer heatwaves than colonies without melatonin supplementation. Mechanistically, melatonin orchestrates a biphasic adaptive response. In an early phase (day 4), melatonin rapidly upregulates heat shock proteins (HSC70-4, CRYAA, l(2)efl) and detoxification enzymes (GST-like), accompanied by reduced reactive oxygen species (ROS) accumulation and enhanced proboscis extension response (PER), indicative of preserved sensory function. This is followed by a later maintenance phase (day 11), characterized by sustained upregulation of fatty acyl-CoA reductases (FAR1, FAR11-like, FARwat) and peroxisomal components (PMP34), which promote lipid remodeling and membrane stabilization. RNA-seq analysis identified differentially expressed genes (DEGs) significantly enriched in pathways related to redox homeostasis, lipid metabolism, detoxification (GSTs, CarEs, CYP450s), and longevity. These molecular responses were associated with enhanced antioxidant capacity, reduced oxidative damage, and sustained foraging activity under thermal stress. Collectively, these results indicate that melatonin serves as a potent nutritional intervention that reprograms redox metabolic networks to mitigate heat-induced damage, extend worker longevity, and enhance colony productivity under climate warming. These findings highlight melatonin's potential as a practical tool to reduce summer colony losses in apiculture.