Evaluating the Potential Immunostimulatory Effects of <i>Cryptomeria japonica</i> Leaf Essential Oil on Honey Bees (<i>Apis mellifera</i>)

This study investigated the effects of Cryptomeria japonica leaf essential oil (CjLEO) on honey bee health, focusing on both toxicity and gene expression modulation. Initial toxicity assessments revealed that high concentrations of CjLEO (75 and 100 ppm) were lethal to honey bees, resulting in complete mortality within a short period. Conversely, a lower concentration of 10 ppm exhibited no significant toxic effects, prompting further investigation into its sublethal impacts. Transcriptome analysis via next-generation sequencing demonstrated that CjLEO at 10 ppm induced significant changes in honey bee gene expression compared to the control group. Principal component analysis (PCA) and differential gene expression (DEG) analysis identified more than 9,000 genes, with notable upregulation of immune-related genes, including hymenoptaecin, abaecin, and apidaecin1. Gene ontology (GO) enrichment analysis indicated that these differentially expressed genes were primarily associated with immune responses, such as defense and innate immune pathways. The chemical composition of CjLEO, characterized by GC-MS, identified 16 compounds, with major components including α-pinene, elemol, α-eudesmol, and kaur-16-ene. These compounds are known for their antimicrobial properties, which likely contribute to the observed immunomodulatory effects. CjLEO at a concentration of 10 ppm enhances honey bee immunity without exhibiting significant toxicity, positioning it a promising candidate for improving honey bee resilience against pathogens. Future research should investigate the mechanisms of immune activation and optimize application methods for practical beekeeping, aiming to improve colony health while reducing dependence on synthetic chemicals.