Low intensity UVB exposure as an immunostimulant in yellow mealworms (Tenebrio molitor)

The yellow mealworm (Tenebrio molitor) synthesizes vitamin D3 de novo when exposed to ultraviolet B (UVB) irradiation. Given the well-established link between UVB, vitamin D3, and immune function in vertebrates, we hypothesized that mealworms exposed to low-intensity UVB would exhibit increased pathogen resistance. When comparing mealworms exposed to prolonged UVB to control groups, we observed significant upregulation of nine transcripts associated with the Toll immune pathway (p < 0.001)—a key signaling pathway mediating bacterial, fungal, and viral immunity in insects. Additionally, the expression of three antimicrobial peptide (AMP) genes was significantly higher in the UVB-exposed group. In a subsequent experiment, involving a pathogen challenge (Beauveria bassiana) after two weeks of UVB treatment (12 hours/day, 3.5 μW/cm²), we found increased expression of tenecin 4 in the fat bodies of UVB-exposed mealworms. However, no significant improvement in pathogen resistance due to UVB exposure was observed. Interestingly, UVB-exposed mealworms experienced stunted growth compared to the control group, gaining 23 mg versus 78 mg over the four-week exposure period (p < 0.001), likely due to the genotoxic effects of UVB. While the phenotypic benefits of low-intensity UVB exposure remain unclear, these novel findings enhance our understanding of insect immunology and may have potential applications in commercial rearing facilities for food and feed, particularly in preventing pathogen-induced colony collapse.