Light as a Chronobiological Zeitgeber in Bombyx mori: Developmental and Reproductive Outcomes under Spectral and Photoperiodic Stress

Light regime and spectral composition are pivotal in synchronising circadian and developmental processes in insects. This study systematically investigated the chronobiological effects of five wavelengths (natural, white, red, yellow, blue, green) and six photoperiod regimes (natural, 12L:12D, 16L:8D, 8L:16D, 24L:0D, 0L:24D) on Bombyx mori L (each group consisted of 50 eggs, n = 50). Survival rates, moulting synchronisation, and reproductive success were measured as key endpoints. Larvae raised under natural, white, or blue light completed their life cycle with high survival (87–93%), exhibited synchronised pupation, and demonstrated oviposition rates exceeding 95%. In contrast, larvae subjected to red, yellow, or green light showed pronounced developmental arrest; survival dropped below 40%, with 100% mortality by the fifth instar for green spectrum exposure. Light-dominant photoperiods (16L:8D, 24L:0D) induced fourth instar arrest with 90% survival and full reproductive capacity. Filament length and cocoon weight were highest in larvae exposed to blue (12L:12D) and natural light (mean body length ≥1070 mm, cocoon weight ≥0.22 g). These findings underscore that prolonged illumination or long-wavelength exposure disrupts hormonal signalling and circadian entrainment, with direct implications for optimising light environments in sericulture to enhance silk yield and reproductive output.