Photoperiod-driven diel and circadian eclosion rhythms in the migratory pest Nilaparvata lugens

Precise timing of emergence in insect pests is critical for understanding circadian regulation and improving time-based control strategies. In this study, we examined the diel and circadian eclosion rhythms of the migratory rice pest Nilaparvata lugens in response to different photoperiodic conditions. Experiments were conducted under a natural geomagnetic field (45 μT) using a custom-built, full-time chronobiological monitoring system for small insects. Insects were entrained to a long-day photoperiod (L14:D10, LD; also used for entrainment before other photoperiod treatments), representative of local outbreak conditions, and then exposed to either continued LD, constant darkness (DD), or constant light (LL). Fifth-instar duration, adult eclosion duration, and population-level emergence rhythms were continuously recorded. Given the relatively long eclosion duration (mean: 32.80~36.48 min), we adopted a 2-hour binning strategy for rhythm analysis. The results showed that eclosion rhythms followed a normal distribution under LD, DD and LL conditions, whereas under continuous light (LL+2), eclosion behavior no longer conformed to a normal distribution. Under all photoperiods, emergence peaks occurred during the dark phase or in circadian phases corresponding to darkness under LD. In the LD condition, peak emergence occurred at Zeitgeber Time (ZT) 22-24, accounting for 20.91% of total eclosion events and showing clear anticipation of lights-on. Under DD, the peak shifted to Circadian Time (CT) 20-22 (24.00%), indicating a free-running period shorter than 24 hours. LL initially preserved the LD-like phase, but rhythmicity was completely lost after two additional days of continuous light, suggesting high sensitivity of the clock system to prolonged illumination. Under LD, both wing morphs and sexes exhibited synchronized emergence peaks in the two hours before lights-on (ZT22-24), with peak proportions ranging from 18.18% to 25.00%. In summary, the eclosion rhythm of N. lugens is jointly regulated by external photoperiodic cues and an internal circadian clock, exhibiting stable and predictable phase characteristics. This rhythm provides a reliable phenotypic marker for investigating its circadian mechanisms and offers a valuable temporal framework for understanding the timing of migration and optimizing precision control strategies in the field.