A changing hierarchy of environmental time cues guide development of the migratory pest moth Loxostege sticticalis

Insects use environmental cycling cues to align developmental progression. In holometabolous insects, the progression from larva to adulthood entails decision points, i.e., larval diapause and adult eclosion, that engage circadian timing mechanisms. What environmental time cues dynamically interact to regulate these developmental checkpoints lying at the intersection of the orthogonal timing systems, remains elusive. We used the seasonally migratory insect Loxostege sticticalis to disentangle the influence of photoperiod regimes and temperature conditions at distinct checkpoints of larval growth and metamorphosis. We here employed sensory conflict paradigms to reveal that the relative dominance of these cues changes over ontogeny. Our results confirm that diapause is induced primarily by the photoperiod during early larval development, rather than by the diel thermoperiod. In contrast, the circadian clock-controlled adult eclosion, normally locked to the first hour of the daybreak, is jointly controlled by light and temperature cycles. An increase in the constant temperature advanced the peak timing of daily eclosion. On the other hand, the interval timing mechanism regulating the duration of the eclosion event is tuned specifically by the night temperatures. Together, photoperiod dominates early developmental decisions, but thermoperiod gains weight at later stages. This shifting hierarchy provides a mechanistic framework for understanding phenological plasticity. Since the timing of eclosion presents a window of increased vulnerability for this pest insect, our results could help to optimize the daily timing of pest management intervention under variable thermal and light conditions.