Photoperiodic clock of diapause induction in Pseudopidorus fasciata (Lepidoptera: Zygaenidae)

Abstract

Pseudopidorus fasciata enters diapause as fourth instar larvae at short day lengths. Using 24-h light–dark cycles, the photoperiodic response curves in this species appeared to be similar with a critical night length of 10.5 h at temperatures below 30°C. At an average temperature of 30.5°C, the critical night length had shifted to between 15 and 17 h. In experiments using non-24-h light–dark cycles, it was clearly demonstrated that the dark period (scotophase) was the decisive phase for a diapause determination. In night interruption experiments using 24-h light–dark cycles, a 1-h light pulse at LD12:12 completely reversed the long night effect and averted diapause in all treatments.. At LD 9:15 light pulses of 1-h, 30- or 15-min also averted diapause effectively when both the pre-interruption (D 1 ) or the post-interruption scotophases ( D 2 ) did not exceed the critical night length. If D 1 or D 2 exceeded the critical night length diapause was induced. The most crucial event for the photoperiodic time measurement in this species is the length of the scotophase. A 10-min light pulse placed in the most photosensitive phase reversed diapause in over 50% of the individuals. Night interruption experiments under non-24-h light–dark cycles indicated that the photoperiodic clock measured only D 1 regardless of the length of D 2 , suggesting that the most inductive cycles are often those in which L+D are close to 24 h. In resonance experiments, this species showed a circadian periodicity at temperatures of 24.5 or 26°C, but not at 30.5 and 23.3°C. On the other hand, Bünsow and skeleton photoperiod experiments failed to reveal the involvement of a circadian system in this photoperiodic clock. These results suggest the photoperiodic clock in this species is a long-night measuring hourglass and the circadian effect found in the final expression of the photoperiodic response in the resonance experiments may be caused by a disturbing effect of the circadian system in unnatural regimes.