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

Pseudopidorus fasciata enters diapause as fourth instar larvae at short day lengths. Using 24-h light-dark cycles, the photoperiodic response curves i n this species appeared to be similar with a critical night length of 10.5 h at temperatures below 30 degreesC. At an average temperature of 30.5 degr eesC, the critical night length had shifted to between 15 and 17 h. In expe riments using non-24-h light-dark cycles, it was clearly demonstrated that the dark period (scotophase) was the decisive phase for a diapause determin ation. 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 aver ted 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 nigh t length. If D-1 or D-2 exceeded the critical night length diapause was ind uced. 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 th e most photosensitive phase reversed diapause in over 50% of the individual s. Ni-ht interruption experiments under non-24-h light-dark. cycles indicat ed 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 cir cadian periodicity at temperatures of 24.5 or 26 degreesC, but not at 30.5 and 23.3 degreesC. On the other hand, Bunsow and skeleton photoperiod exper iments failed to reveal the involvement of a circadian system in this photo periodic clock. These results suggest the photoperiodic clock in this speci es is a long-night measuring hourglass and the circadian effect found in th e final expression of the photoperiodic response in the resonance experimen ts may be caused by a disturbing effect of the circadian system in unnatura l regimes. (C) 2001 Elsevier Science Ltd. All rights reserved.