Photoperiodism of diapause induction and diapause termination in Ostrinia furnacalis

The Asian corn borer, Ostrinia furnacalis (Guenee) (Lepidoptera: Crambidae), enters facultative diapause as fully grown larva in response to short-day conditions during the autumn. Photoperiodic response curves showed a stable critical daylength of ca. 13.5h at 25, 28, and 30 degrees C, which is nearly identical with the critical daylength between 13h 22min and 13h 27min in the field. The incidence of diapause obviously declined during ultra-long nights (14-22h scotophases) and DD (i.e., constant darkness). The required day number (RDN) for a 50% response differed significantly between short- and long-night cycles at different temperatures, indicating that the effect of one short night was equivalent to the effect of three long nights at 25 and 28 degrees C. The third instar was the stage most sensitive to the photoperiod. Night-interruption experiments with a 1-h light pulse at L9:D15 and L12:D12 exhibited a broad trough of diapause inhibition. In experiments using non-24-h light-dark cycles, only the cycles containing a long night of 12h resulted in high incidence of diapause, regardless of the duration of the accompanying light period, indicating that the duration of the dark period was of greater importance than that of the light period. The Nanda-Hamner and Bunsow experiments in O.furnacalis did not show any rhythmic fluctuations with a period of ca. 24h in their photoperiodic response curves; the incidence of diapause was low when the scotophase length exceeded 16h. The rearing daylengths of 11 and 12h evoked a greater intensity of diapause than did 13 h at 25 and 28 degrees C. By transferring naturally overwintering larvae from natural conditions to a short photoperiod of L12:D12 or a long photoperiod of L15:D9 combined with 25 degrees C, photoperiod appeared to have a significant influence on diapause development during the early phase of diapause. Field observations for 2years revealed that pupation and adult emergence of overwintering individuals (50% responses) occurred in early May and mid-May, respectively.