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Effects of skeleton photoperiods on the induction of larval diapause in the Indian meal moth Plodia interpunctella

Abstract The Indian meal moth Plodia interpunctella Hübner (Lepidoptera: Pyralidae) may measure night length to determine its developmental programme with respect to entering diapause or continuing development in the last‐larval instar. Its photoperiodic clock appears to be hourglass‐like, in that the response to a symmetrical skeleton photoperiod consists of two 2‐h light pulses. If the insects read this as lights‐on and lights‐off signals, an oscillator should be involved in the clock system. In the present study, the insects are maintained under a normal photoperiod for 1 day at 25 and 20 °C and then transferred to the skeleton photoperiod or constant darkness (DD) for 1, 2 or 3 days. These photoperiodic regimes are repeated during the larval sensitive stage. There is no difference in the percentage diapause between the effects of skeleton photoperiod and DD treatment. At 25 °C, the incidence of diapause is relatively low under very short days (< 8‐h main photophase) and long (> 14‐h main photophase) days and relatively high under a 12‐h main photophase. At 20 °C, the percentage of diapause is generally high under any photoperiodic treatment. Diapause is prevented slightly under LD 14 : 10 h and 16 : 8 h photoperiods. If the diapause clock is an oscillator that damps rapidly, under DD or constant light (LL) for 5–15 days, it may stop its oscillation but, after exposure to the skeleton photoperiod, the clock system might be reset. There is no difference in the diapause‐inducing effect between the skeleton and normal photoperiodic treatments after DD. The incidence of diapause is moderately high under short days (<12 h) but low under long days (>16 h). On the other hand, when the insects are exposed to the skeleton photoperiod after LL, the induction of diapause is low, regardless of the interval between the two light pulses in skeleton photoperiods. Under normal photoperiodic conditions after LL, a clear photoperiodic response curve is obtained. Thus, P. interpunctella does not show any sign of the oscillatory function in the night‐time measurement and is regarded as being ‘hourglass‐like'.