Hourglass and oscillator expressions of photoperiodic diapause response in the cabbage moth Mamestra brassicae

Abstract. Both oscillator and hourglass features are found in the photoperiodic response that controls the pupal winter diapause of Mamestra brassicae. The expression of oscillatory response to extended long‐night cycles is temperature dependent, i.e. circadian resonance appears at 23 and 25 o C but not at 20 and 28 o C. At 20 o C, scanning of extended scotophases by a short light pulse does not reveal any clear circadian rhythmicity. However, a circadian feature of the photoperiodic response is indicated even at 20 o C by a bistability phenomenon, i.e. either one of the two dark periods in symmetrical skeleton photoperiods determines the diapause response depending on the phase angle with the preceding (entraining) light‐dark cycles. At 20 and 25 o C, the incidence of diapause increases as a function of the number of light–dark cycles regardless of the cycle length (T) , if T is 24 h or 2 X 24h (with a 12 h light period). A non‐diel cycle (r=36h) is less effective, suggesting that disturbance of the circadian organization partly impairs the diapause‐inducing function. The inductive effect of a long night is largely affected by temperature, and becomes saturated with eight cycles at 20 o C and 14 cycles at 25 o C. Presumably, an hourglass mechanism measures the dark time, and a circadian component involved in some later sequence of the photoperiodic response may or may not be expressed depending on the mode of interaction between them.