Effect of light period on dark-time measurement for diapause induction in Mamestra brassicae

Abstract

In the cabbage moth, Mamestra brassicae, the type of pupal development (winter diapause, summer diapause or non-diapause) is detrmined by photoperiod in the larval stage. Interacting effects of photophase and scotophase were examined by exposing larvae to non-24-h cycles in which the two component phases were varied independently of each other. With a photophase of 1–3 h, winter diapause was most effectively prevented by a light pulse at a fixed time (12 h) from lights-on rather than lights-off. With photophases of 4–36 h, the diapause programming was basically dependent on the scotophase length, but the critical scotophase increased as the photophase deviated from 12 h. The quantitative variation in the diapause-inducing effect of scotophases within the inductive range could interact with the available number of cycles restricted by the cycle length, and this might cause apparent shift of the “critical” scotophase. The response to interruption of 12-h scotophase was also affected by photophase length, and the effect of 1 h light pulse was more changeable in the early than in the late scotophase.