PHOTOPERIODIC INDUCTION OF DIAPAUSE IN AN INSECT

1. The European corn borer, Ostrinia nubilalis, is a so-called long-day insect, larval diapause being induced by naturally occurring photoperiods containing scotophases of from 10 to 14 hours. 2. Diapause induction was found to be dependent upon the actual number of hours of the photoperiodic phases. The duration of the scotophase was far more critical than that of the photophase. A 12-hour scotophase was of maximum effectiveness when combined with photophases of from 5 to 18 hours. Significant incidence of diapause occurred when a 12-hour scotophase was combined with photophases of from 4.5 to 32 hours. 3. Diapause induction is a temperature-sensitive phenomenon, with the incidence of diapause tending to be inversely proportional to the ambient temperatures occurring during the scotophase. 4. Interruption of the scotophase by a one-hour period of light modified the photoperiodic response, the effect depending on the position of the light interruption within the scotophase. The effects were interpreted as a demonstration that the insect's photophasic requirement is satisfied by a one-hour light period, but that longer photophases are normally required because of a dark-refractory period following 12-hour scotophases.