Effect of thermoperiod on diapause induction of Sesamia nonagrioides (Lepidoptera-Noctuidae)

We have been studying the factors that control larval diapause of Sesamia n onagrioides (Levebvre), a multivolitine pest of corn, Zea mays L., in the M editerranean. The effect of various combinations of photoperiod and tempera ture on the induction of larval diapause of S. nonagrioides was examined. S hort daylengths (8-12 h) were the main factor inducing diapause in this spe cies, whereas high temperatures could avert the effect of the photoperiodic signal. Daily temperature cycles with different amplitudes (differences be tween the thermophase [high phase] and cryophase [low phase] temperatures), could induce diapause under a photoperiod of 12:12 (L:D) h when the cryoph ase coincided with the scotophase (dark period). In contrast, short dayleng ths had no effect on diapause when the thermophase coincided with the scoto phase. Moreover, greater differences between the high and the low temperatu res of the thermoperiod led to a higher incidence of diapause. Diapause als o occurred under 24-h thermoperiods at constant darkness; the incidence of diapause correlated with the duration of the cryophase. The thermoperiodic response curve of diapause under constant darkness was analogous to that of the photoperiodic response of a type I, or long-day insect. The cryophase of the thermoperiod must be <17.5 degrees C to obtain a high incidence of d iapause. Consequently, it appeared that a thermoperiodic threshold during t he cryophase was demonstrated. Hence, both the duration and the temperature of the cryophase of the thermoperiod were important factors controlling th e diapause response. The ecological implications of this behavior are discu ssed.