Thermoperiodism, photoperiodism and sensitive stage in the diapause induction of Sesamia nonagrioides (Lepidoptera: Noctuidae)

Photoperiod is the major factor governing the induction of larval diapause in Sesamia nonagrioides. Constant temperature and thermoperiod were shown to be additional factors which interact with photoperiod to induce diapause. Constant temperature modified the diapause response curve to daylength from a type III (at 25°C) to a type I (at 18°C). Photoperiodic induction of diapause was enhanced by superimposed thermoperiods regardless of whether the low temperature coincided with the scotophase or with the photophase. However, constant temperature and thermoperiods per se induced diapause in the absence of photoperiodic signals. Low (18°C) constant temperatures induced 100% diapause in larvae reared under continuous darkness. Also in continuous darkness, diapause was induced in a 50% of larvae when they were reared under a thermoperiodic regime [30°C (13 h): 18°C (11 h)] in comparison with 0% diapause if larvae were reared under 25°C constant temperature. Gradually increasing and decreasing photoperiods did not modify the diapause response but the gradual decrease of daylength shortened the larval developmental time. First and second larval instars were the most sensitive to diapause induction by photoperiod. The diapause-inducing effect of short daylengths on young larvae can be reversed by exposure to long daylengths in the late instars.