Photoperiod is a great factor in determining the developmental pattern (continued development vs. suspended development, i.e, diapause) of many insect pests. The cotton leaf worm, Spodoptera littoralis (Boisd.) is non-diapaused insect. Knowledge about how developmental pathway of S. littoralis responds to photoperiod is necessary for mass rearing of the pest and its parasitoids and may suggest new control management. The present work was to investigate the effects of a range of photoperiods under different temperatures on the speed of development and the number of larval instars of S. littoralis in the laboratory conditions. The photoperiods (Light: Dark) 0L: 24D; 6L: 18D; 12L: 12D; 18L: 6D and 24L: 0D) of a constant temperature (15, 20, 25, or 30 °C) affected the developmental speed, live fresh weight and number of instars of S. littoralis larvae. The day length demonstrated its greatest impact at 20 °C, compared to lower (15 °C) or higher temperatures (25 and 30 °C). At 20 °C, short photoperiod significantly accelerated the developmental pattern of both larval (by 14 days) and pupal stages (by 9 days), but long photoperiod slowed down the development. There was significant effect of photoperiod on the developmental time at 20 °C which decreased as the temperature increased to 25, or 30 ºC. The occurrence of extra molt differed significantly among ranges of day lengths at constant rearing temperature. There was also an increased frequency of extra instars at low temperatures of different photoperiods. Extra instars were less common at high temperature (e.g.30 °C). In summary, photoperiod had significant effects on the developmental durations of S. littoralis larvae, fresh pupal weight and number of larval instars.

Keywords: Spodoptera littoralis, non-diapaused insect, photoperiodism, development time, number of larval instars