In order to evaluate the effect of time elapse and field conditions on the efficacy of three bioinsecticides and a methomyl insecticide against cotton leafworm, Spodoptera littoralis, 2^nd and 4^th instar larvae, a semi-field experiment was designated. The study was carried out throughout 2015 and 2016 growing seasons at El-Dakhaly village, the western side of Rashid branch, Kom Hamada Center, Beheira Governorate. Obtained results showed gradual increased mortality rates when 2^nd and 4^th instar larvae of S. littoralis fed on treated cotton leaves with LC₅₀ of tested bioinsecticides until reaching the 5^th day post application. Starting from the 6^th day post application a gradual increased larval mortality rates were observed in both 2^nd and 4^th instar larvae in both growing seasons. Obtained data also showed gradual decrease in mortality rates was observed along nine days post application with LC₂₅ of Kuik^® in both 2015 and 2016 growing seasons.