The present investigation aim to throw light on the efficiency of lethal concentration for 50% (LC50 value) for the novel insecticide pyridalyl on 2nd, 4th,and 6th larval instars of cotton leafworm Spodoptera littoralis ( Boisd.) under laboratory conditions .The results showed that the pyridalyl is more effective on 4th instar larvae due to the larval mortality percent estimated by 78.0%. Also fertility % was 0.0 in comparison to control and the number of eggs/female was the smallest one in comparison with other, estimated by 365.7 eggs. Marked biochemical changes however, being recognized in pest as marked SDS-polyacrylamide gel electrophoresis representing molecular weights in protein showed there are three ,two and one bands were found to be specific to treated 4th, 2nd, and 6th larval instars, respectively. Also the activity of both ᾱ & ß esterases enzymes analysis showed differences in esterase pattern in the treated 4th instars than control. The tested LC50 value of pyridalyl showed highly histopathological disturbance in the epithelium of mid gut. The histochemical observation showed a conspicuous depletion in total protein content in both 4th and 6th treated larval instars.
Keywords: Pyridalyl, Spodoptera littoralis, biological activity, biochemical, histopathology, histochemistry.