The corn stem borer, Sesamia cretica Led. (Lepidoptera: Noctuidae) is one of the most economically
important pests of maize crop in Egypt. The pest is generally controlled by repetitive application of chemicals, resulting in environmental pollution and resistance in pest population. In this study, efficacy of spinosad against different larval instars of S. cretica was evaluated under laboratory and field conditions. The impact of spinosad on larval protein content and glycogen level were also assessed. Data indicated that spinosad had exerted some toxic effect against the tested larval instars and the mortality was in the order of first instar > second instar > third instar > fourth instar > fifth instar, with respect to the LC50 values estimated as 0.008, 0.016, 0.028, 0.044 and 0.159 ml/l for the prementioned instars respectively, 7 days after treatment. The field experiment showed high efficiency of spinosad against S. cretica at the highest three concentrations down to 25% field rate (FR); inducing significant reduction in the number of plants containing either perforated stems or dead hearted cases, number of larvae, tunnels and excavated areas inside infested plants. Regarding the biochemical parameters, results proved that the protein content and glycogen level in the treated larvae were significantly lower those that of the control at all concentrations. The highest reduction in protein content, -54.5% was recorded in 2nd instar larvae exposed to 50% FR (0.25ml/l). Similarly, the highest reduction in glycogen level, -55.8%, was recorded in 2nd instar larvae exposed to 50% FR (0.25 ml/l), meanwhile, it was also noted that impact was concentration dependent.