Effect of two environmentally friendly insecticides on the cotton leafworm, Spodoptera littoralis (Boisduval.) (Lepidoptera: Noctuidae) under laboratory conditions

From the most relative safer groups of insecticides, two compounds lufenuron and spinosad were selected to evaluate its efficacy against the fourth instar larvae of the cotton leafworm, Spodoptera littoralis (Boisduval) (Lepidoptera: Noctuidae). The results of toxicological study by the computed LC50 values of the two tested compounds, showed that their toxicities increased with time post treatment. At 48, 72 and 96 hrs post treatment, LC50 values were 21.75, 1.63 and 0.58 ppm for lufenuron, whereas, for spinosad were 85.48, 29.68 and 19.89 ppm, respectively. According to LC50 values, the results indicated that lufenuron was about 3.9, 18.2 and 34.3 fold toxic than spinosad against the 4th instar larvae of Spodoptera littoralis at 48, 72 and 96 hrs post treatment, respectively under laboratory conditions. In addition the biological effect of the tested compounds was also observed through feeding of 4th larvae on castor-bean leaves treated with LC25 of each compound. It was observed that the tested insecticides suppressed pronouncedly the larval and pupal weight gain and decreased the percentages of pupation, adult emergence and eggs hatchability. Also, the larval duration was highly prolonged comparing with untreated ones.