Under laboratory conditions the initial, latent effect and biometric effects of two IGR's pyriproxyfen (Admiral) and diafenthiuron (Polo) and melbimectin (biocide) were studied on the 6th instar larvae of the cotton leafworm, Spodoptera littoralis (Boisd.) of pyriproxyfen was the most effective, it caused 101.56% increase in duration of the 6th instra larval, followed by diafenthiuron at the recommended rate gave 73.77% increase, in contrast melbimectin was ineffective when all the three chemicals tested at the recommended rate of each. Pyriproxyfen was the most potent compound in increasing the weight of treated larvae (46.15% increase), while diafenthiuron and melbimectin reduced the larval weight (33.33 and 20.51% decrease, respectively). Melbimectin was highly effective against larval stage, where mortality percentages ranged from 30.3 to 54.5 after 48 hrs of treatment. While, it was ineffective during pupal stage, recording from 0.0 to 5.0% mortality. On the other hand, pyriproxyfen and diafenthiuron exhibited poor effect against larval stage, while they were potent during pupal stage. The highest reduction pupation percentages were (38, 52 and 56% pupation), for melbimectin, diafenthiuron and pyriproxyfen, respectively. However, pyriproxyfen was superior to melbimectin and diafenthiuron in forming abnormal pupae. In this respect, pyriproxyfen at the rates 75 and 37.5 ppm exhibited the highest percent pupal malformation (75.00% and 76.32%, respectively). Melbimectin at the rate of 75 ppm and diafenthiuron at the rate of 1500 ppm gave 57.89 and 50.00% pupal malformation, respectively. Pyriproxyfen reduce moths emergence (23.68-50.00%), while diafenthiuron (50.00-89.19%) and melbimectin recorded 42.11-93.33% with different concentrations.