The biochemical energy reserves of three species of entomopathogenic nematodes (EPN) were analysed quantitatively at three infective juvenile ages 0, 20 and 90 days for Steinernema (Steinernema riobrave) , less than 7, 30 and 50 days and 0, 20 and 40 days for Heterorhabditis (Heterorhabditis bacteriophora and Heterorhabditis indica) respectively. Lipid, protein, carbohydrates and glycogen contents of the two species of Heterorhabditis and one species of Steinernema were determined. Results revealed that total lipids are the main component, comprising approximately 42.55 of nematode dry weight for S. riobrave and 50.52%, 53.75% for H. bacteriophora and H. indica, respectively, when freshly emerged from their hosts. These percentages declined significantly at the other two juvenile ages of the three species. Glycogen and protein contents in all species exhibited minor changes during the tested juvenile ages. Estimation of total carbohydrates appeared to be source of energy that only utilized early in the ambushing state of S. riobrave and when juveniles get older in H. indica and H. bacteriophora. Infectivity of the three nematode species at the three tested ages was not affected, where approximately 100% Galleria mellonella mortality was recorded except a slight insignificant decrease (97.33%) in H. indica at the age of 40 days. Penetration rate of H. indica, and H. bacteriophora decreased with the declining of the total lipid content at the three juvenile ages, while the penetration rate in S. riobravae exhibited significant decrease only at 90-day old juveniles. Significant decreases in migration rate were evident in H. indica and H. bacteriophora as the total lipid content declined in the juvenile ages. These results might be helpful in increasing the efficiency of EPN as biological control agents against insect pests by improving shelf-life of nematode juveniles and establishing a simple and easy quality control method for different nematode species or strains.