* Part of thesis submitted to A. P. Agricultural University during 1974 for the award of M.Sc. (Ag.) degree in Entomology. by senior author.
Bio-assay studies of inseeticides with reference to their relative toxicity were conducted in the laboratory with insecticidal spray deposits of 5 insecticides viz. lindane, endosulfan, quinalphos, phorate and chlorpyriphos as contact poisons against the first instar larvae of paddy stem borer, Tryporyza incertulas (W.) at 8 different concentrations of 0.1%, 0.05%, 0.025%, 0.0125% 0.00625%, 0.003125%, 0.0015625%, and 0,00078125%. Mortality counts were taken 48 hours after the treatments. Based on the mortality data LC 50 values and LC 90 values of each insecticide were calculated. The relative toxicity of the insecticides were calculated based on the LC 50 values by taking lindane as unit value. All the four insecticides were found to be more toxic than lindane and the order of toxicity was phorats > quinalphos > chlorpyrip os > endosulfan > lindane.
