Comparative evaluation of larvicidal and insect growth regulator activity of aqueous extracts and solvent extracts (acetone) of brown algal seaweed, Sargassum wightii Greville against rice leaf folder, Cnaphalocrocis medinalis Guenee (Lepidoptera: Crambidae) was conducted in the Seaweed Laboratory, Department of Entomology, Faculty of Agriculture, Annamalai University, India during 2021-2022. Sargassum wightii was collected from the inter tidal and deep sea of Rameswaram coastal ecosystem of Tamil Nadu, India. The aqueous extracts and acetone solvent extracts of the seaweed @ 2, 4, 6, 8 and 10 per cent concentrations were tested for their toxicity against Rice leaf folder and their bioactivity was compared with a standard check neem leaf extract (3%) and untreated control whereas the solvent extract treatments were compared with an additional treatment i.e., one per cent acetone solvent treatment for comparison. Solvent and aqueous extracts @ 8 and 10 per cent demonstrated the higher order of larval mortality with 46.66 and 33.33 per cent larval mortality at 72 hours respectively and were on par with neem leaf extract (3%) influence. The larval to adult conversion ratio of 1: 0.46 in solvent extracts and 1:0.53 in aqueous extracts at 8 and 10 percent were on par with neem leaf extract (3%) whereas in solvent control and untreated control, the ratio was 1:1.00.