The marine algae, popularly known as seaweeds are one of the most important marine resources of the world. The biocidal activity of hexane (HE), chloroform (CH), methanol (ME) and water (WA) extracts (100, 200, 400, 600 and 800 ppm) of brown algae (Ochrophyta), Sargassum wightii (Greville ex J. Agardh) (SW) and Padina pavonica (Linn.) Thivy (PP) was assessed against Dysdercus cingulatus (Fab.). GC-MS results revealed that S. wightii showed the presence of stigmastan-6, 22-dien, 3, 5-dedihydro- (71.34%) whereas P. pavonica showed hexadecanoic acid, methyl ester (43.26%). The chloroform extract of S. wightii caused more nymphal mortality at 96 hrs (LC50= 631.8 ppm) than P. pavonica (LC50= 1062.5 ppm). Further, column chromatographic fractions of S. wightii (F164 - F323) (LC50= 175.2 ppm) and P. pavonica (F800 - F965) (LC50= 292.7 ppm) showed more nymphal mortality. After 96 hrs, live insects were maintained with normal food and life parameters like adult longevity, mating period, fecundity, hatchability and incubation period were recorded. In S. wightii chloroform extract, male longevity and water extracts male longevity (df= 5, 27; F= 8.177; p= 0.005); female longevity (df=5, 24; F= 6.838; p=0.005). Mating period was highly prolonged by the water extracts. Fecundity and hatchability were highly reduced at 800 ppm both by chloroform and methanol extracts. In P. pavonica, mating period was highly prolonged by the water extract; fecundity and hatchability were highly reduced by hexane and the incubation period was slightly increased by extracts. Hence, these algal extracts can be used as biocide in cotton pest management.