Persistance and relative toxicity of different insecticides against Thrips (Scirtothrips dorsalis) and whitefly (Bemisia tabaci) on cotton ecosystem

Residual toxicity of seven different insecticides were tested for their persistence and residual toxicity against thrips and whiteflies on cotton ecosystem. During the investigation all the insecticides except clothianidin @ 0.01 per cent proved toxic to third thrips and whitefly on cotton. Fipronil @ 0.015 per cent and diafenthiuron @ 0.060 per cent exhibited comparatively higher percentage mortality of thrips on cotton (22.35 and 20.22) and (23.21 and 20.98) at 14 days after first and second spraying. The descending order of residual efficacy on the basis of PT values was fipronil 0.015 per cent (878.22) > diafenthiuron 0.060 per cent (824.18) > acetamiprid 0.002 per cent (789.14) > imidacloprid 0.004 per cent (771.26) > thiamethoxam 0.005 per cent (727.02) > dimethoate 0.04 per cent (728) > clothianidin 0.010 per cent (687.12) and fipronil 0.015 per cent (893.06) > diafenthiuron 0.060 per cent (847.98) > acetamiprid 0.002 per cent (802.90) > imidacloprid 0.004 per cent (772.10) > thiamethoxam 0.005 per cent (725.76) > dimethoate 0.04 per cent (685.86) > clothianidin 0.010 per cent (651.56). Highest LT50 value to the extent of 6.39 days and 6.69 days was obtained when the thrips were exposed to cotton leaf sprayed with fipronil after first and second spray respectively. Similarly fipronil @ 0.015 per cent and acetamiprid @ 0.002 per cent gave comparatively higher percentage mortality of whitefly on cotton (20.14) at 14 days after third spraying On the basis of PT values the descending order of persistent toxicity was fipronil 0.015 per cent (889.14) > acetamiprid 0.002 per cent (852.32) > imidacloprid 0.004 per cent (824.32) > diafenthiuron 0.060 per cent (812.98) > thiamethoxam 0.005 per cent (769.86) > dimethoate 0.04 per cent (732.48) > clothianidin 0.010 per cent (700.70). Highest LT50 value to the extent of 6.48 days was obtained when the whiteflies were exposed to cotton leaf sprayed with fipronil after third spray.