Toxicity of Chlorpyrifos and Ditmethoate to Fingerlings of the Nile Tilapia, Oreochromis niloticits-: Cholinesterase Inhibition
A PATHIRATNE and P ATHAUDA
Toxicity of chlorpyrifos and dimethoate, two organophosphorus insecticides commonly used in agricultural pest management in Sri Lanka, to the fingerlings of
Oreochromis niloticus was evaluated. The 96 hour LC50 values for chlorpyrifos and dimethoate were determined to be 0.117 mgl-1 and 14.84 mgl-1 respectively. Exposure to sublethal concentrations of the insecticides for 15 days caused marked inhibition (51-96%) of the acetylcholinesterase activity of brain, skeletal muscle, liver and spleen tissues in the fish. Upon transfer to insecticide free water, acetylcholinesterase activity increased but complete recovery was not attained even after 20 days. The results indicate that chronic exposure to even sublethal concentrations of these insecticides affect the fingerlings of O. niloticus. Of the two insecticides tested, chlorpyrifos was mole toxic than dimethoate.