Joint Toxicity of Pesticides Methomyl and Phoxim to Tilapia(Oreochromis niloticus)

Methomyl and phoxim,widely used to control pest insects both in China and in the world,enter water bodies directly or indirectly,causing toxicity to fishes in aquatic ecosystems.An experiment was carried out to examine the single and joint toxicities of methomyl and phoxim to tilapia(Oreochromis niloticus).The 24 h,48 h,and 96 h-LC50of methomyl and phoxim to tilapia were 1.25,0.75 mg·L-1,and0.43 mg·L-1and 13.30,6.98 mg·L-1,and 4.58 mg·L-1respectively.Their safe concentrations to tilapia were 0.043 mg·L-1for methomyl and0.458 mg·L-1for phoxim,implying high toxicity of methomyl and phoxim to tilapia.Additive indexes of both pesticides were 0.24,0.26 and0.06 at 24 h,48 h and 96 h respectively.The activity of hepatic SOD was inhibited by methomyl and phoxim alone and in combination,with inhibitory effects being increased with pollutant concentrations.The joint effects of methomyl and phoxim were greater than their respective single.It is concluded that co-extistence of methomyl and phoxim in environemnt has greater toxicity to aquatic lives.