Inhibition of Cholinesterase and Ali-Esterase in Parathion and Paraoxon Poisoning in the House Fly

The inhibition of head and body cholinesterase and body ali-esterase was measured in susceptible and parathion-resistant house flies, Musca domestica L., treated either topically or by injection with parathion and paraoxon. Flies were treated with the toxicants only or with the toxicants after pretreatment with the organophosphate synergist DEF (S, S, S-tributyl phosphorotrithioate). Results showed that the inhibition of body cholinesterase provided the best measurement of the effects of the insecticides on treated flies. Inhibition of this enzyme was most closely correlated with the effects of the insecticides and was usually 90% or more at the time of knockdown. The inhibition of head cholinesterase, on the other hand, appeared to be primarily a function of the dosage of toxicants used and was usually not more than 50% at the time of knockdown. Ali-esterase inhibition was not closely correlated with the toxicity of the insecticides, but the importance of this enzyme in detoxication of paraoxon was clearly demonstrated.