STUDIES ON THE MECHANISM OF INSECTICIDAL ACTION OF ORGANO‐PHOSPHORUS COMPOUNDS WITH PARTICULAR REFERENCE TO THEIR ANTIESTERASE ACTIVITY

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Abstract
The preparation of an extract of the mealworm larvae, Tenebrio molitor L. which hydrolyses ethyl butyrate and o‐nitrophenyl acetate, but not acetylcholine, is described. The inhibition of this esterase by TEPP‐containing materials and parathion was determined.An enzyme that hydrolysed o‐nitrophenyl acetate and was inhibited by a TEPP‐containing material was demonstrated in the five other insect species used.The relative toxicities as contact insecticides to adult Tribolium castaneum Hbst. of ten samples of TEPP‐containing materials were compared with their relative activities as esterase inhibitors. There was not an exact quantitative correlation between TEPP content estimated chemically, insecticidal activity and anti‐esterase activity; but the correlation was sufficient to suggest interdependence of these factors.Eggs of Diataraxia oleracea L. and Ephestia kühniella Zell, were shown to contain an enzyme that hydrolysed o‐nitrophenyl acetate and was inhibited by the TEPP‐containing materials. This enzyme was present in eggs less than 24 hr. old, i.e. before there was any visible signs of development. The TEPP was shown to be toxic to these eggs and in high concentrations kills at an early stage of development before differentiation of the nervous system. This, in conjunction with the other evidence, suggests that esterases other than the choline‐esterase of the nervous system are important when considering the toxic action of these compounds.Comparison of the anti‐esterase activity and toxicity of parathion and TEPP‐containing materials as insecticides showed that although the TEPP materials were the more potent enzyme inhibitors, parathion was the more potent contact insecticide to five species of insects. This appears to be due to the relative instability of TEPP. The study of the rates of action of the two poisons applied at different concentrations supports this view.