Metcalf_1963_Bull.World.Health.Organ_29_219

Previous experiments at the University of California Citrus Research Center and Agricultural Experiment Station indicated that compounds incorporating the methylthiophenyl group-such as the insecticide fenthion-were highly susceptible to oxidation. Since the stability of fenthion and the properties of its degradation products have an important bearing not only on the efficacy of the insecticide but also on its toxicity to mammals under operating conditions, studies on the chemical and biological behaviour of fenthion and its residues have recently been carried out at the Center.The activity of fenthion against susceptible and resistant houseflies and mosquitos was compared with that of its principal oxidation products. Most of these products were similar to fenthion in their activity against houseflies, but appeared to be much less water-stable since they were considerably less effective against mosquito larvae. Stability tests showed that, in the presence of sunlight and air, fenthion was completely transformed into its sulfoxide and sulfone oxidation products within one to three days, and that these products were rapidly decomposed to non-insecticidal compounds. Upon heating, fenthion was rapidly isomerized to the S-methyl isomer.