Persistence and degradation of chlorfenvinphos, chlormephos, disulfoton, phorate and pirimiphos‐ethyl following spring and late‐summer soil application

Abstract

AbstractGranular formulations of chlorfenvinphos, chlormephos, disulfoton, phorate and pirimiphos‐ethyl were broadcast at 2 kg a.i./ha and incorporated to 100 mm into a sandy‐loam soil either in May or in September 1971. The relative persistence of their residues, including insecticidally active oxidation products, after both application dates was disulfoton > chlorfenvinphos > phorate > pirimiphos‐ethyl > chlormephos. When applied in September all the insecticides persisted for longer than when applied in May. Degradation was slower during the winter while the mean soil temperature at 100 mm depth remained below 6 to 7°C. Rising soil temperature in the following spring rapidly increased the rates of degradation of chlorfenvinphos, chlormephos and pirimiphos‐ethyl residues but not of disulfoton‐ and phorate‐derived residues, predominantly the parent sulphones, which had been leached deeper. Ten months after the September application, half the total residues derived from disulfoton and phorate were found below the initial incorporation depth, whereas the other insecticides showed relatively little downward movement. It was concluded that late summer/early autumn applications for carrot fly control would leave appreciable residues of some insecticides in the soil at the beginning of the next growing season which may contribute to the terminal residues in crops. These would be minimised if moderately persistent compounds were used or if doses of persistent ones were substantially reduced.