Evidence and implications of accelerated degradation of organophosphorus insecticides in soil∗

The persistence of mephosfolan and phorate in soils treated previously with these insecticides and in previously‐untreated soils was studied in laboratory incubations. Accelerated loss of mephosfolan was induced by a single pre‐treatment. More than 95% of freshly‐applied mephosfolan degraded in previously‐treated hop‐yard soils after 8 weeks at 15°C compared with 23–35% in the corresponding previously‐untreated soils. In organic soils, total phorate residues declined more quickly in the previously‐treated soils than in similar untreated soils, largely because of more rapid degradation of phorate sulphoxide and, especially, of phorate sulphone. It is concluded that accelerated degradation of these insecticides probably limits their performance in some circumstances and that further studies are needed to determine the extent of the phenomenon.