NONSPECIFIC INFLUENCE OF PH ON MICROBIAL ADAPTATION AND INSECTICIDE EFFICACY IN PREVIOUSLY-TREATED FIELD SOILS

The influence of soil pH on the development, stability and expression of accelerated biodegradation of soil-applied insecticides was studied at a single site in four areas, with mean pH values of 5.4, 6.1, 6.3 and 7.5. A brassica crop was treated at sowing with the commercially-r ecommended dose of carbofuran or chlorfenvinphos. After harvesting thi s crop the efficacy of a second application of each insecticide was as sessed 5 and 14 months after the initial application. Microbiological studies were done to determine the numbers of microorganisms able to d egrade carbofuran, carbofuran phenol or chlorfenvinphos as the sole so urce of carbon. With both insecticides there were significant differen ces in behaviour, as well as in their biological performance against l arvae of the cabbage root fly (Delia radicum) in the different plots. In previously-untreated soils, chlorfenvinphos was similarly and highl y effective at all pH values, whereas the performance of carbofuran de clined steadily with increasing pH. In the previously-treated soils, t he efficacy of chlorfenvinphos was reduced only in the pH 7.5 soil, wh ereas that of carbofuran was much reduced at all pH values except pH 5 .4. Microbiological studies showed that carbofuran and carbofuran phen ol-degrading organisms were present in all the previously-treated soil s, but that carbofuran phenol-degrading organisms were predominant in the soils with highest pH. (C) 1997 Elsevier Science Ltd.