A.A. Jukes

Accelerated degradation of aldicarb and its oxidation products in previously treated soils

Abstract The persistence of aldicarb, aldicarb sulphoxide and aldicarb sulphone was studied in soils used for crops grown commercially and in experimental soils with different histories of treatment with a granular formulation of aldicarb. Total aldicarb residues declined more rapidly in incubations in all the previously treated soils (initial 50% loss in 5–17 days at 15°C) than in the corresponding untreated soils (initial 50% loss in 25–50 days). This was largely because of more rapid breakdown of aldicarb sulphoxide and, to a lesser extent, of aldicarb sulphone. Accelerated degradation was induced more readily by pretreatment with carbofuran than with aldicarb or thiofanox and seemed to be influenced at least as much by dose level as by frequency of pretreatment. It is concluded that rotation of chemical usage should suppress development of the phenomenon with commercially recommended doses of aldicarb but that the potential for cross-enhancement is still little understood.

Non-specific influence of pH on microbial adaptation and insecticide efficacy in previously-treated field soils

Abstract 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-recommended dose of carbofuran or chlorfenvinphos. After harvesting this crop the efficacy of a second application of each insecticide was assessed 5 and 14 months after the initial application. Microbiological studies were done to determine the numbers of microorganisms able to degrade carbofuran, carbofuran phenol or chlorfenvinphos as the sole source of carbon. With both insecticides there were significant differences in behaviour, as well as in their biological performance against larvae of the cabbage root fly ( Delia radicum ) in the different plots. In previously-untreated soils, chlorfenvinphos was similarly and highly effective at all pH values, whereas the performance of carbofuran declined steadily with increasing pH. In the previously-treated soils, the efficacy of chlorfenvinphos was reduced only in the pH 7.5 soil, whereas that of carbofuran was much reduced at all pH values except pH 5.4. Microbiological studies showed that carbofuran and carbofuran phenol-degrading organisms were present in all the previously-treated soils, but that carbofuran phenol-degrading organisms were predominant in the soils with highest pH.

Stability of accelerated degradation of soil-applied insecticides: laboratory behaviour of aldicarb and carbofuran in relation to their efficacy against cabbage root fly (Delia radicum) in previously treated field soils

Abstract The behaviour and biological performance of aldicarb and carbofuran was studied over 2 years in soils from 18 sites within a single 200 ha farm. All the soils had been treated with a single application of the recommended dose of carbofuran at some time during the previous 5 years. In laboratory incubation experiments with freshly applied carbofuran, the time required for the rate of change of insecticide concentration to reach a maximum (initial lag phase) ranged from 1.2–1.6 weeks in soils treated 1 or 2 years previously to >4 weeks in soils treated 4 or 5 years earlier. Differences in the behaviour of freshly applied aldicarb in these soils were comparatively small. A simple model was developed to describe the degradation of carbofuran and to calculate the duration of the initial lag phase. In the field, log-dose evaluations of insecticide performance against cabbage root fly (Delia radicum) in the 18 soils showed a strong correlation between the efficacy of carbofuran and the interval between treatments. The most effective treatments were in the soil treated 5 years previously and in the previously untreated soil. A strong association between efficacy and the duration of the initial lag phase indicated that data from laboratory studies could be used to predict field performance. It was concluded that, after a single application of the recommended dose of carbofuran, at least 3 years should be left before re-applying the insecticide. There was no significant relationship between the performance of aldicarb and the interval following carbofuran application.

Some factors influencing the accelerated degradation of mephosfolan in soils.

Abstract Laboratory incubation studies of the stability of freshly applied mephosfolan in 42 hop farm soils showed no consistent correlation between persistence and treatment frequency. However, there was a highly significant correlation between soil pH and the development of accelerated degradation of the insecticide. Thus the 10 soils in which mephosfolan was most persistent had a pH > 5·6 and 13 of the 14 soils in which it was least persistent had a pH of 6·2 or more. Less than 10% of the applied mephosfolan dose was lost from aqueous buffer solutions of pH 5·–7·0 after 14 weeks at 23°C and studied with a previously unterated silty clay loam showed similar prolonged stability of mephosfolan at pH 4·4, 5·3, 6·2 and 7·5. Following a single pretreatment of these soils with 10 mg a.i./kg, rapid loss occurred only from the pH 7·5 soil. Although rates of loss from previously treated and untreated soils declined as concentrations of freshly applied mephosfolan were increased from 1 to 1000 mg a.i. kg −1 , there were differences in the stabilities of individual doses in the two soils. At 1 and 10 mg a.i. kg −1 , > 90% was lost from previously treated soil after 3 weeks, compared with only 60% and 20%, respectively, from previously untreated soil, whereas, at 100 mg a.i. kg −1 , rapid loss occurred only from the former soil after 4 weeks. Field application of the recommended dose to an untreated soil showed that, after 10 weeks, residues in the upper 15 cm of soil comprised 40% of the initial dose and that concentrations in the uppermost soil strata remained above or close to 100 mg a.i. kg −1 for at least 4 weeks after application.

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.

The accelerated biodegradation of phorate in carrot soils in the United Kingdom

Abstract In laboratory incubation studies with 32 farm soils, the initial half-life of freshly-applied phorate ranged from less than 1 to more than 16 weeks. Accelerated degradation was stimulated by a single application of the recommended dose and occurred most readily at pH levels 7.4. It is recommended that the degradative properties of soils are established before insecticide treatments are selected.