DEVELOPMENT AND DURATION OF ACCELERATED DEGRADATION OF NEMATICIDES INDIFFERENT SOILS

The development of accelerated degradation of nematicides was studied in incubation experiments with soils collected from trial plots at thr ee long-term experimental fields. The plots had been treated annually for periods of 3-10 years with aldicarb, oxamyl, ethoprophos, fenamiph os or 1,3-dichloropropene (1,3-D). At one site (sandy soil: pH 7.3) hi ghly accelerated degradation rates were measured for aldicarb, oxamyl and ethoprophos in soil from plots treated three times. After 5 years without further treatments, these high degradation rates were still pr esent for aldicarb and oxamyl, but not for ethoprophos. At another sit e (sandy soil; pH 5.6) little or no increase in the degradation rates of aldicarb, oxamyl and ethoprophos was measured in soil from plots tr eated 10 times. Three annual applications of fenamiphos to a loamy soi l (pH 7.7), doubled the rate of loss of the total toxic residue compar ed with the untreated soil. Fenamiphos and oxamyl did not show cross-e nhancement when incubated in ethoprophos-adapted soils. After a lag-ti me of 4-9 days degradation of 1,3-D was very rapid in all soils includ ing controls which had been treated only once or twice well before the sampling dates. Seven annual applications of 1,3-D resulted in only s lightly shorter lag-times. An interval of 5 years without treatment re sulted in only slightly longer lag-times. Simultaneous incubation of a commercial dose of the fumigant methyl isothiocyanate inhibited the f ast degradation of 1,3-D. (C) 1997 Elsevier Science Ltd.