RESIDUE AND ANALYTICAL ASPECTS
Methamidophos is a widely used organophosphorus insecticide with systemic properties; its residues may also occur as a metabolite of acephate. It was first evaluated in 1976 with further reviews of residue aspects in 1979, 1981, 1984, 1989, 1990, 1994 and 1996. The 1994 JMPR recommended an MRL of 0.5 mg/kg for pome fruit, based on a 21-day PHI. It was held at step 7B by the 29th (1997) Session of the CCPR. The manufacturer has submitted new residue data to support the estimation of a maximum residue level for pome fruit.
The analytical methods employed in supervised trials were based on GLC. Recoveries were >70% and the LOD in all the methods was 0.01 mg/kg.
Studies of the storage stability of residues in several commodities were included in the studies of the stability of acephate residues evaluated in 1996 but no studies of the stability of methamidophos on apples were submitted.
Trials conducted in France and Italy in 1992 on apples and pears were evaluated by the 1994 JMPR and again reviewed by the present Meeting. In three trials on apples according to French GAP (1-2 applications at 0.5 kg ai/ha, 21-day PHI) the residues were 0.01, 0.06 and 0.1 mg/kg. The residue of 0.06 mg/kg was at 28 days; the residue at day 21 was 0.04 mg/kg. Three Italian trials on apples carried out with two applications of methamidophos (Italian GAP allows one) at 0.049 kg ai/hl were evaluated against Greek GAP (1 or 2 applications at 0.045-0.06 kg ai/hl, 21-day PHI). The residues were 0.02, 0.29 and 0.33 mg/kg.
The residues in three trials on apples according to GAP in Greece in 1995/96 were 0.31, 0.4 and 0.49 mg/kg. In similar trials in Spain in 1995 the residues were somewhat lower: 0.08, 0.14 and 0.24 mg/kg at a 21-day PHI. As the application rates were higher than in Spanish GAP the results were evaluated against Greek GAP.
Several of these trials were designed to produce residue decline curves. They showed that when methamidophos was applied twice with an interval of 3 weeks most of the residues resulted from the second application.
Two trials on apples in Israel in 1970 gave residues of 0.04 and 0.23 mg/kg at a 19-day PHI. Since no relevant GAP was reported these results were not considered for the estimation of a maximum residue level.
In a trial in which apples were treated with acephate at an application rate of 1.55-1.6 kg ai/ha the residue of methamidophos at a 15-day PHI was 0.03 mg/kg.
The residues of methamidophos in apples in rank order from the 12 trials according to GAP were 0.01, 0.02, 0.06, 0.08, 0.1, 0.14, 0.24, 0.29, 0.31, 0.33, 0.4 and 0.49 mg/kg.
Pears treated with methamidophos in France in 1992 according to GAP showed residues of 0.15 and 0.21 mg/kg after 21 days.
In view of the identical use patterns on apples and pears the Meeting agreed to evaluate the combined data as applying to pome fruit.
The residues of methamidophos in apples and pears in rank order (median underlined) were 0.01, 0.02, 0.06, 0.08, 0.1, 0.14, 0.15, 0.21, 0.24, 0.29, 0.31, 0.33, 0.4 and 0.49 mg/kg.
The Meeting agreed to confirm the previously estimated maximum residue level of 0.5 mg/kg, and estimated an STMR of 0.18 mg/kg for methamidophos in pome fruit. The Meeting expressed its concern at the long period of storage of many of the samples and the lack of data on the stability of residues during storage, but noted that methamidophos was scheduled for periodic review in 2002.
FURTHER WORK OR INFORMATION
1. Information on methamidophos residues in processed apples.
2. Data on the storage stability of residues of methamidophos for the full duration of studies to be submitted for periodic review in 2002.