PESTICIDE RESIDUES IN FOOD - 1979
Sponsored jointly by FAO and WHO
EVALUATIONS 1979
Joint meeting of the
FAO Panel of Experts on Pesticide Residues
in Food and the Environment
and the
WHO Expert Group on Pesticide Residues
Geneva, 3-12 December 1979
METHIDATHION
Explanation
Methidathion was evaluated in 1972 and 1975 and an ADI and MRLs for a
variety of fruits and vegetables were recorded.
The MRL of 2 mg/kg for citrus was based on data on different varieties
of citrus from many regions. These data included only two trials on
mandarins. Practical experience had shown that residues on mandarins
often exceed 2 mg/kg when the insecticide is used according to good
agricultural practice. These data were brought before the meeting to
consider whether higher MRLs should be established.
RESIDUES IN FOOD AND THEIR EVALUATION
In 1972 when the pesticide was first evaluated it was recognized that
with citrus fruit, methidathion residues are found exclusively in the
peel, and that in small citrus fruits such as lemons, higher residues
are recorded than in larger fruit, as a result of the difference in
surface/weight ratio. Although 92 trial results were available and
were used as a basis for determining an appropriate residue limit,
there were only two sets of data on mandarins.
Mandarin and tangerine hybrids have become important in international
trade, and methidathion is the insecticide of choice for controlling
red scale, an important and destructive pest of citrus crops which
also disfigures the fruit and which in the subject of quarantine
restrictions in many countries. Methidathion appears to gain its
effect from the fact that it is retained in the surface of the fruit,
in a biologically effective form over long periods. It is seldom
necessary to treat more than twice in a growing season which extends
well over 200 days.
Inspection officials have reported that mandarins imported from
Australia have contained methidathion residues above the Codex MRL of
2 mg/kg. Their findings are given in Table 1.
Table 1. Methidathion Residues in mandarins analysed at point of
Import (Finland, 1979)
Date Residue (mg/kg)
19.6.78 2.4
14.7.78 2.3, 2.4, 3.0, 2.4
4.9.78 2.5, 2.3, 2.9, 3.9, 2.4
7.9.79 3.1, 3.0
The Australian authorities who have monitored methidathion residues in
mandarins have found that 2 of 47 samples examined contain residues in
excess of 2 mg/kg. Investigations have revealed that these samples
came from orchards where the insecticide was applied in strict
accordance with the registered label (Snelson, 1979).
Table 2. Methidathion Residues in Mandarins - Results of Australian
Survey (Snelson, 1979)
Range (mg/kg) No. of samples
0.10-0.5 18
0.51-1.0 18
1.01-2.0 9
>2.0 2
Investigations were carried out to determine whether approved
agricultural practice in the use of methidathion for the control of
red scale on mandarins violated the Australian, Codex and JMPR maximum
residue limit of 2 mg/kg for methidathion residues on citrus.
Hamilton (1979) reports studies in which three mandarin orchards were
sprayed once, twice and three times with the approved concentration of
methidathion emulsion (0.05%) applied by standard orchard spray
equipment, and one orchard where the same quantity of spray was
applied per hectare but in the form of a more concentrated spray. The
results of these investigations are summarised in Table 3. The
results indicate that irrespective of whether one, two or three
treatments are applied by standard high volume spray booms, the
residue in the mandarins at harvest is generally close to, or
significantly above, the limit of 2 mg/kg. When the spray is applied
by low volume sprayer, applying approximately the same quantity of
insecticide per hectare, but in the form of a more concentrated spray
at the rate of 900 L/ha, the residues range well above 2 mg/kg
averaging 4.1 mg/kg in the particular trial.
Table 3. Methidathion Residues on Mandarins - Results of Supervised Trials
Number Days between Method of Residues mg/kg
of application Application Replicates Mean
Sprays and Harvest
1 193 (oscillating boom) 2.6, 2.1, 2.2, 1.7 2.1
2 193, 124 (applying 70-90 L per) 1.7, 1.9, 1.7, 2.0 1.8
3 193, 124, 93 (tree, 0.05% emulsion) 2.1, 2.0, 2.6, 2.3 2.3
2 223, 154 Low volume sprayer 5.6, 4.6, 3.1, 3.3 4.1
delivering 900 L/ha
Flesh from the mandarins from each of the four trials was also
analysed separately and the residue was found to be less than 0.05
mg/kg. This bears out the understanding that the residue is retained
exclusively in the flavido layer of the peel.
APPRAISAL
Experience has shown that the maximum residue limit of 2 mg/kg for
methidathion residues on citrus fruits is not adequate to cover the
residues which occur on mandarins which, because of their surface to
weight ratio, thin peel and high oil content of the flavido retain
methidathion for periods in excess of 200 days at levels above 2
mg/kg.
The residue level is significantly higher if the spray is applied by
means of low volume spray equipment not withstanding the fact that the
same amount of insecticide is applied to each tree.
Studies have confirmed that the residues remain exclusively in the
peel, the residue in the flesh being below the limit of determination.
RECOMMENDATION
The Meeting recommends that the maximum residue limit proposed for
methidathion on citrus fruit be modified as follows:
Citrus fruit (except mandarins) - 2 mg/kg
Mandarins - 5 mg/kg
REFERENCES
Finland. Finnish Customs Laboratory, Helsinki District Customs
Office. (1979) Various reports on analysis of imported mandarins.
Hamilton, D. (1979) Report on methidathion residue studies on
mandarins. Agricultural Chemicals Laboratory. Department of Primary
Industries, Brisbane. October 1979.
Snelson, J.T. Pesticide Residue Survey. Department of Primary
Industry, Canberra, Australia. November 1979.