Sponsored jointly by FAO and WHO


    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 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.


    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.


    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

    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.


    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


    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.

    See Also:
       Toxicological Abbreviations
       Methidathion (ICSC)
       Methidathion (WHO Pesticide Residues Series 2)
       Methidathion (WHO Pesticide Residues Series 5)
       Methidathion (Pesticide residues in food: 1992 evaluations Part II Toxicology)
       Methidathion (Pesticide residues in food: 1997 evaluations Part II Toxicological & Environmental)