OMETHOATE      JMPR 1975


         Omethoate was evaluated by the Joint Meeting in 1971 (FAO/WHO,
    1972). Since the previous evaluation, additional residue data have
    become available, and information on tolerances and pre-harvest
    intervals has been obtained from several countries.

         At the eighth session of the Codex Committee on Pesticide
    Residues, several delegations were of the opinion that the proposed
    temporary tolerances were too high. The Committee requested the Joint
    Meeting to consider residues of omethoate arising from the use of
    omethoate, dimethoate or formothion (Alinorm, 76/24, para. 150).

         Further data available since the previous review are summarized
    in this monograph addendum.



    Special studies on mutagenicity


         A dominant lethal study was performed with male mice to evaluate
    the potential effect of omethoate on spermatogenesis. Groups of male
    mice (20 mice/group) were administered omethoate at doses of 0 and 5
    mg/kg orally and mated to groups of 3 virgin females weekly for 8
    weeks over the period of spermatogenesis. Omethoate gave no suggestion
    of mutagenic hazard by this test at this acute dose as evidenced by
    pre- and post-implanatation mortality (Machemer, 1974).

    Special studies on neurotoxicity


         Groups of hens (10 hens/treatment group and 5 hens/positive
    control) were administered omethoate or ToCP orally in a study to
    evaluate the potential for delayed neurotoxicity. Omethoate was
    administered at the LD50 level (92 mg/kg) with atropine and ToCP was
    administered at a dose of 350 mg/kg orally. Although several hens died
    following omethoate, none showed clinical signs of delayed
    neurotoxicity. Clinical signs were observed with the ToCP group
    (Kimmerle, 1972). Histological examination of nervous tissue (using
    the H and E stain) showed degeneration in the ToCP treated group. No
    such evidence was noted with omethoate (Newman et al., 1972).

    Special studies on teratogenicity

         Groups of pregnant rats (20-24 rats/group) were orally
    administered omethoate at dosages of 0, 0.3, 1.0 and 3.0 mg/kg from
    day 6 to 15 of gestation. On the 20th day of gestation the animals
    were sacrificed and the foetuses examined for skeletal and tissue
    abnormalities. The foetuses and placentas of the 3.0 mg/kg group
    weighed less than the control. The other reproductive parameters were
    unaffected. No teratogenic effect was observed with respect to
    malformations of foetuses (Machemer, 1975).


         Omethoate was reviewed by the Joint Meeting in 1971 and a
    temporary ADI was estimated. It was recognized that omethoate is a
    metabolic of a common pesticide, dimethoate, for which an ADI had been
    allocated and on which several significant studies had been performed.
    It was also recognized that while animals exposed to dimethoate were
    also exposed to omethoate that long-term studies were needed to give
    an assurance of safety to the ADI. Studies were reported on
    neurotoxicity, mutagenesis and teratogenesis testing. No adverse
    effects were noted in these studies. No long-term studies on omethoate
    were reported, although information was received of appropriate
    studies in progress. Although the temporary ADI is currently based on
    short-term studies, the further studies reported gave the Meeting
    added assurance of safety to maintain the temporary ADI until
    long-term studies are complete.


    Level causing no toxicological effect

         Rat: 1.0 ppm in the diet equivalent to 0.05 mg/kg body weight

         Dog: 1.6 ppm in the diet equivalent to 0.12 mg/kg body weight

    Estimation of temporary ADI for man

         0-0.0005 mg/kg body weight



         Additional residue data, summarized in Table 1, are available on
    orange, hop, sugar beet, potato and apple (Bayer, 1975). Some of these
    are data required by the 1971 Joint Meeting (FAO/WHO, 1972).

        TABLE 1.  Omethoate residues in crops resulting from supervised trials


                   a.i. kg per     No. of          Days after             Residue (mg/kg)
    Crop           hectare         applications    last applic.
                                                                     Fruit     Peel      Total

    Orange         0.75                1                  7          0.04      0.7       0.25
                                                       ca50          0.12      0.07      0.11
                                                         66          0.03      0.05      0.03

    Apple          0.75                1                 39          0.10      0.14      0.11
                   1.28                1                  0                              \¾0.1
                                                          1                              <0.1
                                                          2                              <0.1
                                                          3                              <0.1

    Plum           1.28                1                  0                              <0.1
                                                          1                              <0.1
                                                          2                              <0.1
                                                          3                              <0.1


    Hop            1.5                 1                  0                54.4, 45.8
    (dried)                                               7                44.3, 23.5
                                                         14                10.4, 25.0
                                                         21                0.6, 2.3
                                                         28                0.6, 2.1
                   10                  1                161                0.05, 0.08

                                                                     Root     Leaves
    Sugar beet
                   0.4                 2                 21          n.d.  0.18, 0.48
                                                         49          n.d.  0.08, 0.025
                                                         72          n.d.  n.d., 0.03
                                                         93          n.d.  n.d.

                   0.6/0.5/0.4         3                 16                n.d. - 0.01
                                                         21                n.d. - 0.01
                                                         54                n.d. - 0.01
                                                         63                    n.d.
                                                         76                    n.d.
    Witloof chicory
                   1.28                1                  0                <0.1, 0.1
                                                          7                     <0.1

    In plants

         As mentioned previously (FAO/WHO, 1972) omethoate is relatively
    persistent in plants although oxons are usually hydrolyzed more
    readily than thionates. This fact should be borne in mind when the
    residues of omethoate derived from dimethoate and formothion in plants
    are evaluated. Radio-tracer studios showed that the residues arising
    from the use of dimethoate and formothion are identical.

         Data on residues in various crops resulting from treatment with
    formothion (Environment Agency, Japan, 1974) or dimethoate (Wit, 1972,
    Broeke and Dornseiffen, 1973) showed that formothion residues were
    always very low <0.01 mg/kg) and the main residue was generally
    dimethoate. Residues of omethoate derived from either insecticide were
    lower than those of dimethoate except in tomato and witloof chicory
    (Tables 2 and 3).

         Following the treatment of grapes with dimethoate (Steller and
    Brand, 1974), residues of dimethoate decreased from a maximum of 18
    mg/kg to about 0.5 mg/kg during a seven-week period, with no
    accumulation of omethoate (maximum of 0.3-0.4 mg/kg)

    In soils

         The persistence of omethoate applied to soils of three localities
    at a rate of 3 kg a.i./ha was measured. Half-life values of omethoate
    ranged from 3.8 to 25 days. A close correlation was found between the
    dissipation of omethoate and the rainfall during the experimental
    period, indicating that the water solubility of omethoate was one of
    the factors affecting the dissipation of omethoate in soils. (Bayer,

    In processing and cooking

         Witloof chicory treated with dimethoate was sampled 36 days after
    treatment and analysed before and after washing and cooking (Broeke
    and Dornseiffen, 1973). Washing alone removed about one third of both
    the dimethoate and omethoate, and washing and cooking together removed
    about two thirds of both residues (Table 4).


         Further information on changes in and additions to tolerances and
    pre-harvest intervals for omethoate was obtained from several
    countries (Table 5). Tolerances and pre-harvest intervals vary
    considerably from country to country for the same crop, owing to the
    differences in agricultural practice, food consumption patterns,
    climatic conditions, etc., all of which influence the rate of
    dissipation of omethoate residues.

    TABLE 2.  Residues of formothion, dimethoate and omethoate in crops treated with formothion


    Crop           Formulation    Application    Number of      Pre-harvest                     Residues (mg/kg)
                                  rate, %        treatments     interval (days)     formothion     dimethoate     omethoate

    Cabbage        EC             0.036                         8                   0.004          0.332          0.053
                                                 3              15                  <0.003         0.068          <0.003
                                                                22                  0.007          0.153          0.014

                                                                8                   <0.003         0.119          0.022
                                                 5              15                  0.008          0.216          0.038
                                                                22                  0.005          0.147          0.023

    Tomato         EC             0.036                         6                   <0.005         0.046          0.027
                                                 3              13                  <0.005         0.006          0.021
                                                                20                  <0.005         <0.005         0.015

                                                                6                   <0.005         0.081          0.042
                                                 5              13                  <  "           0.016          0.035
                                                                20                  <  "           <0.005         <0.005

    Eggplant       EC             0.036                         7                   <0.005         0.006          <0.005
                                                 3              14                  <  "           < "            <  "
                                                                21                  <  "           < "            <  "

                                                                7                   <0.005         0.028          0.008
                                                 5              14                  <  "           < "            <  "
                                                                21                  <  "           < "            <  "

    Orange         EC             0.045                         49                  <0.005         0.013          0.014
                                                 1              62                  <  "           0.023          0.008

                                                 2              49                  <0.005         0.016          0.016
                                                                62                  <  "           0.007          0.008


         Additional data on the residues of omethoate on sugar beets,
    potatoes, hops, witloof chicory, oranges, apples and plums have become
    available. Residues in dried hops remain high for two weeks following
    application, but by three weeks have declined considerably.

    TABLE 3.  Residues of dimethoate and omethoate in fruit and witloof
              chicory treated with dimethoate

              Application,  No. of       interval,        Residues, mg/kg
    Crop      kg a.i./ha    treatments   days          dimethoate   omethoate

    Apples       0.3           1           0           0.19, 0.20    <0.1

                                           1           0.09, 0.10    <0.1

                                           2           0.07, 0.29    <0.1

                                           3           0.07, 0.14    <0.1

    Plums        0.3           1           0           0.03, 0.19    <0.1

                                           1           0.01, 0.02    <0.1

                                           2           <0.01, 0.11  <0.1

    Witloof   0.5 g/m2         1          36           0.14          0.15
    chicory   sprayed on                  50           0.15          0.19
              top of roots
              before forcing

    TABLE 4.  Effect of washing and cooking on residues of dimethoate and
              omethoate in witloof chicory


                                 Residue level, mg/kg, in
                                                          Washed and cooked
    Residue           Raw chicory    Washed chicory       chicory

    Dimethoate          0.14              0.09                  0.05

    Omethoate           0.15              0.12                  0.06

    TABLE 5.  National tolerances and pre-harvest intervals


                                                  Tolerance,   Pre-harvest
    Country            Crop                          mg/kg     interval, days

    Australia          tomatoes, peppers             1.0

    ("recommended")    all other vegetables,
                       tree fruit                    2.0            21

                       pastures, forage crops
                       cereal crops, oil seed
                       crops                         0

                       citrus fruit, pome fruit      -              21

    Federal            beets
    Republic of        fruit and vegetables          0.4
    Germany            all other plant-food          0.05

    Finland            general                                      21

    France             fruit, vegetables             0.4
                       general                                      21

    Israel             citrus fruit                  0.4            30

    Netherlands        fruit and vegetables          0.6
    (dimethoate +      potatoes                      0.05
    omethoate from
    the use of

    (omethoate from    witloof chicory               0.4
    direct             other vegetables              0.1
    application)       fruit                         0.1

    South Africa       apricots                      -              28
                       peas, alfalfa, clover         -              7

    USA ("dimethoate   corn fodder and forage        1.0
    and its oxygen
    analogue")         corn grain (n.r.)             0.1
                       eggs and meat, fat and
                       meat byproducts of            0.02
                       cattle, goats, horses,
                       poultry, sheep and

    TABLE 5.  (Cont'd.)


                                                  Tolerance,   Pre-harvest
    Country            Crop                          mg/kg     interval, days

                       alfalfa                       2.0            10

                       cottonseed                    0.1            14

                       grapefruit, tangerines        2.0            15

                       grapefruit, tangerines
                       (dried citrus pulp for        5.0
                       cattle feed)

                       grapes                        1.0            28

                       lemons, oranges (dried
                       citrus pulp for cattle        5.0            15

                       safflower (seed)              0.1            14

                       sorghum (grain)               0.1            28
                       sorghum (forage)              0.2

    UK                 apples, hops                                 21

         Residues were below 0.2 mg/kg in the other commodities except in
    sugar beet leaves and orange peel, where they reached 0.5-1 mg/kg. The
    residue data on various crops following treatment with formothion or
    dimethoate showed that the main residue was generally dimethoate.
    Omethoate residues derived from either of these insecticides were
    always lower than those of dimethoate except in tomato and witloof
    chicory. Formothion residues were invariably below 0.01 mg/kg. The
    effect of cooking on omethoate residues was reported for witloof
    chicory, indicating that washing and cooking remained about two thirds
    of the original residue. Dissipation of omethoate in soils seems to be
    related to rainfall, indicating that leaching from soils takes place
    owing probably to high solubility in water.

         Omethoate is used in many situations where dimethoate and
    formothion are also used and it is a residue from treatment with
    dimethoate or formothion. For this reason, and because omethoate is
    determined by the analytical methods used for these parent compounds,
    the Meeting recommends that the limits previously recommended for
    dimethoate should apply also to omethoate. (The Joint Meeting has
    already recommended that residues arising from the use of formothion
    should be determined as dimethoate + omethoate and expressed as
    dimethoate). Some additional limits covering direct uses of omethoate
    are recommended below.


         The previously recommended temporary tolerances for omethoate are
    deleted and are replaced by temporary maximum residue limits identical
    to the tolerances previously recommended for dimethoate (FAO/WHO
    1973a, 1974a). The following additional recommendations for temporary
    maximum residue limits apply to both omethoate and dimethoate. They
    are based on residues likely to be found at harvest following the
    application of omethoate or dimethoate according to current use
    patterns. Residues of omethoate arising from treatment with dimethoate
    or omethoate are to be expressed as dimethoate. The recommendation of
    the 1972 Joint Meeting (FAO/WHO, 1973) that residues of formothion
    should be determined as dimethoate + omethoate and expressed as
    dimethoate remains in force.



    Commodity                          Limit (mg/kg,     Pre-harvest
                                       expressed         interval
                                       as dimethoate)    on which
                                                         are based

    Sugar beet (roots),
    potatoes                           0.05              21

    Sugar beet (leaves)                1                 21

    Grapes                             2                 21

    Hops (dried)                       3                 21


    REQUIRED (by 30 June 1978)

    1.   Long-term; studies in at least one species.


    1.   Information on residues occurring in food in commerce.


    Bayer, (1975) Unpublished report

    Environment Agency, Japan. (1974) Unpublished residue data.

    Broeke, R. T. and Dornseiffen, J. W. (1973) Residues of dimethoate and
    omethoate in Witloof in various stages of household preparation. KvW
    173/WRV (73) Food Inspection Service, Amsterdam,

    Kimmerle, G. (1972) Folimat / Akute Neurotoxizitätsuntersuchungen an
    Hühnern. Unpublished report from Institue für Toxikologie,
    Wuppertal-Elberfeld, submitted to the World Health Organization by
    Bayer A.G., FRG.

    Machemer, L. (1974) Präparat S 6876/ Dominant-Letal-Test an der
    männlichen Maus zur Prüfung auf mutagene Wirkung. Unpublished report
    from Institut für Toxikologie, Wuppertal-Elberfeld, submitted to the
    World Health Organization by Bayer A.G., FRG.

    Machemer, L. (1975) Präparat S 6876/ Untersuchung auf exbryotoxische
    und teratogene Wirkung nach oraler Verabreichung an der Ratte.
    Unpublished report from Institut für Toxikologie, Wuppertal-Elberfeld,
    submitted to the World Health Organization by Bayer A.G., FRG.

    Newman, A. J., Cherry, C. and Urwin, C. (1972) Pathology Report of
    Folimat Active Substances in Hens (Add. to Rep. No. 3439). Unpublished
    report from the Huntingdon Research Centre, England, submitted to the
    World Health Organization by Bayer A.G., FRG.

    Steller, W. A. and Brand, W. W. (1974) Analysis of Dimethoate-treated
    grapes for the N-Hydroxymethyl and De-N-methyl Metabolites and for
    their sugar adducts. J. Agr. Food Chem., 22(3): 445-449

    Wit, S. L. (1972) Residues of the insecticides dimethoate and
    omethoate in apples and plums. (Original in Dutch) Rep. 144/72.
    TOX-ROB. National Institute of Public Health, Netherlands.

    See Also:
       Toxicological Abbreviations
       Omethoate (WHO Pesticide Residues Series 1)
       Omethoate (Pesticide residues in food: 1978 evaluations)
       Omethoate (Pesticide residues in food: 1979 evaluations)
       Omethoate (Pesticide residues in food: 1980 evaluations)
       Omethoate (Pesticide residues in food: 1981 evaluations)
       Omethoate (Pesticide residues in food: 1984 evaluations)
       Omethoate (Pesticide residues in food: 1985 evaluations Part II Toxicology)