WHO Pesticide Residues Series, No. 1



    The evaluations contained in these monographs were prepared by the
    Joint Meeting of the FAO Working Party of Experts on Pesticide
    Residues and the WHO Expert Committee on Pesticide Residues that met
    in Geneva from 22 to 29 November 1971.1

    World Health Organization



    1 Pesticide Residues in Food: Report of the 1971 Joint Meeting of
    the FAO Working Party of Experts on Pesticide Residues and the WHO
    Expert Committee on Pesticide Residues, Wld Hlth Org. techn. Rep.
    Ser., No. 502; FAO Agricultural Studies, 1972, No. 88.

    These monographs are also issued by the Food and Agriculture
    Organization of the United Nations, Rome, as document AGP-1971/M/9/1.

    FAO and WHO 1972



    Chemical names

    dimethyl S-methylcarbamoylmethyl phosphorothiolate.

    O,O-dimethyl S-(N-methylcarbamoylmethyl) phosphorothioate.

    O,O-dimethyl S-(2-oxo-3-aza-butyl)-monothiophosphate.


    Folimat (R), Bayer 45, 432, S 6876, P-O-dimethoate.

    Empirical formula

    C5H12NO4PS (213.1)

    Structural formula


    Physical and chemical properties

    Colourless to slightly yellowish oil, b.p. ca. 135°C (decomposes when
    distilled); v.p. 2.5 × 10-5 mm Hg at 20°C; volatility at 20°C, 0.29
    mg/m3; d20 1.32; nD20 1.4987; readily soluble in water,
    alcohol and acetone; slightly soluble in ethyl ether; almost insoluble
    in petroleum ether. Stability to hydrolysis: half-life period, 611
    hours at pH 7 and 24°C. The velocity of decomposition of the active
    ingredient is essentially greater at higher temperatures or at pH
    values 8-10.


    Composition of a typical technical omethoate:

        content of active ingredient                         94.0 - 96.0%
        O-methyl S-methylcarbamoylmethyl 
                       phosphorothiolate                       1.0 - 2.0%
        dimethylphosphate                                      1.0 - 2.0%
        1,2-dichloroethane                                      max. 3.0%


    Biochemical aspects


    Dauterman et al. (1959) treated rats orally with radioactive
    omethoate. The urine from two male rats treated with a dose of 50
    mg/kg was collected at 12, 24 and 48 hours. The cumulative percentages
    of the administered radioactivity excreted over the indicated times
    were 16, 19 and 30. Utilizing ion exchange chromatography the
    metabolites found in a 24 hr/urine composite were:

         O,O - dimethyl phosphoric acid            34%

               Unknown A                           52%

         O,O - dimethyl phosphorothioic acid       9.5%

               Unknown B                           4.5%

    Following treatment of male rats with dimethoate, 81% of the
    administered dose was excreted in the urine in 24 hours, while
    following treatment with omethoate, only 19% was excreted in 24 hours
    (Dauterman et al., 1959).

    Apart from this preliminary study the biotransformation of omethoate
    has not been examined although the fate of dimethoate, the
    phosphorodithioate which is oxidized in vivo to omethoate, is well
    documented (FAO/WHO, 1968). It is probable that the metabolic route of
    omethoate will follow that observed for dimethoate in plants and
    animals although the rate of the individual reactions may differ as
    indicated in the study on male rats. In animals metabolism is
    oxidative and hydrolytic and should yield compounds as follows:


    (For reference to dimethoate metabolism in animals see: Hassan et al.
    (1969), Lucier and Menzer (1970), FAO/WHO (1968), Brady and Arthur
    (1963), and Dauterman et al. (1959); and in plants see: Morikawa and

    Saito (1966) and Lucier and Menzer (1968; 1970).) Apparently all
    metabolites of dimethoate observed in mammals are found in plants. In
    plants a major reaction pathway includes demothoxylation which yields
    a product that is not a major metabolite in mammals. (Morikawa and
    Saito, 1966). However, in vitro studies with dimethoate using liver
    and insect homogenates and in vivo studies with insects have shown
    the presence of the dimethoxyl derivative. Dealkylation of omethoate
    is a significant detoxication mechanism as evidence by fly head
    cholinesterase bimolecular rate constant K1 values for omethoate of
    9.2 × 10-5 L mole-1min-1 and 1.39 L mole-1min-1 for
    demethoxylomethoate (Aharoni and O'Brien, 1968). Oxidative metabolism
    of omethoate results in the de-N-methyl derivative which is as toxic
    as the parent compound although less active as a cholinesterase
    inhibitor (Lucier and Menzer, 1970). Hassan et al. (1969) whilst
    investigating the metabolic fate of dimethoate in the rat concluded
    that oxidation to omethoate occurred in vivo. They suggested two
    major metabolic pathways for both dimethoate and omethoate. The first
    involved cleavage of the C-N bond by a carboxy amidase, while the
    second proceeded through esterase action on the S-C bond. Kinetic data
    indicated that reaction between acetycholinesterase and omethoate was
    irreversible and bimolecular. Omethoate was found to be 75-100 times
    more potent than dimethoate in inhibiting rat brain

    Effects on enzymes and other biochemical parameters

    Omethoate is a direct inhibitor of acetyl cholinesterase from various
    sources. Sensitivity to inhibition is greater with invertebrate than
    with mammalian sources. This is reflected in the omethoate bimolecular
    rate constant (Ki) for rat brain of 1.65 × 10-3 L mole-1min-1
    (Hassan et al., 1969) and house fly head of 9.2 × 10-5
    L mole-1min-1 (Abaroni and O'Brien, 1968) as well as the 150 valve
    (molar concentration of omethoate producing 50% inhibition of enzyme
    activity) as seen in the following table:

    Enzyme source       I50 (M)             Reference
    Rat brain           1.2 × 10-5          Hassan et al., 1969
                        1.1 × 10-5          Sanderson and Edson, 1964

    Bovine (RBC)        3.9 × 10-5          Santi and Pietri-Tonelli, 1969

    Bovine liver        5.7 × 10-5          Villeneuve and McKinley, 1968

    Human plasma        6.3 × 10-5          Lucier and Menzer, 1970
                        1.2 × 10-4          Sutherland, 1962

    Housefly            1.7 × 10-7          Santi and Pietri-Tonelli, 1969
    Following oral administrations of dimethoate to a cow at 10 mg/kg, red
    blood cell cholinesterase was depressed in two stages; for six days
    inhibition was constant at less than 20% and then slowly declined to
    40% at 12 days (Dauterman et al., 1959), This biphasic inhibition was
    also noted in sheep by Beck et al. (1968) who found that
    cholinesterase activity continued to decrease for periods of time
    after dimethoate treatment stopped. This is apparently due to slow
    release of dimethoate and/or conversion rate of dimethoate to

    Following acute oral administration to rabbits (20 mg/kg) various
    enzymatic tests for liver function were not affected. These include:
    serum glutamic oxaloacetic transaminase (SGOT), serum glutamic pyruvic
    transaminase (SGPT), serum sorbitol dehydrogenase (SDH) and
    bromophthalein test (Kimmerle, 1962).


    Special studies

    (a)  Toxicity of the metabolites

    Compound                Species   Route   LD50       150 (M)           pI50
                                              (mg/kg)    (human plasma)
    Dimethoate              Mouse     ip      151        >0.1              <1
    N-hydroxymethyl P(S)    n.d.                         >0.1              <1
    Des-N-methyl P(S)       Mouse     ip      190        >0.1              <1
    Omethoate               Mouse     ip      13         6.5 × 10-5        4.2
    N-hydroxymethyl P(O)    n.d.                         2.0 × 10-4        3.7
    Des-N-methyl P(O)       Mouse     ip      10         4.0 × 10-4        3.4
    n.d. = not determined.
    Data from Lucier and Menzer (1970).
    Deamination of omethoate results in a significantly less toxic
    compound than omethoate. Carboxyomethoate ((CH3O)2 P(O)
    (SCH2C(O)OH)), the deamination product of omethoate, was fed to rats
    for 33 days at levels of 0, 330, 1000 and 3000 ppm in the diet. Plasma
    cholinesterase activity was not affected. Red blood cell
    cholinesterase was depressed at 1000 and 3000 ppm in both sexes and to
    a slight degree in female rats at 330 ppm. Brain cholinesterase was
    slightly, but significantly decreased in female rats at 3000 ppm,
    Weight gain and food intake was depressed at 3000 ppm. No effects were
    noted on survival, appearance, organ weights or histological
    examination of organs and tissues (Levinskas and Shaffer, 1965a).

    (b)  Potentiation

    Omethoate was observed to influence the acute oral toxicity of
    malathion in rats. When administered at 1/2 LD50 levels the
    mortality observed was slightly greater than theoretically anticipated
    (Kimmerle and Lorke, 1967). Pretreatment of rats with phenobarbital
    resulted in a threefold increase in the acute toxicity of omethoate
    (Menzor and Best, 1968).

    (c)  Reproduction

    Eighty male and 80 female rats were administered omethoate orally at a
    daily dose of 0.5 mg/kg for six weeks and at the conclusion of
    treatment they were mated. The pups were raised on normal diets for
    four weeks after weaning and subsequently treated in a similar manner
    for six weeks as the Fo generation. Daily oral treatment for six
    weeks to the Fo generation had no effect on reproduction and the
    same treatment did not grossly affect the F1 generation. Red blood
    cell cholinesterase levels during the administration of omethoate
    remained constant at 40-50% of normal while the plasma cholinesterase
    was unaffected. This treatment did not affect growth of Fo or F1.
    (Kimmerle, 1969).

    (d)  Neurotoxicity

    Hens were dosed with 50-200 mg/kg of omethoate by intraperitoneal
    injection under the protection of 2-PAM and atropine. Of 17 hens
    treated, 14 were given 150 mg/kg and five died of acute intoxication.
    The hens which survived were kept under observation for six weeks and
    no signs of neurotoxic effect was observed (Kimmerle, 1962).

    Hens (six hens/group) were fed omethoate at 0, 60, 120 and 240 ppm for
    four weeks. At the end of the study three hens were sacrificed and
    examined histologically (brain, thoracic spinal cord and sciatic
    nerve) for myelin degeneration. The remainder were maintained on
    normal diets for four weeks. No evidence was found, clinically or
    histologically, for delayed neurotoxicity or demyelination (Levinskas
    and Shaffer, 1965b).

    (e)  Antidotes

    Administration of atropine alone and in combination with 2-PAM or BH-6
    was effective in reducing the oral LD50 values for rats (Kimmerle,
    1962; Kimmerle and Lorke, 1967). Administration of 2-PAM or BH-6 alone
    was not effective although TMB-4 alone reduced the acute toxicity. The
    optimum antidotal effects were noted with combinations of atropine and
    oxime reactivators. At the first signs of poisoning with omethoate,
    2-PAM or toxogonin and/or atropine were effective therapeutic agents
    increasing the LD50 of omethoate from 50 to 200% (Lorke and
    Kimmerle, 1969).

        Acute toxicity

    Animal       Sex          Route        LD50           Reference
    Mouse        M            Oral         36             Kimmerle, 1968
                                           27             Santi and Pietri-Tonelli, 1969
                              Ip           13             Lucier and Menzer, 1970
                              Iv           23             Kimmerle, 1962

    Rat          M and F      Oral         28-65          Kimmerle and Lorke, 1967
                                                          Dauterman, 1959
                                                          Kimmerle, 1966; 1968
                                                          Kimmerle, 1962
                                           50             Ben-Dyke, 1970
                 M            Ip           14             Kimmerle, 1968
                              Ip           38             Kimmerle, 1962

    Rabbit                    Oral         50             Kimmerle, 1962

    Cat                       Oral         50             Kimmerle, 1962

    Guinea-pig                Oral         100            Kimmerle, 1962

    Hen                       Oral         125            Kimmerle, 1962
                                           100            Levinskas and Shaffer, 1965b

    Signs of poisoning are typical of cholinergic stimulation as elicited
    by other organophosphorus esters. The signs appear in from 5-60
    minutes following poisoning and include salivation, lacrymation,
    tremors, etc. The signs of poisoning may persist for one to three days
    following intoxication (Kimmerle, 1968).

    Rats (groups of three males per dose) were orally administered
    omethoate at 2.5, 5 and 10 mg/kg. cholinesterase assay indicated
    maximal depression at three hours (first interval examined after
    exposure) with recovery in one to three days (Kimmerle, 1962),

    Groups of rats (five male and five female/group) were orally
    administered omethoate at a single dose of 0, 0.1, 0.5, 2.5, 5.0, 10.0
    and 25 mg/kg and examined one, three, and seven days after treatment
    for cholinesterase inhibition. Red blood cell cholinesterase was
    depressed to a greater extent and duration than plasma. The response
    of the RBC cholinesterase was the same in both sexes. With plasma,
    females were more susceptible than males. Plasma was completely
    recovered at all dose levels by day seven from a maximum of 67%
    inhibition at day one at 25 mg/kg (female). Red blood cell
    cholinesterase was still depressed at the conclusion of the test
    (seven days) at doses of 10 mg/kg and above. Maximum inhibition
    observed at 10 mg/kg on day one was 63%. The higher dose did not give
    a greater reduction of activity (Kimmerle, 1969).

    Short-term studies


    Rats (20 males) were administered omethoate orally for five days/week
    for eight weeks (42 doses) at 5 mg/kg/day. Tremors were transient
    after each application. Cholinesterase was depressed 50-70% of normal
    and quickly recovered after treatment ended (Kimmerle, 1962).

    Rats (10 males per group) were orally administered omethoate at doses
    of 0, 1, 2, 4, 8 and 16 mg/kg, five days/week for eight weeks. The
    survivors were sacrificed four weeks after treatment ended. Mortality
    occurred at 8 and 16 mg/kg. Cholinergic symptoms evident at 4 mg/kg,
    decreased as time progressed. Slight trembling was observed at the
    lowest doses. Organ weights, growth and gross and microscopic
    examination of tissues were not affected (Kimmerle, 1962).

    Groups of rats (10 male and 10 female per group) were administered
    omethoate orally for 14 days at doses of 0.1 and 0.5 mg/kg/ day.
    Plasma cholinesterase was not depressed during the 14-day interval.
    Red blood cell cholinesterase was slightly depressed in males and
    females at 0.5 mg/kg and this was maintained for the 14-day period. No
    differences in sex were observed with regard to red blood cell enzyme
    inhibition (Kimmerle, 1969).

    Rats (10 male and 10 female per group) were fed omethoate (which was
    mixed with the feed every day) for four weeks at 0, 2.5 and 15 ppm.
    Growth was temporarily depressed at 15 ppm and cholinesterase was
    depressed at both feeding levels. Red blood cell cholinesterase
    activity was inhibited at 2.5 ppm and plasma at 15 ppm with the female
    rats showing the higher sensitivity of plasma cholinesterase (Loser,

    Rats (15 males and 15 females per group, 30 of each sex were controls)
    were fed omethoate at 0, 2.5, 5, 15, 50 and 150 ppm for four months.
    Cholinergic stimulation was evident at 15 ppm and above.
    Cholinesterase was depressed at 50 ppm and 150 ppm in females and 5
    ppm and above in males. No effects were noted on growth, organ
    weights, blood parameters or urinalysis at the feeding levels
    including 50 ppm. At 150 ppm some animals died, the bodyweight and
    food consumption was depressed and the relative liver weight in males
    was increased (Loser and Lorke, 1967).

    Groups of rats (80 male and 80 female per group) were administered
    omethoate daily at an oral dose of 0.5 mg/kg for six weeks. RBC
    cholinesterase was inhibited similarly in males and females at 40-50%
    of normal. When placed on a normal diet for two weeks the enzyme
    recovered to normal values. No significant adverse effects were
    observed on growth or plasma cholinesterase activity (Kimmerle, 1969).

    Rats (15 male and 15 female per group) were fed omethoate (mixed with
    feed daily) at levels of 0, 0.5, 1.0, 2.0 and 4.0 ppm for three
    months. Clinical signs of cholinergic stimulation were evident at 4
    ppm. Cholinesterase (red blood cell) was depressed at 2 ppm though in
    females only slightly. At 4 ppm the inhibition was 30-50%. No effects
    were noted on growth, food consumption, blood parameters, liver and
    kidney function tests, organ weights (Loser, 1968a) and histological
    examination of tissues (Vince and Spicer, 1971).


    Dogs (four male and four female per group) were fed omethoate for 14
    weeks at dietary concentrations of 0.4 ppm, 0.8 ppm and 1.6 ppm. No
    adverse effects were observed on red blood cell and plasma
    cholinesterase activity or other parameters measured including:
    growth, survival, food consumption, blood analyses (haematocrit,
    haemoglobin and total and differential leukocyte counts) and clinical
    chemistry (alkaline phosphatase, glucose, urea nitrogen and
    bromsulphthalein (BSP) retention) (Hutchison et al,, 1968).

    Long-term studies

    No data available.


    Omethoate is an organophosphorus insecticide with a high acute
    toxicity. Its toxicity is considerably higher than is phosphorothioate
    analogue dimethoate, which was evaluated in 1967 and for which an
    acceptable daily intake has been established. Although dimethoate is
    converted to omethoate, quantitative data on the rate of conversion
    following administration have not been reported.

    Adequate short-term studies are available in both rats and dogs. The
    lack of an acceptable reproduction study is partly offset by the
    existence of an acceptable study of this kind in the case of
    dimethoate. No long-term studies have been reported and there are no
    observations in man comparable with those for dimethoate.

    In view of the information available from dimethoate the Meeting
    agreed that a temporary ADI for omethoate could be established based
    on the 90-day study on omethoate in the rat. It was stressed however,
    that long-term studies are required.


    Level causing no toxicological effect

    Rat: 1.0 ppm in the dry diet equivalent to 0.05 mg/kg bodyweight per

    Dog: 1.6 ppm in the moist diet equivalent to 0.12 mg/kg body-weight
         per day

    Estimate of temporary acceptable daily intake for man

    0-0.0005 mg/kg body-weight


    Use pattern

    It is a systemic, organophosphorus insecticide and acaricide with a
    broad sphere of action and good plant compatibility. It acts against
    both sucking and biting insect pests and is active against spider
    mites which are resistant to some other organophosphorus pesticides;
    in this respect it differs from dimethoate, of which it is the oxygen
    analogue. Its chief uses are for pre-harvest treatments of:

         fruit (pome, stone and citrus)                      65%
         field crops (pasture, cotton, sugar cane, etc.)     25%
         vegetables                                           5%
         miscellaneous (e.g. ornamentals)                     5%

    It is available as 50% and 80% w/v emulsifiable concentrates and is
    applied at about 0.025 to 0.075% w/v of active ingredient, equivalent
    to up to 1500 g/ha a.i.

    Residues resulting from supervised trials

    The behaviour of omethoate on and in plants is characterized by a rate
    of degradation which is relatively slow for an organophosphorus
    pesticide. It seems that the surface condition of the crop, perhaps in
    association with weather factors, can have a bearing on this matter.
    Information on available residue data is given in Table 1 (Bayer,
    1971). The application rates are expressed in percentage concentration
    of active ingredient; the normal volume applied being about 2000 l/ha
    for fruit crops and between 600-1000 l/ha for vegetable crops. Data on
    the behaviour of residues in storage and processing and on the level
    of residues in food moving in commerce are not available. Residues of
    omethoate were not observed during a total diet study for
    organophosphorus pesticide residues (Abbot et al, 1970).

    Crop           a.i.%      No. of         Weeks after       Residues found
                   w/v        applications   last                 (ppm)
                                             application     Range          Mean
    Apples         0.03       1              0               0.66-1.0       0.8
                                             1               0.16-0.4       0.3
                                             2               0.04-0.2       0.1
                                             3               0.01-0.14      0.07

    Apples         0.05       3              0               1.55-2.07      1.75
                                             1               0.32-0.6       0.45
                                             2               0.11-0.17      0.15
                                             3               -              0.10
                                             4               0.07-0.09      0.08
                                             5               -              <0.05

    Apples         0.064      1              0               -              3.3
                                             1               -              1.9
                                             2               -              1.6
                                             3               -              1.2
                                             4               -              0.4

    Apples         0.075      2              0               2.3-5.2        3.75
                                             2               1.1-2.4        1.62
                                             3               0.75-1.3       1.02
                                             4               0.55-1.4       0.95
                                             5               0.5-1.1        0.70
                                             6               0.25-0.85      0.51

    Pears          0.064      1              0               -              2.15
                                             1               -              2.0
                                             2               -              1.45
                                             3               -              1.05
                                             4               -              0.9

    TABLE I. (Continued)
    Crop           a.i.%      No. of         Weeks after       Residues found
                   w/v        applications   last                 (ppm)
                                             application     Range          Mean
    Pears          0.075      2              0               4.05-6.4       5.5
                                             2               1.7-3.05       2.2
                                             3               0.5-2.25       1.1
                                             4               0.35-1.3       0.9
                                             5               0.17-1.1       0.55
                                             6               0.16-1.1       0.4

    Pears          0.075      2-3            0               2.4-6.7        4.05
                                             1               2.2-3.8        3.0
                                             2               1.4-1.5        1.45
                                             3               1.3-1.6        1.5
                                             4               0.8-1.3        1.05

    Apricots       0.075      1              0               -              3.1
                                             1               -              1.8
                                             2               -              1.8
                                             3               -              0.4

    Cherries       0.05       1              0               -              1.1
                                             2               -              0.2

    Grapes         0.05       1              0               -              5.7
                                             1               -              3.9
                                             2               -              2.35
                                             3               -              1.8
                                             4               -              1.25

    Peaches        0.075      1              0               3.3-7.3        5.3
                                             1               2.6-6.2        4.4
                                             2               1.8-3.8        2.8
                                             3               1.4-1.7        1.55

    Plums          0.075      1              0               0.7-0.85       0.8
                                             1               0.7-1.05       0.85
                                             2               0.6-0.9        0.75
                                             3               0.6-0.75       0.68
                                             4               -              0.5

    Lettuce*       0.03       1              0               3.3-3.7        3.5
                                             1               0.3-0.9        0.6
                                             2               0.11-0.17      0.15
                                             3               -              0.03

    Potatoes*      0.03       2              2               n.d.-0.21      0.11

    TABLE I. (Continued)
    Crop           a.i.%      No. of         Weeks after       Residues found
                   w/v        applications   last                 (ppm)
                                             application     Range          Mean
    Sugar beets*   0.03       1              2               -              1.4
                                             3               -              0.1
                                             7               -              0.1

    Sugar beets*   0.03       1              0               -              2.7
    leaves                                   2               -              0.3
                                             3               -              0.23
                                             7               -              0.02

    Hops (dried)   0.05       1              0               -              35
                                             2               -              6.8
                                             3               0.5-6.5        3.5
                   0.05       5-6            2“              3.8-11.7       7.8
                                             6               -              0.07
                   0.1        1              2               -              9.4

    Note: slight underdosages used on these crops.
    Fate of residues

    In animals

    Beck et al. (1968) fed 0.50 mg/kg dimethoate and 0.05 mg/kg of
    omethoate for 14 days to cattle but found no residues in the milk.
    When double these amounts were fed for an additional 14 days, residues
    of omethoate ranging from 0.004 to 0.125 ppm were detected but
    dimethoate was still absent; three days after ceasing the treatment no
    omethoate was detected in the milk. See also "Biochemical aspects".

    In plants

    Dauterman et al. (1960) studied the metabolism of 32P-dimethoate
    (surface and absorbed residues) after foliar treatment of corn,
    cotton, pea, and potato plants. The amount of dimethoate, omethoate
    and water-soluble metabolites on the surface of the plants was
    investigated two and 12 days after treatment. With the longer time
    interval there was a decrease in the total amount of omethoate on the
    surface but an increase in the proportionate amount with respect to
    dimethoate. The compound (4) (see Fig. 1) accounted for the largest
    proportion of the water-soluble metabolites (up to 94%). Inside the
    leaf, there was also a decrease with time in the total amount of
    omethoate but a proportionate increase with respect to dimethoate.
    Here, compound (4) accounted for no more than 10% of the water-soluble

    Hydrolisis at the alkoxy group occurred mainly inside the leaves,
    indicating an enzymatic action at this site, which also held for
    omethoate. As none of the thio-carboxy derivative (4a) was found in or
    on plants, formation of (4) probably proceeded by oxidation of
    dimethoate to omethoate followed by hydrolysis of the carbamoyl group.
    As (4) was found in the largest amount on the surface, this cleavage
    was probably non-enzymatic. This is an important indication of the
    mode of transformation of omethoate on the plant. However, these
    considerations are not supported by the observations of Hacskaylo and
    Bull (1963) who, using the excised-leaf technique, found large amounts
    of (4a) as a metabolite whilst the presence of even slight amounts of
    (4) could not be established with any certainty.

    Lucier and Menzer (1968, 1970) studied the metabolism of dimethoate on
    bean plants. The most important neutral metabolites which they found
    were compound (3) and an unidentified compound (X) although only in
    small amounts as compared with omethoate, as is shown below
    (proportions expressed in relation to omethoate = 1X%).

                           Days after treatment
                           1        3        6

                   (2)    1.0%     0.4%     0.4%
                   (3)    7.7%     7.2%     5.7%
                   (X)    29.0%    5.0%     2.2%

    The ratio of phosphorodithioates to phosphorothioates in the
    N-demethylation scheme was approximately 1:1, possibly indicating a
    reduced rate of hydrolytic metabolism for the desulfurated compounds.
    However, the small proportions of the compounds (X), (2), and (3), in
    relation to omethoate, suggests that these compounds had merely a
    transitory character. It is interesting to note that compound (4) was
    not found in these studies.

    The simple phosphates and thiophosphates (6) to (11) have also been
    identified as metabolites in plants, or presumed to be present, after
    application of dimethoate:

    (6), (7), (11); Dautermann et al. (1960); Hacskaylo and Bull (1963)
    (6); Rowlands (1966); Lucier and Menzer (1968)
    (8), not significant; Hacskaylo and Bull (1963)
    (8), (9), (10), (11) not significant; Looter and Menzer (1968)

    In vitro hydrolysis of omethoate by grain extracts (Rowlands, 1966)
    provided clear evidence of the formation of (4) in addition to (6);
    compound (5) was also found.

    FIGURE 1

    The behaviour of omethoate in the plant is characterized by a
    relatively high degree of persistence, which at first seems surprising
    in view of the fact that excuse usually hydrolyze more readily than
    thionates. In actual fact, omethoate hydrolyzes at pH 6 and 25°C about
    four times faster than dimethoate (Table II). Omethoate was found to
    have a biological half-life of 9.3 days as against 3.2 days for
    dimethoate in tomato plants which were placed in a solution of
    32P-labelled compound for 24 hours and then held in distilled water
    for 14 days (Grimmer et al., 1968b). Confirmation of this relatively
    slow breakdown of omethoate on and in plants has been obtained
    following field application of the compound.

    (GRIMMER ET AL., 1968a)
    pH        Dimethoate     Omethoate    (4a)+       (4)+
    2         176            62           2           0.02

    4         160            74           14          0.33

    6         122            32.5         124         10

    8         38             2.8          20          2.25

    9++       5.8            0.3          -           -

    + The acids (4a) and (4) are stabilized by formation of
      salt in neutral and weakly alkaline media.

    ++ At 21°C; Santi and de Pietri-Tonelli (1960).

    Methods of residue analysis

    Most of the methods proposed for the determination of omethoate
    residues have been developed in association with methods for
    dimethoate, of which it is the oxygen-analogue; for reviews of these
    see de Pietri-Tonelli et al. (1965); Smart (1966); Bazzi (1966); Joint
    Dimethoate Residues Panel (1968); FAO/WHO (1968).

    All methods for the extraction of omethoate from plant material must
    make allowance for the extremely high solubility of the compound in
    water. The distribution coefficients for omethoate between water and
    some organic solvents are as follows (Grimmer et al., 1968a):

              chloroform               1.15 <(0.7*)
              n-butanol                1.55
              ethyl acetate            0.5
              benzene                  0.25 <(0.02*)

                   * Santi and de Pietri-Tonelli (1960)

    Ethyl acetate in a strongly mineral-acid solution is suitable for the
    extraction of compound (4) (Grimmer at al., 1968a).

    The total phosphorus method is suitable for samples from supervised
    trials which have not been treated with other phosphoric esters. A
    simple method was recommended by the Joint Dimethoate Residues Panel
    (maceration with acetone; extraction with chloroform; Al2O3 column
    (grade V), elution with chloroform/carbon tetrachloride (1 + 1); wet
    ashing; colorimetric phosphorus determination). After an additional
    clean-up step (acetonitrile/n-hexane partitioning), the residues can
    be identified by two-dimensional thin-layer chromatography. This
    method of analysis has since been modified (Frehse, 1969) in order to
    obtain better recoveries. The aqueous extract from the plant material
    is extracted four times with chloroform; the Al2O3 column is
    eluted with 150 ml of a mixture of chloroform/methanol (1 + 1) which
    contains 1% glacial acetic acid to stabilize the residues. In most
    cases, plant material poor in chlorophyll can be analysed without
    using the Al2O3 column.

    Steller and Curry (1964) describe a total phosphorus method
    incorporating thin-layer chromatographic isolation for the
    determination of dimethoate and omethoate. The method includes an
    extraction of hexane, which is done before shaking out with
    chloroform; extracts with a very high content of chlorophyll must be
    further cleaned-up with diatomaceous earth and charcoal. A
    column-chromatographic clean-up need not be done before the thin-layer
    chromatography. The compound-containing spots are scraped, eluted and
    analysed for phosphorus. The recoveries are between 55 and 72% for
    apples (0.4-0.8 ppm), 81-94% for green tomatoes (0.16-0.32 ppm), and
    72-87% for alfalfa (0.16-0.4 ppm). Details of the thin-layer
    chromatography of omethoate are by Ackermann (1966) and Mendoza et al.

    Sissons and Telling (1970) describe a total phosphorus procedure which
    is similar to the above-mentioned method recommended by the panel. The
    extraction is done without adding acetone from the acidified aqueous
    phase. The authors point out that the reason for the recoveries for
    omethoate being lower than for other organophosphorus compounds was
    due to significant volatilization losses which occurred during
    Kaderna-Danish evaporation of the extracts. The recoveries obtained by
    the proposed method are between 70 and 80% for an addition of 0.1-0.2

    With the increasing use of gas chromatography, some methods had been
    proposed for determination of dimethoate and omethoate residues but
    they were not sufficiently developed at the time the Joint Dimethoate
    Residues Panol (1968) concluded its deliberations; however, several
    promising stationary phases were described.

    Bache and Lisk (1966) used the emission detector for determining
    omethoate residues in soil. The soil is blended with chloroform and
    the filtered solution, concentrated to a small volume, is directly
    injected into the gas chromatograph: 6 ft × 3/16 in i.d. glass column

    packed with 5% FFAP on 80/100 mesh Gas Chrom Q, 130°C, retention time
    9.8 min. Recovery at the 5 ppm level was 86%; the limit of detection
    was reported to be about 0.02 ppm. Beck et al. (1968) developed a
    method for corn silage and milk. Silage is extracted with ethyl
    acetate, and filtered. The solvent is removed and the residue
    partitioned twice between 10 ml volumes of iso-octane and 1:4 (v/v)
    water/acetonitrile. The iso-octane layers are discarded, and 5 ml of
    methylene chloride is added to the acetonitrile layer; the mixture is
    shaken. The lower layer (methylene chloride acetonitrile) is cleaned
    up with Norite SG-extra, filtered through gelite and the filter cake
    worked with ethyl acetate. The filtrate is evaporated just to dryness
    and then diluted with ethyl acetate to a suitable volume for gas
    chromatography: 4 ft glass column, 1/4 in o.d.; 5% Carbowax 20 M on
    80/90 mesh Chromosorb W, acid washed and silanized, 120°C, flame
    photometric detector (526 mµ), retention time six minutes, limit of
    determination, 0.01 ppm. For milk analysis, 100 g aliquots are heated
    in a 60°C water bath for 20 minutes to facilitate protein
    precipitation. The samples are homogenized at room temperature with
    200 ml of acetonitrile and are then subjected to a similar clean-up as
    for the silage extracts. The limit of determination is stated to be
    0.001 ppm.

    Ruzicka et al. (1967) were able to determine omethoate with the aid of
    a thermionic detector on a glass column (150 cm × 5 mm o.d.) packed
    with 3% Apiezon L + 0.2% Epikote 1001 on
    dimethyl-dichlorosilane-treated Chromosorb G, 70/80 mesh, at 165°C.
    Bache and Lisk (1968), using the micro-wave powered helium emission
    detector and an EC detector, obtained for omethoate, at 160°C, a
    retention time of 3.6 minutes on a 2 ft × 3/16 in i.d. column packed
    with 10% OV-17 on 80/100 mesh Gas Chrom Q; a slight but perceptible
    tailing was observed. Sissons and Telling (1970) used a 5 ft × 1/8 in
    glass column packed with 3% OV-17 on AWDMCS Chromosorb G, 80/100 mesh,
    and obtained a retention time of 2.5 minutes at 225°C.

    After comparing the behaviour of over 60 organophosphorus compounds on
    three GDC columns, using a KCI thermionic detector, Watts and Storherr
    (1969) found the following conditions to be the most suitable for

         6 ft × 4 mm i.d. glass column, 2% diethylene glycol succinate
         (C6 stabilized) on 80/100 mesh Gas Chrom Q, 210°C (retention
         time of 3.7 minutes).

    Watts et al. (1969) recommended an extraction with ethyl acetate
    followed by charcoal clean-up for many organophosphorus pesticide
    residues in crop extracts, especially in kale. Omethoate was included
    in the studies and a recovery of 98% was obtained at the 0.5 ppm level
    by gas chromatography using the above-mentioned method of Watts and
    Storherr (1969).

    Abbott et al. (1970) included omethoate, too, in their method for the
    determination of organophosphorus pesticide residues in total diet
    samples. Extracts from six groups of foods and milk are cleaned up and

    examined using four different types of column and thermionic detectors
    with caesium bromide tips. The authors chiefly report on their results
    and unfortunately do not state which of the columns used is the most
    suitable for omethoate. The AOAC multi-residue method is not suitable
    for determination of omethoate because the compound is not eluted from
    the column with the elution solvents used in this method (Pardue,

    A simple method (Frehse, 1970) for fruits (including citrus fruits)
    and vegetables operates as follows: 200 g of the sample is macerated
    twice (500, 250 ml) with acetone, the cleaned-up filtrates are freed
    from acetone on the rotary evaporator, the aqueous remainder is
    rapidly cooled (at first under running tap water, then for one hour in
    a refrigerator), filtered and extracted four times with 200 ml
    portions of chloroform. The combined chloroform extracts are
    evaporated to dryness and made up to a volume of 10 ml with acetone.
    The gas-chromatographic determination is done on a 100 × 1.5 mm i.d.
    glass column packed with 1.6% DC-200 + 0.6% Reoplex on 60/80 mesh
    Chromosorb G, 185°C, using a thermionic detector giving a retention
    time of 2.2 minutes. The recoveries are between 80 and 90% at the
    0.05 - 1.0 ppm level. With some cole crops an interference peak may
    appear at the position of omethoate which must be removed by
    additional clean-up steps (see e.g. Watts et al. (1969)).

    Fechner at al. (1969) published a method for determining dimethoate
    and omethoate in milk. The milk sample is freed from fat by
    centrifuging and extracted with methylene chloride at 5°C. The extract
    is filtered and evaporated almost to dryness (0.25 ml), Aliquots of
    the concentrate are chromatographed on Kieselgel G plates with
    benzene/acetone (2+1); the compound spots are visualized
    enzymatically, and evaluated semiquantitatively by comparison with
    standards. In this way, omethoate residues can be determined down to a
    level of 0.002 - 0.004 ppm. Fischer (1969) has described a thin-layer
    chromatographic procedure for the identification and quantitative
    determination of omethoate in biological material. Omethoate can be
    semiquantitatively determined by thin-layer chromatography in
    cucumbers, cherries and apples (Anonymous, 1970). After extraction
    with ethanol and clean-up, the concentrated extracts are
    chromatographed on Kieselgel G (pre-development with
    chloroform/acetone 3 + 1), then chromatographed twice with
    toluene/n-heptane/acetone (1 + 2 + 4). The compound spots are
    visualized with thymol (bright spots on brownish background); 2 µg of
    omethoate can be determined.

    For regulatory purposes, a gas chromatographic procedure using either
    flame photometric or phosphorus-sensitive thermionic detection would
    be the method of choice. Thin-layer chromatography can be used to
    obtain additional evidence of identity of the residue found.

    Country        Crop                                  Tolerance        Safety
                                                         in ppm           interval
                                                                          in days
    Australia      Apples, citrus fruits, peaches        2.0              21
    Austria        General                               -                35
    Belgium        General                               -                21
                   Apples, pears                         0.5              -
    Brazila        Deciduous fruits                      2.0              -
                   Tomatoes, peppers                     1.0              -
                   Vegetables, potatoes, beets           2.0              -
    Bulgaria       Tobacco                               -                0
                   Pome and stone fruit                  -                0
    France         General                               -                21
    Italy          General                               -                20
    Netherlands    General, field-grown                  0.1              21
    New Zealand    General                               0.5              21
    South Africa   General                               1.5
                   Apples, pears                         -                28
                   Vines                                 -                28
    Spain          General                               -                28
    United         Apples, pears                         2                28
    States of      Beans (green, lima, snap, dry)        2                -
    Americaa       Broccoli, cauliflower                 2                7
                   Cabbage                               2                3
                   Lemons, oranges                       2                15-45
                   Collards, endive, kale, lettuce )
                    (leaf), mustard greens,        )
                    spinach, Swiss chard, turnips  )     2                14
                    (greens and roots)             )
                   Lettuce (head), tomatoes              2                7
                   Peas, peppers                         2                -
                   Melons                                1                3
                   Potatoes                              0.2              -
                   Pecans                                0.1              21
                   Milk                                  0.002            -
                   Meat, fat and meat by-products
                    of cattle                            0.02             -
                   Wheat (grain)                         0.04             60
                   Wheat (green fodder and straw)        2                14
                   (Alfalfa, cotton, tobacco)            -                14-28
    a "....dimethoate and its oxygen analogue".

    Omethoate, which is the oxygen analogue of dimethoate, is a systemic,
    organophosphorus insecticide and acaricide, active against both
    sucking and biting pests. Its chief uses are for pre-harvest
    treatments of tree fruits, especially apples and pears, field crops
    and vegetables. Omethoate is more persistent than dimethoate in plant
    tissue but it degrades similarly to give esters of phosphoric and
    thiophosphoric acid; metabolism in animals is also similar to that of
    dimethoate. Residues are therefore determined only in terms of the
    parent omethoate. Information was available regarding residues
    resulting from supervised trials but no residue data from foodstuffs
    moving in commerce or total diet studies was presented. Gas
    chromatographic procedures are available for the determination of
    residues of omethoate which can be adapted for regulatory purposes as


    Temporary tolerances

    The following temporary tolerances are recommended for omethoate
    residues. The pre-harvest interval shown is an indication of the basis
    on which the recommendation is made:


                                    Tolerance      Pre-harvest
              Fruit                   (ppm)      interval (days)

    Apples, grapes, peaches, pears      2              21
    Apricots, cherries, plums           2               7

    Insufficient information was available to support recommendations for
    tolerances for omethoate residues on lettuce, potatoes, sugar beet or

    Further work or information

    Required before 30 June 1975

    A long-term feeding study in at least one species of animal.


    1.   Relevant observations in man.

    2.   More information on the quantitative aspect of the metabolism of
         omethoate as compared with that of dimethoate.

    3.   Reproduction study in a non-rodent species.

    4.   Studies on the fate of the compound during storage, processing
         and preparation of food for consumption.

    5.   Further data on residues occurring in use on lettuce, potatoes,
         sugar beet and hops.

    6.   Information on residues occurring in food in commerce and in
         total diet studies.


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    See Also:
       Toxicological Abbreviations
       Omethoate (WHO Pesticide Residues Series 5)
       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)