Sponsored jointly by FAO and WHO


    Data and recommendations of the 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, 5 - 14 December 1983

    Food and Agriculture Organization of the United Nations
    Rome 1985




    Methiocarb was evaluated by the 1981 Meeting and a full acceptable
    daily intake (ADI) was allocated.1/ Observations in humans,
    considered by the previous Meeting to be desirable information, are
    still not available. Other additional toxicological data have been
    submitted and are summarized and discussed in the following monograph

    In 1981, recommendations were made for maximum residue levels (MRLs)
    (and temporary MRLs pending further information on GAP) in a range of
    commodities on which methiocarb was used for snail and slug control,
    spray application as a bird repellent or as a seed treatment.
    Information on good agricultural practice for commodities on which
    bait applications are recommended or used, was required, and it was
    noted that additional metabolism and residue data would be necessary
    if future uses of the compound on animal feeds could result in
    potential residues in meat, milk, poultry and eggs. Information on
    levels of methiocarb residues in foods in commerce or at consumption
    was also listed as desirable.

    A considerable amount of additional information, not all relevant to
    the above requests was evaluated by the meeting and is presented in
    this monograph addendum.



    Absorption, Distribution, Elimination and Biotransformation

    In male Wistar rats given a single oral dose of approximately 5 mg/rat
    (body weight of the animals not given) of methiocarb, only a small
    amount of the administered dose was eliminated in the urine, mainly
    within 48 hours of treatment, as the unchanged parent compound
    (<2.3 percent) and its metabolites (3.3 percent) (van Hoof &


    1/ See Annex 2 for FAO and WHO documentation.

    Under aerobic conditions and in the presence of NADPH, methiocarb was
    oxidized by the flavin-adenine dinucleotide-dependent monoxygenase of
    pig liver microsomes. The rate of sulphoxidation of methiocarb was low
    as compared to other thioether-containing pesticides such as
    disulfoton, an organophosphate, and thiofanox, a carbamate (Hajjar &
    Hodgson 1982).

    Effects on Enzymes and Other Biochemical Parameters


    Groups of 15 female Sprague-Dawley-derived rats were intubated with
    technical methiocarb (97 percent pure) or methiocarb sulphoxide (95.2
    percent pure) in Carbowax at 0, 0.5 or 2 mg/kg b.w./day five times
    weekly for four weeks. Determination of cholinesterase activity 30
    minutes after dosing on days 7, 14, 21 and 28 (i.e. following the two-
    day withdrawal period) showed a significant dose-related inhibition of
    plasma cholinesterase (21-61 percent) and erythrocyte cholinesterase
    (22-46 percent) at practically all intervals following methiocarb
    sulphoxide dosing. Treatment with technical methiocarb resulted in
    plasma cholinesterase inhibition on days 0, 7 and 14 (23-41 percent)
    following the 2 mg/kg b.w. dose and on days 0 (21 percent) following
    the 0.5 mg/kg b.w. dose. Erythrocyte cholinesterase was depressed by
    29 percent on day 0 with 2 mg/kg b.w. of technical methiocarb. Four
    hours after dosing on days 4, 11 and 18, only rats treated with 2
    mg/kg b.w. of methiocarb sulphoxide showed a decrease (>20 percent)
    in activity of plasma cholinesterase (days 11 and 18) and erythrocyte
    cholinesterase (day 11) (Hixson 1981).


    Groups of two male and two female beagles were given daily 0.05 or 0.5
    mg/kg b.w. of technical methiocarb (97 percent pure) or methiocarb
    sulphoxide (95.2 percent pure) in gelatin capsules for 29 days.
    Control groups comprised two males and two females. Slight to heavy
    salivation or vomiting was observed in animals (both sexes) given
    either test material at 0.5 mg/kg b.w. One female treated with 0.05
    mg/kg b.w. of the sulphoxide also exhibited slight salivation on one
    occasion. Assay of plasma and erythrocyte cholinesterase from unfasted
    animals, using an unspecified method, at various intervals after the
    first and second or third dosing each week showed inhibition (> 20
    percent) of the enzyme in plasma or erythrocytes by both test
    compounds at 0.5 mg/kg b.w. Depression of cholinesterase peaked at 
    1-2 h and was essentially reversible at 6 h, post-treatment. In
    general, the extent of inhibition, which was greater with plasma
    cholinesterase than with erythrocyte cholinesterase, did not increase
    with time with either compound. Data indicated 0.05 mg/kg b.w. to be a
    marginal no-effect level (NOEL) on cholinesterase for technical
    methiocarb. A no-effect level for methiocarb sulphoxide was, however,
    not demonstrated (Hayes 1981).


    Special Study on Teratogenicity


    Groups of 17-19 female rabbits (New Zealand White), artificially
    inseminated, were intubated with methiocarb (a mixture of five
    batches, 97.3 percent pure) as a suspension (in distilled water
    containing 0.5 percent carboxymethyl cellulose and 0.5 percent
    Tween 80) at 0, 1, 3 or 10 mg/kg b.w./day from day 6 to 18 inclusive
    of gestation (day 0 = day of insemination). The does were sacrificed
    on day 29 of gestation and uterine contents were examined. Foetuses
    were examined for external, skeletal and internal abnormalities. Three
    to six females per group, including the control, died or were
    sacrificed in extremis 6 to 25 days after insemination, mainly owing
    to respiratory tract infection and/or gastrointestinal disorder or
    accidental tracheal intubation. The number of does found to be
    pregnant with viable young on day 29 was only 11 in control group, 13
    at 1 mg/kg b.w., 12 at 3 mg/kg b.w. and 10 at 10 mg/kg b.w. Toxic
    signs such as increased respiratory rate, muscular tremors, pupillary
    constriction, incoordination and prostration were noted at 10 mg/kg
    b.w. in maternal animals. Increased respiratory rate was also seen,
    although infrequently, at 3 mg/kg b.w. Does of the top-dosage group
    exhibited actual weight loss during the first two days of treatment
    and growth depression thereafter throughout the dosing period.
    Premature parturition occurred in one doe at 3 mg/kg b.w. on day 28
    and two does at 10 mg/kg b.w. aborted on day 25. The incidence of
    pregnant does with early resorptions was elevated at 10 mg/kg b.w. but
    the mean number of early resorptions per litter in this dosage group
    was within the ranges of background data submitted.

    There were no treatment-related effects on the number of corpora
    lutea, implantations, viable young or late resorptions or on pre-
    and post- implantation loss, foetal weight and placental weight. An
    increase in incidence of foetuses with pale areas in the liver was
    noted at 10 mg/kg b.w. but frequency of foetal malformations was not
    affected by treatment. The study appeared to give no evidence
    suggestive of teratogenic activity of methiocarb under the conditions
    of the experiment. Maternal and/or foetal toxicity, however, occurred
    at 3 mg/kg b.w. and above (Tesh et al 1981).

    Acute Toxicity

    The four-hour inhalation LC50 of an aerosol of technical methiocarb
    (97.9 percent pure) in male and female Wistar rats (Bor: WISW (Spf
    (Cpb)) was greater than 322 mg/cu m air, in terms of analytical
    concentration of the test material in the inhalation chamber. No
    information was given on the particle size of the aerosol (Thyssen

    Short-Term Studies

    In Wistar rats (Bor: WISW (Spf (Cpb)) exposed to an aerosol (particle
    size not specified) of technical methiocarb (97.9 percent pure) at
    chamber (analytical) concentrations of 0, 27, 92 or 298 mg cu m air, 6
    h/day for-five days, the LC50 was greater than 298 mg/cu m air in
    males and approximately 300 mg/cu m air in females (Thyssen 1982).


    The cholinesterase studies in both rats and dogs indicated methiocarb
    sulphoxide to be a more potent inhibitor than methiocarb. No evidence
    of teratogenicity of methiocarb was observed in the rabbit teratology

    The Meeting confirmed the ADI estimated at the 1981 Meeting and
    further desirable work was recommended.


    Level Causing no Toxicological Effect

    Rat: 25 ppm in the diet, equivalent to 1.3 mg/kg b.w./day.

    Dog: 5 ppm in the diet, equivalent to 0.125 mg/kg b.w./day.

    Estimation of Acceptable Daily Intake for Man

    0 - 0.001 mg/kg b.w.



    1. Data to clarify the significance of the inhibition of plasma
    cholinesterase noted in the dog study which led to the estimation of
    the no-effect level by the 1981 Meeting, bearing in mind the views of
    the 1982 Meeting on the function of plasma cholinesterase.

    2. Information on the method of analysis used to estimate plasma and
    erythrocyte cholinesterase activities in vivo in the above dog

    3. Observations in humans.


    Hajjar, N.P. & Hodgson, E.    Sulfoxidation of thioether-containing
    1982                          pesticides by the flavin-adeninine
                                  dinucleotide-dependent monooxygenase of
                                  pig liver microsomes. Biochem. Pharmacol
                                  31: 745-752.

    Hayes, R.H.                   Cholinesterase evaluation study of
    1981                          methiocarb technical and methiocarb
                                  sulphoxide in dogs. Report from Mobay
                                  Chemical Corp. submitted to WHO by Bayer
                                  AG. (Unpublished)

    Hixson, E.J.                  Cholinesterase no-effect level of
    1981                          RMesurol and RMesurol sulphoxide in
                                  female rats. Report from Mobay Chemical
                                  Corp. submitted to WHO by Bayer AG.

    Tesh, J.M., Rose, F.W.,       H321: effects of oral administration
    Secker, R.C. & Wilby, O.K.    upon pregnancy in the rabbit. 2. Main
    1981                          study. Report from Life Science
                                  Research, England, submitted to WHO by
                                  Bayer AG. (Unpublished)

    Thyssen, J.                   H321 (Mesurol active ingredient). Acute
    1982                          inhalation toxicity. Report from Bayer
                                  AG, submitted to WHO by Bayer AG.

    van Hoof, F. & Heyndrickx.    The excretion in urine of four
    1975                          insecticidal carbamates and their
                                  phenolic metabolites after oral
                                  administration to rats. Arch. Toxicol.
                                  34: 81-88.




    Further detailed information on use patterns in various countries only
    served to confirm those identified in 1981. The only additional uses
    brought to light were on sorghum in Africa and South America, where a
    single application of 1 - 1.5 kg/ha is made at the dough stage and the
    uses in Eastern Africa mentioned below.

    Bruggers et al. (1981) summarized the results of field evaluations,
    during several years, of methiocarb when used to protect ripening
    crops of rice, wheat, sorghum and sunflowers in Eastern Africa from
    losses due to birds. The positive results and favourable
    cost/effectiveness obtained in nearly all situations justifies the use
    of methiocarb under typical farming conditions in Africa. Treatment
    rates are in the range of 1-2 kg/ha and application is made at the
    dough stage when the crop first becomes vulnerable to damage by birds.

    There is world-wide acceptance of methiocarb for use as a bait against
    slugs and snails. The baits contain either 2 percent or 4 percent
    methiocarb, together with wheat bran or a similar attractant,
    formulated into small pellets designed to be scattered by hand or
    machine. The treatment is designed to deposit the pellets on the soil
    between the crop plants so as to be readily accessible to foraging
    molluscs. It is recognized that some pellets will impact upon the
    leaves and stems of crop plants and that some may lodge between leaves
    of crops such as leafy vegetables. The structure of the pellets is
    sufficiently strong that they do not disintegrate and thus contaminate
    plant parts to a significant extent. The pellets are generally
    coloured distinctively and can be dislodged in preparing crops for
    market and cooking.

    When methiocarb is used as bait, the rate of application is in the
    range of 120-240 g a.i./ha. One application is generally sufficient
    but a second may be required. Treatment is usually made early in the
    crop cycle to prevent damage to young seedlings or emerging plants. In
    the event of snails and slugs invading from adjacent fields, perimeter
    baiting is usually effective.

    In 1981, the United Stated Environmental Protection Agency granted
    authority to use methiocarb to repel birds depredating grapes. The
    rate of application is 4 kg/ha with a maximum of four applications per
    season. Maximum residue limits (MRLs) were established for grapes,
    raisins, raisin waste and grape pomace together with secondary MRLs in
    foods of animal origin. This use was extended in 1982. In 1983, almost
    900 tonnes of methiocarb active ingredient were used on grapes in 15
    states, in addition to quantities used to repel birds from
    blueberries, cherries, peaches and cereal grains (Anonymous 1983).


    Extensive data from supervised trials were evaluated in 1981. The
    following additional reports were considered by the Meeting.


    A trial carried out in Germany in 1981 involved the application of 4
    percent methiocarb granular snail bait, on two occasions 16 days
    apart, at the rate of 0.12 kg a.i./ha. Samples of leaves, flowers and
    stalks taken 0, 4, 7, 14 and 28 days after the second treatment were
    analysed by a method sensitive to 0.05 mg/kg of methiocarb, its
    sulphoxide and sulphone. Though the deposit on the day of treatment

    ranged up to 2 mg/kg on stalks, by the fourth day the residue was at
    or slightly above the limit of determination. By the seventh day there
    was no detectable residue in flowers, leaves or stalks (Bayer 1983).


    Methiocarb is used as a seed treatment against fruit flies and as a
    bird repellent. The treatment involves the application of 300 ml of
    formulation (500 g/l) per 100 kg of seed. The treated seed is
    distributed at a rate equivalent to 150 g of methiocarb/ha. Green
    maize plants cut for silage approximately 90 days after planting were
    analysed, as was the ripe grain harvested approximately 180 days after
    planting. In none of six trials conducted in various regions of
    Germany or of two similar trials in the United Kingdom was methiocarb
    or its metabolites detected when samples were analysed by methods
    sensitive to 0.05 mg/kg or 0.01 mg/kg (Bayer 1983).


    Pelissier (1978) working in Senegal showed that when methiocarb was
    applied to ripening sorghum at the rate of 2 kg/ha, together with an
    adhesive, the pesticide had a half-life of six to seven days. Residue
    levels in grain were less than 3.6 mg/kg 20 days after application.

    Gras et al. (1981) conducted trials with methiocarb to determine the
    level and fate of residues when the pesticide was applied to ripening
    sorghum as a bird repellent. Such treatments are finding favour in
    West Africa to reduce the depredations wrought by the red-billed
    Quelea. Methiocarb was applied at the rate of 2 kg/ha in aqueous
    suspension when the sorghym grain was at the "milk" stage. The crop
    was subjected to overhead irrigation. Samples (heads on stalk) were
    collected 12 and 24 hours, and 5, 10, 15, 20, 25 and 30 days after
    treatment. Residues were determined in the entire head (grain and
    glume) and in the grain only. The results are given in Table 1.

    The normal harvest date is approximately 21 days after treatment (milk
    stage). The glumes are usually removed during threshing or prior to
    and during milling.

    A mathematical analysis of these data indicates that the residual
    half-life is 6 days on grain and 7.5 days on grain plus glumes. The
    authors drew attention to the fact that sorghum is always cooked prior
    to eating. In view of the sensitivity of methiocarb to heat it is
    entirely likely that any residues remaining on the sorghum grain would
    be partially or wholly degraded during preparing and cooking. However,
    the data on residues in grain plus glumes leads to the conclusion that
    the residue level on the stubble after harvest would be high. The
    consequences of feeding such stubble to livestock have not been


    Workers in Belgium studied the use of methiocarb sprays for the
    protection of cherries against damage by starlings. The concentration
    of the residues was determined on cherries treated with methiocarb
    spray once, 23 days before harvest and twice, 23 and 6 days before
    harvest. The maximum concentrations of methiocarb (13.4 mg/kg) and
    methiocarb sulphoxide (1.25 mg/kg) were recorded on cherries sprayed
    twice. No methiocarb sulphone was detected in any samples (Hoyoux &
    Zenon-Roland, 1979).

    Information concerning the use of methiocarb and the residues
    resulting from such use was received from the Government of Thailand.
    This information is summarized in Table 2. With the exception of
    Chinese radish the results were within the MRLs recommended in 1981,
    notwithstanding the fact that the treatments had been made by
    spraying. In the case of Chinese radish the residue reported (4.25
    mg/kg) appears to be high for a root crop but there may be special
    reasons for this result. The Meeting proposed an MRL for Chinese



    The 1981 evaluation of methiocarb (FAO/WHO 1982b) included a review of
    the fate of its residues, which is supplemented below. Revised
    versions of the list of identified metabolites and of the diagram of
    metabolic pathways given in 1981 are reproduced here for convenience
    (Table 3 and Fig. 1, respectively).

    In Animals

    Some studies on the fate of methiocarb in rats, dogs, cattle, poultry
    and fish were evaluated in 1981. It should be noted that the last
    sentence describing the study on dogs in FAO/WHO 1982 (p.318) should
    end "... tissue residues were below 0.4 mg/kg." Additional studies
    are reviewed below.

    FIGURE 1

    Table 1. Persistence of methiocarb on sorghum, Bambey, Senegal, 1978.


    Sample               Interval         Methiocarb residue (mg/kg)
    No.                  after            Thin-layer        Gas
                         application1     chromatography    chromatography
    Seed only

    1                        5                20                36
    2                       10                17                13
    3                       15                 3.8               8.8
    4                       20                 3.5               4.3
    5                       25                 3.0               4.0
    6                       30                 0.9               2.2

    Seed and glumes

    7                        0               180               200
    8                        0.5             140               150
    9                        1               150               170
    10                       2               100               140
    11                       5                92               140
    12                      10               100               140
    13                      15                11                76
    14                      20                24                56
    15                      25                17                24
    16                      30                 8                11

    1 Application rate was 2 kg/ha spray.

        Table 2. Residues Resulting from Supervised Trials of Methiocarb, Thailand

    Crop             Pest                 Rate       No.        Pre-harvest      Residue
                                          (kg/ha)               Interval         (mg/kg)

    Cabbage          Diamond back moth    1.0        1             7             0.16

    Chinese Kale     Aphis                1.0        2             5             N.D.2
                     Phyllotreta sp.

    Edible Rape      Thrips               1.0        1             5             0.14
                     Phyllotreta sp.

    Chinese          Thrips               1.0        2             7             4.25
    Radish           Phyllotreta sp.

    Broccoli         Aphis                1.0        1             7             0.13

    Yard long bean   Aphis                1.45       1             7             N.D.

    Multiplier       Thrips               2.5        1             7             0.02

    Mung bean        Aphis                1.0        2            12             N.D.

    1 50% wettable powder used in all treatments.
    2 N.D. = not detectable

        Table 3. Chemical Names, Structures and Designations of Methiocarb and its Metabolites Identified in Animals


    Designation                    Chemical name                                Structure


    Methiocarb                     3,5-dimethyl-4-(methylthio)=phenyl           R1 = -O-C-NH-CH3
                                                                                R2 = -SCH3

    Methiocarb sulphoxide          3,5-dimethyl-4-(methylsul=phenyl)phenyl      R1 = -O-C-NH-CH3
                                                                                R2 = -SCH3

    Methiocarb sulphone            3,5-dimethyl-4-(methylsul=phonyl)phenyl      R1 = -O-C-NH-CH3
                                                                                R2 = -SCH3

    N-hydroxymethyl-methiocarb     3,5-dimethyl-4-(methylthio)=phenyl           R1 = -O-C-NH-CH2OH
                                                                                R2 = -SCH3

    Table 3 (continued)

    Designation                    Chemical name                                Structure

    N-hydroxymethyl-methiocarb     3,5-dimethyl-4-(mathylsul=phinyl)phenyl      R1 = -O-C-NH-CH2OH
      sulphoxide                   N-hydroxymethyl)carbamate
                                                                                R2 = -SCH3

    Methiocarb phenol              3,5-dimethyl-4-(methythio)=phenol            R1 = -OH

                                                                                R2 = -SCH3

    Methiocarb sulphoxide phenol   3,5-dimethyl-4-(methylsul=phinyl)phenol      R1 = OH

                                                                                R2 = -SCH3

    Methiocarb sulphone phenol     3,5-dimethyl-4-(methylsul=phonyl)phenol      R1 = -OH

                                                                                R2 = -SCH3

    Krishna & Casida (1966) studied the elimination of carbonyl-14C
    methiocarb after intraperitoneal (i.p.) administration to rats. Within
    48 hours, 66.1 percent of the applied radioactivity was expired as
    14CO2, 22.3 percent excreted in the urine and 2.5 percent eliminated
    in the faeces. Only 8.9 percent remained in the body.

    The excretion and distribution of radioactivity were examined in
    foetal and maternal tissues of rats after i.p. injection of 
    carbonyl-14C methiocarb (Wheeler & Strother 1974). The behaviour was
    characterized by rapid maternal distribution and placental transfer.
    Foetal kidney and heart contained the most radioactivity. Liver was
    the maternal tissue with the highest 14C content. From the total
    radioactivity recovered in the urine, faeces and exhaled air after 8
    h, it appeared that the pregnant rat retained about 10 percent more
    radioactivity than the nonpregnant animal.

    Following a single oral application of unlabelled methiocarb to rats,
    only minor quantities of the unchanged product and its phenol were
    excreted in the urine during the 48 hours following administration
    (van Hoof & Hendrickx 1975).

    In Vitro Studies

    The in vitro metabolism of methiocarb by liver, kidney and blood
    fractions was studied in man, dog and rat (Strother 1970, 1972;
    Wheeler & Strother 1971). Methiocarb was degraded in all preparations.
    All species produced essentially the same major metabolites
    (methiocarb sulphoxide and N-hydroxymethyl-methiocarb) although
    quantitative differences were observed.

    In studies with microsomal-NADPH2 systems of houseflies (Tsukamoto &
    Casida 1967) and rats (Oonnithan & Casida 1968), sulphoxidation of
    methiocarb was also found to be the main degradation pathway. In
    vitro studies on houseflies revealed the formation of 
    N-hydroxymethyl-methiocarb as a further metabolite (Metcalf et al

    The request of the 1981 Meeting for analysis of ruminant liver and
    kidney for N-hydroxy-methyl-methiocarb was reconsidered since, in one
    of the studies cited by that Meeting, essentially all of the
    radioactive residue in those tissues was accounted for and none of it
    was that metabolite. Even if N-hydroxymethyl-methiocarb were present,
    it would be measured by the recommended residue method for meat and

    In view of the need to apply methiocarb to grain crops to prevent
    destruction by plagues of birds, consideration needs to be given to
    the consequences of feeding the straw to livestock or grazing
    livestock on the stubbles. Such limited information as is available
    suggests that the residues on the waste plant material would be quite
    substantial. The animal transfer studies evaluated by the Meeting in

    1981 were not adequate to demonstrate the fate of such residues. The
    Meeting therefore was unable to make recommendations for MRLs in meat,
    milk and eggs to deal with such practices. Further studies are needed.

    In Plants


    Following both foliar application and stem injection, carbonyl -14C
    methiocarb was rapidly converted to the sulphoxide and sulphone
    (Abdel-Wahab et al 1966). Six days after stem injection, a 64
    percent loss of radioactivity was measured, which the authors
    considered possibly resulted from expiration as 14CO2 (Table 4).

    In Soil

    Methiocarb is readily absorbed by soil but not desorbed. The parent
    compound is classified as having only low mobility in leaching
    studies. The degradation products of methiocarb also appear in only
    very small amounts in the leachate.

    In laboratory studies, which were conducted in accordance with 
    BBA-Merkblatt No. 37 (Biologische Bundesanstalt für Land- und
    Forstwirtschaft 1980), the leaching behaviour of methiocarb,
    formulated as 50 percent WP and as slug bait, methiocarb sulphoxide
    and methiocarb sulphone was investigated. After the application of
    methiocarb or its sulphone and addition of water equivalent to about
    200 mm rainfall in 60 h, residues found in the leachate amounted to
    less than 2.5 percent of the applied dose (Bayer 1974a, 1978a). In the
    studies with methiocarb sulphoxide, the residues in the leachate
    amounted to 3.8 percent (Bayer 1978G).

    Laboratory degradation studies with the two standard soils specified
    in BBA-Merkblatt No. 36 (Biologische Bundesanstalt für Land- und
    Forstwirtschaft, 1976) gave half-lives of about four and six weeks for
    methiocarb (Bayer 1974b).

    A study of the adsorption of methiocarb sulphoxide from water by sandy
    loam (Strankowski & Murphy 1982) was complicated by rapid degradation.
    It was not feasible to measure the adsorption because the methiocarb
    sulphoxide was rapidly hydrolysed to its phenol. Within half an hour,
    24 percent decomposition had occurred in the soil/water system. A
    maximum of 4 percent of the methiocarb sulphoxide was adsorbed, and
    this decreased to 1 percent during the four-hour study.

    In Rotational Crops

    Wheat, sugarbeet and spinach were planted as rotational crops one year
    after bare soil (sandy loam) had been treated with ring-1-14C
    methiocarb at a field application rate of 5.6 kg a.i./ha (Strankowski
    & Kottman 1979). Maize was used as the original crop in this soil. At
    the time of planting the rotational crops, the soil contained total

    radioactivity of 2.62 mg/kg methiocarb equivalents. The rotational
    crops were grown to maturity and, at the time of harvest, each crop
    matrix contained an average total radioactivity of slightly less than
    0.1 mg/kg methiocarb equivalents. Samples of the immature crops
    usually contained radioactivity equivalent to more than 0.1 mg/kg. The
    carbamate residues found in each of the rotational crops by thin-layer
    chromatography were <0.08 mg/kg (wheat heads, 0.022 mg/kg; wheat
    stalks, 0.025 mg/kg; wheat forage, 0.080 mg/kg; spinach, 0.016 mg/kg;
    sugarbeet roots, <0.014 mg/kg). Methiocarb metabolites identified in
    these crops included N-hydroxymethyl-methiocarb, methiocarb
    sulphoxide, N-hydroxymethyl-methiocarb sulphoxide, methiocarb
    sulphoxide phenol, methiocarb sulphone and methiocarb sulphone phenol.
    No methiocarb was detected. The radioactivity in the soil at the final
    harvest was 1.57 mg/kg methiocarb equivalents.

    In Water

    Studies of methiocarb hydrolysis and its degradation in a pond
    water/sediment combination were evaluated in 1981 (FAO/WHO 1982).

    A further hydrolysis study was carried out according to the OECD Test
    Method for determining hydrolysis as a function of pH in compliance
    with the requirements of the German Biologische Bundesanstalt für
    Land- und Forstwirtschaft (Wilmes 1983). The half-lives at different
    temperatures and pH values were as follows:

    pH                        7         7          9          9

    Temperature (°C)         40        50         20         30

    Half-life (hours)        26.5       4.6        8.5        1.5


    Methiocarb in solution and in the adsorbed state is degradable by
    light in the laboratory. Owing to the ultraviolet adsorption
    properties of the pesticide, direct photodegradation should also be
    possible under environmental conditions. The most important primary
    degradation reactions are hydrolysis to the phenol and oxidation to
    the sulphoxide and sulphone.

    Kumar et al (1974) examined the photolysis of methiocarb in aerated
    and degassed ethanol and cyclohexane solutions. The exciting
    wavelength was above 300 mm. Photolysis yielded only one major
    product, methiocarb phenol.

    Degradation by light on glass, silica gel and soil surfaces
    (Houseworth & Tweedy 1974) was mentioned in the 1981 evaluation.
    Crosby et al (1965) irradiated ethanolic solutions of methiocarb in
    sunlight and found a number of cholinesterase-inhibiting products,
    which were not identified.

        Table 4. Distributien of Radioactivity among Methiocarb and its Metabolites in Bean Plants

                                                                        % of applied label
                                                       In organic extract
    Treatment                Time after     Methiocarb     Methiocarb     methiocarb     In             Unextracted    Loss
                             last                          sulphoxide     sulphone       water
                             application                                                 phase

    Foliage Treatment        0 h            99.3           0.4            0.1
    14CO                     2 h            93.8           4.6            0.2
    80 µg - 190 mg/kg        4 h            84.9           8.7            0.5
                             8 h            70.8           9.3            0.8
                             1 day          68.8           9.9            1.1
                             2 days         60.0           8.3            2.2
                             3 days         55.4           6.7            1.9

    Stem Injection           0 h            98.0           1.3            <0.1           0.3            0.4
    14CO                     12 h           49.0           19.8           2.3            4.2            11.5           13.2
    25 µg = 12 mg/kg         1 day          19.8           23.3           8.2            7.2            18.0           23.5
                             2 days         2.2            5.5            5.5            8.2            25.3           53.3
                             3 days         <0.1           4.7            1.8            8.7            24.0           60.8
                             4 days         <0.1                3.21                     8.1            26.1           62.6
                             6 days         <0.1                1.31                     7.7            27.5           63.5

    1 Total found in chloroform extract after column clean-up. Thin-layer chromatography omitted.

    In addition to the methods reviewed in 1981 (FAO/WHO 1982) the Meeting
    was aware of several other developments that were considered useful.

    Blass (1974) described a multi-residue method for the determination of
    methiocarb and five other carbamate insecticides in plant material.
    This method offers the possibility of determining the unchanged parent
    compound following acylation or the total residue after phosporylation
    of the phenols. Recoveries from lettuce, potatoes, apples and carrots
    in the 0.5-5 mg/kg range were 70-104 percent.

    Muth & Erro (1980) described a rapid multi-residue procedure for
    determining carbamate residues in vegetable crops, using water as the
    extracting solvent and adsorbing the proteinaceous material and
    particulate matter on Filter-Cel before filtration. The clarified
    extract was passed through a reverse-phase Sep-pak cartridge from
    which the residue was eluted with methanol and determined by high
    precision liquid chromatography (HPLC). Fifteen crops spiked at a
    level of 5 mg/kg were analysed. Interferences were minimal and the
    recovery of methiocarb was generally in the range 80-107 percent,
    although parsley, artichokes and onions showed lower recoveries.

    The method of Strankowski & Stanley, which was reviewed in FAO/WHO
    1983, has since been published (Strankowski & Stanley 1981).

    A gas-chromatographic method for the determination of carbaryl,
    propoxur and methiocarb in vegetables and fruit has been elaborated at
    the National Health Institute of The Netherlands (Anon. 1976). It
    involves extraction with dichloromethane, followed by hydrolysis with
    sodium hydroxide, steam distillation, derivatization with 
    1-chloro-2,6-dinitro-4-trifluoromethylbenzene and measurement by 
    gas-liquid chromatography (GLC). The lower limit of determination is
    0.1 mg/kg. The sulphoxide and sulphone metabolites are not determined
    by this method.


    A considerable amount of information has been received since the
    original evaluation was made in 1981. Some of this information was
    directly related to the requests made on that occasion. It is clear
    that many countries recognize the use of methiocarb bait (4 percent)
    against slugs and snails in vegetables, grains, fruit and sugarbeets.

    Some delegations to the 15th Session of the Codex Committee on
    Pesticide Residues (CCPR) questioned whether it was good agricultural
    practice to use methiocarb as a bird repellent to prevent destruction
    of such crops as blueberries, cherries, currants and grapes. There can
    be no doubt that numerous countries have accepted such uses as Being
    essential to the protection of these crops. Methiocarb is also used to
    protect cereal grain crops from attack by birds in Africa, New

    Zealand, the United States and elsewhere. Studies made in Belgium
    indicated that when methiocarb is used to protect cherries from birds,
    the residues can be up to 15 mg/kg.

    The Meeting gave special consideration to the toxicological
    significance of the residues on apples, blueberries, cherries,
    currants, grapes and peaches arising from the use of methiocarb as a
    bird repellent. While recognizing that the majority of the fruits
    would normally contain methiocarb residues at concentrations
    considerably lower than the MRLs, the Meeting considered the
    possibility that these commodities, consumed fresh, might contain
    residues that could lead to an intake of methiocarb that exceeded the
    demonstrated NOEL for plasma cholinesterase in dogs noted by the 1981
    Meeting. The Meeting did not have available for review the details of
    this dog experiment and therefore was unable to evaluate fully the
    significance of this demonstrated inhibition of plasma cholinesterase.
    Attention was drawn to para 3.2 of the report of the 1982 Meeting
    dealing with the function of plasma cholinesterase. The Meeting agreed
    to request that the relevant information should be made available to
    it in 1984 to allow full consideration of the safety of the observed
    residue levels. The Meeting recommended that the MRLs for the
    commodities in question should be withdrawn pending this further

    Information concerning the use of methiocarb and the resulting
    residues was received from Thailand. This supported several of the
    recommendations already made for MRLs but also allowed the Meeting to
    estimate a maximum residue level for Chinese radish.

    Further information was available on the fate of methiocarb in
    animals, plants, soil and water and the effect of light on deposits
    and solutions. This only served to confirm the evaluations made in
    1981. It is clear that methiocarb is rapidly degraded in plants, water
    and soil so that there is little possibility of carry-over in crop
    rotation. Similarly, because it is rapidly metabolized in animals,
    there is unlikely to be any transfer of residues into foods of animal
    origin following the feeding of forage or food offals to livestock.
    However, this applies only to current practices and does not allow for
    the possibility of livestock being grazed or fed on stubbles or other
    plant parts from grain crops sprayed with methiocarb to repel birds.

    Further methods of residue analysis have been noted. It was suggested
    at the 15th Session of the CCPR that the limit of determination in
    regulatory practice should be 0.1 mg/kg, not 0.02 mg/kg as proposed in
    1981. The Meeting reconsidered this question and noted that in the
    case of several commodities the apparent residue on untreated control
    samples was frequently higher than that reported on the treated
    samples. It was reasonable to expect that food control officials would
    find difficulty in working to a limit of determination as low as 0.02
    mg/kg. Bearing in mind the decision published in para. 2.4 of the 1981
    Meeting report, the Meeting agreed not to recommend MRLs lower than
    0.05 mg/kg.


    The further information evaluated enabled the Meeting to recommend
    additional MRLs. The previously recommended MRLs for strawberries,
    sugarbeets and citrus fruit are increased to reflect the re-evaluated
    limit of determination. Some MRLs are withdrawn pending toxicological
    re-evaluation. MRLs refer to the sum of methiocarb, its sulphoxide and
    its sulphone.


    Commodity             MRL            Pre-harvest intervals
                          mg/kg          on which recommendations
                                         are based, days

    *Chinese radishes     5                      7

    * Sorghum             5                     21
    Strawberries          0.05**                14
    Sugarbeets            0.05**                 7
    Citrus fruit          0.05**                30
    *Meat                 0.05**1
    *Milk                 0.05**1
    *Eggs                 0.05**1
    *Poultry              0.05**1

    Cherries                     MRLs withdrawn pending
    Currants(red)                toxicological re-evaluation
    Grapes                       by JMPR.
    * new recommendations
    ** at or about the limit of determination

    1 These recommendations do not allow for the possible feeding of
    straw and similar fodder from crops sprayed with methiocarb to
    repel birds.


    Abdel-Wahah, A.M., Kuhr, R.J. &    Fate of C14 -carbonyl-labelled
    Casida, J.E.                       aryl methyl-carbamate insecticide
    1966                               chemicals in and on bean plants,
                                       J. Agric. Food Chem., 14: 290-298.

    Anonymous.                         The gas-chromatographic
    1976                               determination of carbaryl, propoxur
                                       and methiocarb in vegetables and
                                       fruit (In Dutch). Report 39/76
                                       TOX-ROB, February 1976.
                                       Rijksinstituut Voor de
                                       Volksgezondheid, Utrecht,
                                       The Netherlands.

    Anonymous.                         Pesticide and Toxic Chemical News,
    1983                               17 August, p. 20-21.

    Bayer.                             Reports Nos. 2154-2159/74.

    Bayer.                             Report No. 2153/74.

    Bayer.                             Reports Nos. 2113-2115/78.

    Bayer.                             Reports Nos. 2110-2112/78.

    Bayer.                             Data on methiocarb residues from
    1983                               supervised trials on cauliflower
                                       and maize. (Unpublished)

    Biologische Bundesanstalt für      Unterlagen zum Verhalten von
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    1976                               Boden im Rahmen des
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                                       39: 1337-1358.

    Bruggers, R., Matee, J.,           Reduction of bird damage to field
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    Jaeger, M., Jackson, W.B. &        methiocarb. Trop. Pest Manage
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    Crosby, D.G., Leitis, E. &         Photodecomposition of carbamate
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    1965                               13: 204-207.

    Gras, G., Hasselman, C.,           Residue analysis of methiocarb
    Pellissier, C. & Bruggers, R.      applied to ripening sorghum as a
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    Houseworth, L.D. & Tweedy, B.G.    Photodecomposition of MESUROL-ring-
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    Hoyoux, J.M. & Zenon-Roland, L.    Protection of cherries against
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    Krishna, J.G., Casida, J.E.:       Fate in rats of the radiocarbon
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    Kumar, Y., Semeluk, G.P.,          The Photochemistry of Carbamates
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    1974                               23-27.

    Metcalf, R.L., Osman, M.G.,        Metabolism of C14 -labelled
    Fukuto, T.R.:                      carbamate insecticides to C14O2
    1967                               in the housefly. J. econ. Entomol.
                                       60, 445-450.

    Muth, G.L., Erro, F.:              A rapid carbamate multiresidue
    1980                               procedure for vegetable crops
                                       Bull. Environm. Contam. Toxicol
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    Oonnithan, E.S., Casida, J.E.:     Oxidation of methyl-and
    1968                               dimethylcarbamate insecticide
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    Pelissier, C.,                     Analyse dans les graminées de
    1978                               variétés sorghum sp. des résidues
                                       d'un répulsif utilisé dans la lutte
                                       contre le quelia: Le methiocarb
                                       Diplome d'Etudes Approfondes,
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    Strankowski, K.J., Kottman, R.F.:  Radioactive residues of MESUROL in
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    Strankowski, K.J., Murphy, J.J.:   Adsorption of MESUROL Sulfoxide by
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    Strankowski, K.J., and             Determination of Residues of
    Stanley, C.W.                      Mesurol and its sulphoxide and
    1981                               sulphone in plant, animal and soil
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    Strother, A.:                      Comparative metabolism of selected
    1970                               N-methylcarbamates by human and rat
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    Strother, A.:                      In vitro metabolism of
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    Tsukamoto, M., Casida, J.E.:       Metabolism of methylcarbamate
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    van Hoof, F. & Henydrickx, A.      The excretion in urine of four
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    See Also:
       Toxicological Abbreviations
       Methiocarb (ICSC)
       Methiocarb (Pesticide residues in food: 1981 evaluations)
       Methiocarb  (JMPR Evaluations 1998 Part II Toxicological)