Pirimiphos-methyl was evaluated in 1974 and 1976 (FAO/WHO 1975 ,
    1977), an acceptable daily intake of 0.01 mg/kg for humans was
    established and maximum residue limits in a range of commodities were
    recommended. In 1976 a request was made for results from commercial
    trials on other commodities. The 9th session of the Codex Committee an
    Pesticide Residues considered these proposals and requested
    clarification of the question of intake by humans and the distinction
    between commodities destined for animal feed and for human consumption
    (ALINORM 78/24 Para. 154).

    Additional information concerning the level and fate of
    pirimiphos-methyl residues on sorghum and in milled products from
    wheat has come available and is summarized the following monograph

    No new toxicological information has become available since the 1976



    In 1974 the usefulness of pirimiphos-methyl as a grain protectant for
    barley, oats, maize, rice, rye and wheat was evaluated and maximum
    residue limits in these grains were recommended. Since then there has
    been extensive experience from bulk trials and commercial use of
    pirimiphos-methyl on these raw cereal varieties and also on sorghum.

    The rate of application depends upon a number of considerations of
    which the following are the most important: -

         a)   anticipated period of storage

         b)   temperature of grain

         c)   moisture content of grain (as an indicator of relative
              humidity in the inter-grain space)

         d)   type of storage structure

         e)   severity of insect pest problems

         f)   intended use of the grain

    There are many situations where cool (temperature below 15°C), dry
    (moisture content less than 10%) grain is to be stored for short
    periods in sound storage structures where insect pest numbers are
    relatively low. Under such conditions the application of
    pirimiphosmethyl at rates of the order of 4 mg/kg or less would

    provide adequate protection against most, if not all, of the
    stored-product pests likely to be encountered. This applies equally to
    all varieties of grains though it is known that some varieties are
    preferred by certain species of insects.

    However, in many countries, particularly in the tropics and
    semi-tropics, grain with a high temperature (above 30°C and ranging
    above 40°C) and high relative humidity must be stored for long periods
    (one year or more) under conditions that lead to high incidence and
    high levels of insect infestation. Under such conditions many
    countries sustain losses of over 20% of their harvested crop.

    It is therefore apparent that some lots of barley, maize and oats may
    need to be treated with pirimiphos-methyl at rates that could produce
    residues higher than the maximum residue limit of 7 mg/kg recommended
    in 1974.


    Because of the development of resistant strains and the failure of
    malathion and dichlor vos to effectively protect sorghum from insect
    attack, pirimiphos-methyl has been evaluated as a grain protectant for
    sorghum. Bengston at al. (1977c) evaluated pirimiphosmethyl along with
    a number of other organophosphorus insecticides in combination with
    bio-resmethrin, d-phenothrin and pyrethrins. Pirimiphos-methyl was
    applied at the nominal rate of 4 mg/kg at two separate sites. The
    sorghum contained 11.8 -12.6% moisture and had a temperature of 25 -
    27°C. The rate of decline of pirimiphos-methyl residues is given in
    Table 1.

    TABLE 1. Pirimiphos-methyl residues on sorghum held in storage


    Site/Weeks     1       4       8       12      18      24

    A              3.9     3.6     3.5     3.1     3.1     3.0
    B              3.9     4.9*    3.5     3.4     2.4     4.9*

    * samples contained appreciable quantities of grain dust

    Bioassay tests indicated that at the end of 12 weeks the treated
    sorghum would still withstand infestation by a variety of
    stored-product pests selected to be resistant to malathion. The work
    is continuing.

        TABLE 2. Pirimiphos-methyl residues in stored wheat, 1976-77 Pilot studies (Australia)
    nominal application rate 4 mg/kg


                                                         Residue, mg/kg
    Site       No.       Dec.    Jan.    Feb.    Mar.     Apr.      May       June      July     Aug.     Sept.

    A          8         1.8     2.4                      1.8       2.1                          1.6      1.5
    B          1         1.6             1.1
    C          8         2.4
    D          8         2.4     2.4             2.2                1.6       1.8
    E                                    4.5     6.2      5.5                 5.6
    F          1-3                       3.1     3.3                4.3       3.8                         2.5
    "          2-3                       3.0     2.9                4.2       3.6                         2.5
    "          3-3                       2.9     2.4                3.6       3.3                         3.3
    G          93        2.2     3.4                      2.4       2.6                          2.1
    "          98                0.8                      1.2       1.0       1.2                1.0
    "          99                        1.0              0.9       1.1       1.2                1.0
    H                                            2.3                1.1                                   1.8
    I                                            2.6      3.2       1.9                          3.4
    J                                            3.1      2.6       2.2       2.5                         3.0
    K                                                               0.9       1.2                4.2
    L                                                               1.8       1.2                1.8      1.6
    M                                                               1.8       2.1                2.1      2.9
    N                                                                                                     3.6


    Bengston at al. (1975) have reported on the performance and fate of
    pirimiphos-methyl as a grain protectant against malathion-resistant
    insects in stored wheat in laboratory studies and Bengston at al.
    (1977a,b) have reported the results of extensive bulk trials and
    commercial scale use of pirimiphos-methyl on wheat in Australia. Prom
    these studies it was calculated that the half-life of
    pirimiphos-methyl in grain at 25°C is of the order of 80 weeks. Table
    2 gives a section of the analytical data from 18 separate storages
    over a period of 9 months. In these studies the application rate was
    deliberately low in order that the likelihood of failure to control
    resistant species could be determined without waiting for
    exceptionally long times.

    Cerná and Bénes (1977) have provided results of a study carried out in
    Czechoslovakia wherein 400 tons of wheat containing 12.6 -14.2%
    moisture at a temperature of 8-10°C was treated with pirimiphos-methyl
    at a rate equivalent to 4 mg/kg. The wheat was analysed at the time of
    treatment and at intervals thereafter until discharge for milling at
    the end of 286 days. The results of these analyses are given in Table

    TABLE 3. Pirimiphos-methyl residues after treatment of wheat
    in the silo at 4 mg/kg


    Days after treatment          Sampling                 Residues, mg/kg

    0                       during treatment                 3.70
    51                      top                              3.11
                            bottom                           3.35

    105                     top                              4.51
                            bottom                           2.50
    160                     bottom                           2.27
    286                     (average before grinding)        1.62

    In a trial in the Philippines pirimiphos-methyl was applied either as
    a dust or emulsifiable solution to maize (shelled) at rates calculated
    to leave 10, 15 and 30 mg/kg on the grain. Samples were analysed after
    3, 6 and 9 months. The results obtained are given in Table 4
    (Kagallona, 1977).

        TABLE 4. Pirimiphos-methyl residues on maize (Philippines)


    Formulation          Rate of application         Residue found after
                                  mg/kg              3          6         9 months

    E.C.                          10                 1.3                  N.D.
                                  15                 1.5        1.5       N.D.
                                  30                 2.2        2.5       N.D.

    Dust                          10                 0.6        0.4       N.D.
                                  15                 1.5        1.2       N.D.
                                  30                 2.8        2.5       N.D.


    In stored products

    Residues of pirimiphos-methyl on grains are degraded and detoxified by
    hydrolysis of the phosphorus-ester side chain, the rate of hydrolysis
    increasing with increases in moisture and temperature (FAO/WHO 1975,
    Page 502).

    Desmarchelier (1977) has shown by an extensive series of experiments
    with different grains held at a series of controlled temperatures at
    different relative humidities that the loss of insecticide from
    post-harvest application to various grains is predictable. These
    studies have shown that the loss is a second-order process with the
    rate of loss being proportional, at a fixed temperature, to the amount
    of insecticide and the equilibrium partial pressure of watervapor, or
    in the inter-grain space.

    The predictions of Desmarchelier (1977) have been plotted against
    residues of pirimiphos-methyl expressed as a percentage of the
    application rate using a log scale. Fig. 1 shows the rate of
    degradation predicted for "typical Australian conditions" and based on
    80% recovery of the calculated application against the mean of
    analytical results from many separate samples from 20 separate sites.
    Temperature and humidity conditions were neither uniform nor
    consistent for all sites.

    From this work it is clear that the rate of degradation doubles for
    each 5°C increase in temperature over the range 15 -35°C. In other
    words the residual life of deposits is twice as long at 25°C as at

    30°C. This information will enable grain storage authorities to treat
    their grain with protection whilst possibly avoiding excessive
    residues. However, it is often impossible to determine in advance how
    long the grain will remain in store.

    In processing

    Results of experiments in Czechoslovakia (Cerná and Bénes 1977)
    indicate that the residues in grain which has been in store for 9
    months are substantially removed by the milling process. Table 5 shows
    both the residue level and total weight of pirimiphos-methyl in the
    various mill fractions. The bulk of the residue was removed in the
    bran, there being no substantial difference in the concentration in
    the different bran fractions. When the white flour was made into white
    bread there was a further loss of approximately 50% so that the
    concentration of the residue in the bread was only 10-15% of its
    concentration in the raw grain. Only 10-18% of the total weight of
    residue on the raw grain found its way into the white flour. Some
    residue was lost (destroyed?) during milling.

    In bran

    The Codex Committee on Pesticide Residues at its 9th Session asked for
    reconsideration of the recommended maximum residue limit in bran, in
    view of the use of bran in human diets (ALINORM 78/24 para. 154).

    With the exception of the study by Cerná and Bénes (1977) there are no
    data on the level and fate of residues in diet bran additional to that
    reported in the 1974 monograph. Most workers, including those whose
    work was reviewed in 1974 and 1976, have concentrated on studying the
    transfer of residues from grain to bread, biscuit, pasta and similar
    cereal foods or wholemeal products. The small dietetic use of bran has
    not apparently been studied separately.

    In the 1974 monograph it was reported that although bran represents
    only about 25% of the mass of the original grain the concentration of
    residues in the bran was of the order of 2.5 times the concentration
    in the whole wheat. In the studies reported by Cerná and Bénes the
    concentration in bran was 2.8 times that in the whole wheat from which
    it was milled. Since the rate of application of pirimiphos-methyl
    ranges from 4 to 6 mg/kg, with higher rates only where high
    temperature and moisture are likely to contribute to rapid
    degradation, it can be assumed that in commercial practice with
    typical storage periods, crude bran of the type used for dietetic
    purposes will contain about 5 mg/kg or less of pirimiphos-methyl.
    Wheat from several sources is usually blended in order to obtain the
    qualities desired in the flour and baked goods. Such blending will
    usually lead to a dilution of the residues, especially those
    segregated in bran.

    FIGURE 8

        TABLE 5. Pirimiphos-methyl residues in wheat, milling fractions and white bread


                                                     A                                      B
                                             pirimiphos-methyl                      pirimiphos-methyl

                             Weight of      mg/kg     Total weight   Weight of      mg/kg          Total weight
    Fraction                 fraction,                in fraction,   fraction,                     in fraction,
                             g                        mg             g                             mg

    wheat before grinding    1000.0         1.62      1.62           1000.0         1.94           1.94
    crude bran               172.4          4.64      0.80           207.6          4.84           1.00
    fine bran                61.2           4.90      0.30           77.4           3.01           0.23
    pollard flour            215.7          0.32      0.07           215.0          0.45           0.10
    white flour              550.7          0.525     0.29           500.0          0.371          0.19
    white bread                             0.27                                    0.19
    total after grinding     1000                     1.46           1000                          1.52
    decrease by grinding                              0.16                                         0.42
                                                      (9.9%)                                       (21.7%)

    A: wheat treated in the silo (dose 4 mg/kg) 286 days earlier
    B: wheat treated in the laboratory (dose 4 mg/kg)



    Desmarchelier at al. (1977) as the result of a collaborative study of
    the analysis of residues of a number of grain protectant insecticides,
    have pointed to the importance of the extraction solvent and procedure
    in obtaining consistently high recoveries of residues from grain and
    cereal products. They found a simple shaking of unmilled grain with
    methanol at room temperature to be the most convenient and consistent
    procedure. The extract can be used for direct injection into the gas
    chromatograph without further clean-up.


    The following national maximum residue limits have been reported to
    the Meeting.

    Country             Commodity                MRL mg/kg

    Australia           Bran                     20
                        Wheat, rye, rice
                        (in husk)                10
                        Barley, maize, oats      7
                        Wholemeal flour          5
                        Rice (dehusked),
                        Wheat flour (white)      2
                        Bread (wholemeal),
                        rice (polished)          1
                        Bread (white)            0.5
                        Meat, milk and eggs      0.05*
    * at or about limit of determination


    In 1976 a request was made for results from commercial trials with
    pirimiphos-methyl in further commodities and the Codex Committee on
    Pesticide Residues at its 9th (1977) Session sought clarification of
    the intake by humans, the high level of residues in bran and the
    distinction between commodities destined for animal feed and human

    Additional information concerning the level and fate of
    pirimiphos-methyl residues on Sorghum, maize and in milled products
    from wheat has become available.

    The rate of application of pirimiphos-methyl post-harvest for grain
    protection depends upon the degree of protection desired, the length
    of storage, the temperature and humidity of the grain and the
    anticipated period of storage. At 25°C and low relative humidity the
    half-life of pirimiphos-methyl on grain is of the order of 80 weeks.
    Under hot humid conditions, the half-life is comparatively short and

    therefore higher initial deposits are required to achieve acceptable
    protection against insect damage.

    A model has been developed whereby it is possible, given the
    temperature, relative humidity and proposed application rate, to
    predict the storage life of the deposit and to estimate the residue
    level at any desired date. Residue data from extensive pilot and
    commercial trails have been compared with the model and there has been
    remarkably good agreement.

    The fate if residues of pirimiphos-methyl on grain subjected to
    milling has been revealed in several studies. These show that the bulk
    of the residue is removed in the bran and that only about 10% of the
    amount present on the grain finds its way into bread. It is recognized
    that small amounts of bran are used for dietetic purposes but it is
    considered that the amount contributed to the intake from this source
    is acceptable because only occasional batches of grain will contain
    residues approaching the recommended maximum residue limit.

    Attention is drawn to the importance of the extracting solvent and
    extraction technique on the recovery of residues from raw grains,
    milling products and cooked cereals.


    The existing maximum residue limits for barley, maize, oats, rice (in
    husk), rye and wheat are replaced by a limit for raw cereals.

              Commodity                                  Limit, mg/kg
         Raw cereals (except rice, hulled or polished)       10



    1.   Results of studies now in progress on the residues in peanuts and
         peanut products.
    2.   Results trim commercial trials on other commodities.
    3.   Further information on the level and fate of residues in food at
         the point of consumption following the use of pirimiphos-methyl
         for the control of various stored product pests.


    Bengston, Cooper, L.M., and Grant-Taylor, F.J. (1975) A comparison of
    bioresmethrin, chlorpyrifos-methyl and pirimiphos-methyl as grain
    protectants against malathion resistant insects. Queensland J. agric.
    anim. Sci. 32, 51-78.

    Bengston, Connell, M., Crook, I.D., Desmarchelier, J.M., Hart, R.J.,
    Phillips, M., Snelson, J.T. and Sticka, R. (1977a) Field trials to
    compare chlorpyrifos-methyl, fenithrothion, pirimiphos-methyl

    malathion and methacrifos for the control of malathion-resistant
    insects infesting wheat in Australia. J. Stored Prod. Res. (In press).

    Bengston, M., Connell, M., Davies, R., Desmarchelier, J., Elder, B.T.,
    Hart, R., Phillips, M., Ridley, E., Ripp, E., Snelson, J. and Sticka,
    R. (1977b) Chlorpyrifos-methyl plus bioresmethrin, methacrifos,
    pirimiphos-methyl plus bioresmethrin and synergised bioresmethrin as
    grain protectants for wheat (In press).

    Bengston, M., Cooper, L.M., Davies, R.A.H., Desmarchelier, J.M., Hart,
    R/J. and Phillips, M. (1977c) Grain protectants for the control of
    malathion resistant insects in sorghum. J. Stored Prod. Res. (in

    Cerná, V. and Bénes, V. (1977) Residues of pirimiphos-methyl in wheat
    mill products and white bread. Report by Czechoslovakian Institute of
    Hygiene and Epidemiology. Prague - June 30, 1977.

    Desmarchelier, J.M. (1977) (CSIRO, Division of Entomology, Canberra,
    Australia.) Loss of insecticide deposits on grains in storage. (In

    Desmarchelier, J., Bengston, M., Connell, M., Minett, W., Moore, B.,
    Phillips, M., Snelson, J., Stiaka, R., and Tucker, K. (1977a) A
    collaborative study of residue of CGA 20168, chlorpyrifos-methyl,
    fenitrothion, malathion and pirimiphos-methyl.
    (1977b) II. Rates of decay - Pestic. Sec. (in press).

    Magallona, E. (1977) Fate of pirimiphos-methyl applied to stored corn.
    Report to FAO from Pesticide Residue Laboratory University of the

    FAO/WHO (1975) 1974 evaluations of some pesticide residues in food.
    AGP:1974/14/11; WHO Pesticide Residues Series, No. 4.

    FAO/WHO (1977) 1976 evaluations of some pesticide residues in food.

    See Also:
       Toxicological Abbreviations
       Pirimiphos-methyl (WHO Pesticide Residues Series 4)
       Pirimiphos-methyl (Pesticide residues in food: 1976 evaluations)
       Pirimiphos-methyl (Pesticide residues in food: 1979 evaluations)
       Pirimiphos-methyl (Pesticide residues in food: 1983 evaluations)
       Pirimiphos-methyl (Pesticide residues in food: 1992 evaluations Part II Toxicology)