FAO, PL:CP/15
    WHO/Food Add./67.32


    The content of this document is the result of the deliberations of the
    Joint Meeting of the FAO Working Party and the WHO Expert Committee on
    Pesticide Residues, which met in Geneva, 14-21 November 1966.1

    1 Report of a Joint Meeting of the FAO Working Party and the WHO
    Expert Committee on Pesticide Residues, FAO Agricultural Studies, in
    press; Wld Hlth Org. techn. Rep. Ser., 1967, in press




        carbophos, malathion

    Chemical name

    S-[1,2-di(ethoxycarbonyl)ethyl] dimethyl phosphorothiolothionate or
    S-[1,2-di(ethoxycarbonyl)ethyl] 00-dimethyl phosphorodithioate




    Biochemical aspects

    Malathion is rapidly absorbed from the intestinal tract. Its
    metabolism has been studied in the hen, mouse, rat, cow and man.
    Malathion is oxidized to malaoxon, the active form of the compound,
    and is also hydrolyzed to less toxic metabolites. Six to eight
    metabolites have been found, the main ones being in the urine,
    malathion mono- and di-acids. Malathion or its metabolites were
    recovered from eggs from treated hens and milk from cows treated with
    malathion (March at al., 1956; O'Brien at al., 1961).

    Malathion labelled with 32P was given to a lactating cow at 1.3
    mg/kg bodyweight per day for three days. Malathion metabolites were
    rapidly excreted in the urine, 69 per cent of the total radioactivity
    being excreted in four days after the first dose, after which the
    excretion rate decreased. After seven days 77.2 per cent of the dose
    was recovered, 69 per cent of which was in the urine, 8 per cent in
    the faeces, and 0.2 per cent in the milk. The principal metabolite in
    early urine samples was the mono-acid of malathion. In later samples
    it was the di-acid. Demethyl malathion was a significant component in
    early and late samples. Dimethyl phosphate and
    0,0-dimethylphosphorothioate were present in very small amounts. In
    the faeces, 85 per cent, of the labelled material was malathion and 12
    per cent was malaoxon (O'Brien at al., 1961).

    In a lactating cow fed 1.3 mg/kg body-weight daily for three days
    there was no significant inhibition of erythrocyte cholinesterase
    activity (O'Brien et al., 1961).

    Malaoxon is a cholinesterase inhibitor in vivo and in vitro (I50
    7 × 10-7) (O'Brien, 1957).

    The half time for the conversion in vivo of the reversibly inhibited
    form of the dimethylphosphorylated cholinesterase to the irreversibly
    inhibited form of this enzyme in the brain of chicken given malathion
    has been found to be 2 hours. The same half time was observed in
    vitro with the brain homogenate inhibited with paraoxon (Witter &
    Gaines, 1963).

    After single intraperitoneal or oral doses of malathion, trichlorofon
    or dioxathion in rats, an increase in the activities of liver tyrosine
    transaminase and alkaline phosphatase, as well as a decrease in the
    level of adrenal ascorbic acid were found. Further results of this
    experiment support the hypothesis that acute poisoning may produce
    metabolic alterations which are mediated through the pituitary-adrenal
    system (Murphy, 1966).

    Simultaneous administration of malathion and ethyl p-nitrophenyl
    thionobenzenephosphate (EPN) results in a potentiation of the
    cholinesterase inhibitory effect of malathion in the mouse, rat and
    dog (Frawley et al., 1957).

        Acute toxicity
    Animal        Route            LD50                         References
                             mg/kg body-weight
                             90%            99%
                             technical      technical

    Rat, male     Oral       940-1156*      4700-5843*          American Cyanamid Co., 1955
                                                                Hazleton & Holland, 1953

    Rat, male     Oral       390-480*       1400-1845*          American Cyanamid Co., 1955
                                                                Frawley et al., 1957
                                                                Hazleton & Holland, 1953

    Mouse, male   Oral       720-886        3300-4060           American Cyanamid Co., 1955
                                                                Hazleton & Holland, 1953

    Mouse, male   Oral                      2700-3320           American Cyanamid Co., 1955
                                                                Hazleton & Holland, 1953

    Mouse, male   i.p.       420-474                            Hazleton & Holland, 1953

    Chicken       Oral       >850(95%)                          American Cyanamid Co., 1955

    Calf          Oral       80 (95%)                           American Cyanamid Co., 1955

    Cow           Oral       560(95%)                           American Cyanamid Co., 1955

    * Differences due to use of different vehicles.

    In a colony of rats showing an oral LD50 of 925 mg/kg for adults,
    the intragastric LD50 for newborn rats was approximately 124 mg/kg
    (Lu et al., 1965).

    Simultaneous oral administration of malathion and fenitrothion to male
    rats resulted in potentiation when one-half the LD50 doses were
    given. However, no potentiation was seen when one-tenth the LD50
    doses were given (Benes & Cerná, 1966).

    Short-term studies

    Mouse. When malathion was added to the diet as 500 or 5000 ppm for 6
    weeks or after the administration, of 5 oral doses of 500 mg/kg the
    production of antibodies against B. pertussis was not affected
    (Benes et al., 1963).

    Rat. Groups of 10 males were given malathion at 100 or 500 ppm in
    the diet or trichlorofon at 60 or 300 ppm for 6 weeks and this was
    followed by the administration of both compounds at the same time.
    During the experiment erythrocyte cholinesterase fluctuated around 100
    per cent of the initial values. At the end of the experiment, in
    comparison with the control group, the adrenals weighed more and
    showed hypertrophy of both cortex and medulla, the intensity of which
    was related to the concentration of the two substances in the diet
    (Benes & Cerná, 1965).

    In another experiment 95 per cent technical malathion was fed to 3
    groups of male rats, 10 animals per group, for 33 days at the levels
    of 100, 1000 and 5000 ppm. No sign of toxicity was observed, nor any
    deaths. Food intake and weight gain in the groups fed 100 and 1000 ppm
    were higher than in the control group; groups fed 5000 ppm showed no
    difference from the controls. Cholinesterase activity was determined
    in 6 animals from each group. Activity was normal in the 100 ppm
    group. Erythrocyte cholinesterase activity was 68 per cent of normal
    in the 1000 ppm group, and in the 5000 ppm group plasma cholinesterase
    activity was 78 per cent and erythrocyte activity 22 per cent of
    normal. At all levels no depression of brain cholinesterase activity
    was found (American Cyanamid Co., 1955).

    Ninety-eight per cent technical malathion was fed to groups of 5 rats
    for 8 weeks at levels of 100 and 500 ppm without any inhibition of
    whole-blood cholinesterase activity (Frawley et al., 1957).

    Ninety-five per cent technical malathion was fed to 40 male and 40
    female rats for 5 months in a daily dose of 240 mg/kg body-weight
    (4000 ppm in the diet). Growth was normal and no signs of intoxication
    occurred. Ten weeks after the beginning of the experiment, 18 females
    and 12 males were used for breeding. The average litter size from the
    treated females was smaller than in the controls and the number of
    newborn alive after 7 and 21 days was about half the number in the
    litters of the controls (Kalow & Marton, 1961).

    Chick. Ninety-five per cent technical malathion was fed to day-old
    chicks for 2 weeks at a level of 10 ppm. For the following 10 weeks
    they were divided into groups of 10 and fed 100, 1000 and 5000 ppm in
    their diets. The groups on 100 and 1000 ppm behaved normally and
    showed a similar growth rate and food consumption to the controls.
    Four animals died in the 5000 ppm group, and signs of intoxication and
    growth retardation were observed. At necropsy, no pathological lesions
    were found. Plasma and brain cholinesterase activities were
    significantly lowered in the 5000 ppm group (American Cyanamid Co.,

    In a two-year study, 21 females were fed 250 ppm and 21 females and 6
    males 2500 ppm. The 250 ppm group did not differ significantly from
    the controls. At the 2500 ppm level a decrease in plasma
    cholinesterase activity was found between the 195th and 465th day of
    experiment. The test hens came into production later and laid slightly
    fewer eggs, but the hatchability was not influenced. The offspring
    showed no deformities. At necropsy no gross or microscopical lesions
    were found (American Cyanamid Co., 1960).

    Man. Five male volunteers, 23-36 years old, took 6 mg of malathion
    in gelatin capsules daily for 32 days. No effect on plasma or
    erythrocyte cholinesterase activity could be detected. Five males took
    16 mg daily for 47 days, also without any significant effect on
    cholinesterase activity. A daily dose of 24 mg taken by 5 males for 56
    days was followed by depression of the plasma cholinesterase activity
    2 weeks, after the first administration. Maximum depression amounting
    to about 25 per cent of the plasma cholinesterase activity occurred
    approximately 3 weeks after the cessation of administration. No
    clinically manifest side-effects were reported. Simultaneous intake of
    16 mg of malathion and 5 mg of EPN per day caused a slight inhibition
    of cholinesterase activity (Moeller & Rider, 1962).

    No plasma or RBC cholinesterase depression was noted in 10 humans
    ingesting 3 mg EPN or 8 mg malathion daily for 32 days, nor in 5
    humans receiving 6 mg EPN for 88 days and 8 mg malathion for the last
    44 days, nor in 5 humans ingesting 16 mg malathion for 86 days and 3
    mg EPN for the last 41 days. However 10 humans ingesting 6 mg EPN and
    16 mg malathion daily for 42 days showed a slight depression of both
    the plasma and the RBC cholinesterase (Rider et al., 1959).

    Long-term studies

    Rat. Sixty-five per cent technical malathion as a 10 per cent or 25
    per cent wettable powder was mixed in the diets of groups of 20 male
    rats at the levels 100, 1000 and 5000 ppm, and fed for 2 years. The
    mortality rate was not influenced, and at the 2 lower levels weight
    gain and food intake were comparable to those of the controls. Five
    thousand ppm reduced food intake and decreased weight gain.
    Cholinesterase determinations showed no inhibition at the 100 ppm
    level; with a diet containing 1000 ppm, 36 per cent inhibition of
    cholinesterase activity was found in the plasma, 73 per cent in the

    erythrocytes and 37 per cent in the brain, while at the 5000 ppm
    level, the plasma samples showed 80 per cent, the erythrocytes 100 per
    cent and the brain 77 per cent inhibition. Neither gross nor
    microscopic examination revealed any pathological changes attributable
    to malathion (American Cyanamid Co., 1955; Hazleton & Holland, 1953).

    Ninety per cent technical malathion was fed as 25 per cent wettable
    powder in the diet to 20 males at a concentration of 100 ppm, to 20
    males and 10 females at 1000 ppm, and to 20 males at 5000 ppm for 2
    years. Mortality rate, growth response and food intake were not
    influenced by any of these diets, except that there was some growth
    retardation when the concentration was 5000 ppm. Terminal
    cholinesterase determinations revealed 10-30 per cent inhibition of
    cholinesterase activity in the plasma, erythrocytes and brain at 100
    ppm. At 1000 ppm, 60-95 per cent inhibition of erythrocyte
    cholinesterase activity was observed. The 5000 ppm group showed total
    inhibition of erythrocyte cholinesterase activity and 60-95 per cent
    inhibition of cholinesterase activity in plasma and brain (American 
    Cyanamid Co., 1955, Hazleton & Holland, 1953).

    Ninety-nine per cent technical malathion was fed for 2 years to groups
    of 3-4 rats and produced, at 1000 and 5000 ppm levels, inhibition of
    erythrocyte cholinesterase activity of the same order as did the 90
    per cent compound. The decrease in plasma and brain cholinesterase
    activity, however, was much less than that produced by 90 per cent
    technical malathion (American Cyanamid Co, 1955, Hazleton & Holland,

    A two-year rat feeding experiment with combinations of six pesticides
    (DDT, aldrin, pyrethrin, piperonyl butoxide, 2,4-D and malathion) and
    eight flavouring agents (allyl heptylate, anethole, amyl butyrate,
    cinnamic aldehyde, citral, ethyl methyl phenyl glycidate, eugenol, and
    methyl salicylate) did not show significantly different toxic effects
    compared with the effects of the compounds administered separately
    (Fitzhugh, 1966).


    The studies are extensive and have been carried out in several species
    including man.

    In view of the very high doses used in the short-term breeding
    experiments in the rat, the results of these experiments were not
    taken into account in arriving at the maximum acceptable daily intake
    for man.

    It would be desirable to carry out reproduction studies in at least
    two species, and biochemical studies, particularly with regard to the
    influence of other chemicals on the metabolism of malathion.


    Level causing no toxicological effect

    Rat. 100 ppm in the diet, equivalent to 5 mg/kg/day.

    Man. 16 mg a day, equivalent to 0.2 mg/kg/day.

    Estimate of acceptable daily intake for man

     0.002 mg/kg/body-weight


    Use pattern

    (a) Pre-harvest treatments

    Malathion is used in many countries against aphids, scales and other
    insects on a wide range of fruits and vegetables in agriculture and
    horticulture. Crops treated include stone fruits (e.g. plums), pome
    fruits (e.g. apples and pears) and soft fruits; carrots, turnips,
    tomatoes and leafy vegetables.

    Malathion is also used fairly widely in the veterinary field on
    poultry (lice, mites, fleas), cattle and pigs (lice and flies).

    (b) Post-harvest treatments

    Malathion is used on a fairly large range of products during storage.
    In some instances, it is applied directly to the raw agricultural
    product (e.g. cereals, oilseeds, nuts, beans); in others, its use on
    foodstuff (e.g. as a general warehouse spray) is incidental to the
    hygiene of storage.

    (c) Other uses

    Malathion is a common ingredient of pesticides used against various
    public health and domestic insect pests (flies, mosquitos, roaches,
    etc.). It is also used quite extensively as a home garden insecticide,
    when it may be sprayed on to both food crops and ornamental plants.


    Product               Country                Tolerance ppm

    General               Austria                7

    Tolerances (cont'd)

    Product               Country                Tolerance ppm

    Cereals               Brazil                 8

    Cereals               Germany                3

    Cereals               Kenya                  12.5

    Flour                 France                 2

    Fruits, green         Comeco                 5
    vegetables            (Bulgaria,
                          Roumania, East
                          Germany, Poland

    Apples, pears         USA                    8
    peaches, plums,       Canada

    Leafy vegetables      USA                    8

    Meat (beef, pork,     USA                    4
    poultry, etc.)        Canada

    Residues resulting from supervised trials

    (a) Pre-harvest treatments

    Many data are available on a variety of food crops from different
    rates of application, modes of application, and times between
    application and harvest. As examples Waites & Van Middelem (1958)
    sprayed turnip tops and collards at various rates and found a maximum
    of 3.9 ppm three days after the application. Tew & Sillibourne (1960)
    measuring residues in apples and soft fruits, found a half life period
    of only 1-1/2 to two days. Under their conditions useful applications
    of the pesticide were unlikely to result in residues above 0.5 ppm at
    harvest. Eheart (1962) investigated the persistence of various
    pesticides on vegetable crops and judged that collards could be

    consumed three days after spraying. From these and other data,
    including some supplied by manufacturers of malathion, it is possible
    to summarize the amounts expected to remain after useful applications
    of the insecticide, as follows:

    Type of food             Pre-harvest              Residue
                             period                   (ppm)


    Cabbage                  2                        2 to 30
                             7                        <0.5

    Potatoes                    No residues detected

    Lettuce                  0                        21
                             7                        5

    Kale                     2                        3
                             7                        <0.5

    Beans                    0                        1 to 50
                             3                        0.5

    Beet                     1                        15
                             4                        9


    Berries (cane)           1                        1
                             3                        0.1

    Apples and pears         0                        5
                             3                        1.5
                             7                        0.5

    Cherries and plums       0                        Up to 10
                             7                        1.5

    Grapes                   1                        2
                             7                        0.5

    Peaches                  0                        Up to 18
                             7			      3	

    Tomatoes                 0                        Up to 6
                             3 to 4                   0.5

    Type of food             Pre-harvest              Residue
                             period                   (ppm)

    Citrus                   1                        3.5
                             7                        1.5
                             21                       0.5

    Cereals (pre-harvest)    3                        Up to 4
                             7                        <0.5

    The fairly high levels are from relatively short pre-harvest intervals
    and longer intervals result in lower residues. Data are not extensive
    for residues from pre-harvest use on cereals. Nevertheless, various
    workers have shown that malathion disappears rapidly on plants. For
    example, Tomizawa et al. (1960), using labelled malathion, found less
    than five per cent remaining on rice plants two days after
    application: Koivistoinen (1961) found half life periods of two days
    or less for residues on a fairly wide range of plants. Tomizawa & Sato
    (1962) examined the mechanisms by which the insecticide disappeared
    from rice during growth of the plant. From this work it is evidence
    that applications during the growing period and up to seven days from
    harvesting of cereals should leave residues which are much lower than
    those which are added in many countries during post-harvest
    treatments. Pre-harvest treatments therefore are a minor source of
    residues in cereals in commerce.

    Investigations of the residues resulting from uses in veterinary
    practice have been fairly reassuring. Goulding & Terriere (1959) could
    barely detect residues in milk and found none in flesh of cattle
    treated for the control of horn fly. Claborn et al. (1960) compared
    the effects from this and other insecticides used as sprays on
    livestock: only traces of malathion could be detected in milk.
    Pasarela et al. (1962) also did not detect malathion in various
    tissues of cattle receiving food containing 200 ppm for 41 to 44 days,
    although traces were found in the livers of two calves sacrificed
    after 14 days. It was also not possible to detect residues in the milk
    from cows, each receiving a daily 12 lb ration of dairy chow
    containing up to 800 ppm. Adkins & Hair (1965) also were not able to
    detect residues after the application of malathion to cattle via back

    (b) Post-harvest treatments

    As outlined in the second report of the FAO Working Party on Pesticide
    Residues (PL/1965/12), malathion is widely used for controlling
    insects in stored cereals and residues up to about 8 ppm result from
    this usage.

    Residues in food moving in commerce

    (a) Cereal grains

    Samples taken within countries where treatments have been carried out
    (e.g. Australia, Britain, USA) show up to 8 ppm, which is the
    recommended dosage for effective treatment. This is an average figure
    and some spread has been observed due to uneven admixture in some
    cases. Lower figures have been found after periods of storage and at
    the termination of sea voyages. For example, examinations in the
    United Kingdom of 70 samples representing nine shipments from
    Australia known to have been treated at between 8 and 10 ppm showed a
    mean of 5.1 in the ships before discharging.

    Malathion is not very stable, is relatively volatile and soluble in
    water. Therefore, it would not be expected to remain long in any
    product where it is exposed to air and moisture. On the other hand it
    is soluble and would be expected to remain much longer in oily
    products. For example, it dissolves in the oil glands of citrus peel
    where it may remain for a long period.

    Fate of residue during storage and processing

    (a) In plants and animals

    The rapid losses which occur on the plant before harvesting appear to
    be due to a number of factors (Koivistoinen, 1961) including
    evaporation, chemical decomposition of surface deposits and metabolism
    within the plant. In cereals, metabolism appears to lead to the
    formation of thiophosphoric acid and to the mono- and dicarboxylic
    acids of malathion.

    The fate and toxicity of malathion in the animal body was reviewed in
    the report of the first joint meeting of FAO Committee on Pesticides
    and the WHO Committee on Pesticide Residues (FAO/WHO 1964) and in the
    second report of the FAO Working Party on Pesticide Residues
    (PL/1965/12). No significant residues have been found in the milk or
    other tissues of animals receiving the pesticide.

    Koivistoinen (1961) has shown that an enzyme system in plants rapidly
    hydrolyzes malathion similar to that reported by Cook, Blake &
    Williams (1957) in liver tissues.

    (b) In storage and processing

    The losses occurring during the storage of cereals appear to be almost
    entirely due to hydrolysis to relatively inert derivatives (Rowlands,
    1964, 1965). During the preparation of flour from treated wheat much
    of the residue is removed and the residues in the order of one tenth
    of those in the whole wheat have been found (Schesser, Priddle &
    Ferrell, 1958; Allessandrini, 1965; Acton & Parouchais, 1966). The

    main losses apparently occur during the cleaning process. The figure
    for certain of the by-products, such as bran, may be greater than
    those in the original wheat; but this does not seem very important
    bearing in mind that residues of malathion have not been found in the
    milk or fat of animals to which the insecticide has been fed.

    Malathion is not stable to heating in the presence of moisture,
    particularly under neutral or alkaline conditions. It does not readily
    stand up to cooking. Alessandrini (1965) found that bread prepared
    from flour containing known amounts of malathion had residues of from
    eight to 16 per cent of that originally present. She concluded that
    the residues would be negligible in bread from flour prepared from
    wheat treated at commercial rates. In cooked pasta the residues were
    not detectable.

    Koivistoinen et al. (1964) investigated the stability of residues
    during storage and processing of various fresh fruits and vegetable.
    Residues on the surfaces decreased almost as rapidly as on the growing
    plants (i.e. 50 per cent loss in from one to two days). Residues were
    much more stable on or in deep frozen foods but during the preparation
    of juice for storage or of jam from 54 to 86 per cent of the residues
    present were destroyed; 1.1 ppm was the highest residue present in any
    of this jam.

    Methods of residue analysis

    A number of methods are available for the determination of malathion
    in foodstuffs. For cereals, the method suggested in the Second Report
    of the FAO Working Party (PL/1965/12) is suitable. Even though that
    method is adequate for the determination of the levels of malathion in
    the commodities included in this monograph, there seems good promise
    of more sensitive methods for malathion based on gas liquid
    chromatography. However, the extraction and clean-up procedures
    adequate for gas chromatography of malathion have not been
    sufficiently developed for the working party to make a recommendation
    at this time. For those interested in using GLC with a thermionic
    detector, a paper by Storherr et al. (1964) will be useful. The
    methods are sensitive to 0.1 ppm malathion in most foods.


    The range of foodstuffs that could conceivably contain residues of
    malathion is very wide. It could include cereal products, various
    fruits and vegetables and dairy or other animal products. However,
    there is no evidence of any residues being found in meat or dairy

    The recommendation for tolerances is as follows:


              Fruit          8.0

              Dried fruit    8.0

              Nuts           8.0

              Citrus         4.0

              Cereals and
              products       8.0

              Vegetables     6.0

              Vegetables     3.0
              (other than

    These values are those resulting from good pest control practice. They
    are predicated on a large loss in storage in processing. As an
    example, during the storing, transporting and milling of treated
    cereals, considerable reductions occur from the initial dosage levels
    of about 8 ppm. Cooking results in further substantial losses. The
    amounts found in bread or finished pasta made from wheat treated in
    this way (i.e. below 0.4 ppm). As these amounts are well within the
    permissible level, the residue resulting from good agricultural
    practice, viz. 8 ppm, may be accepted as a tolerance for raw cereals.


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    See Also:
       Toxicological Abbreviations
       Malathion (ICSC)
       Malathion (FAO Meeting Report PL/1965/10/1)
       Malathion (FAO/PL:1967/M/11/1)
       Malathion (JMPR Evaluations 2003 Part II Toxicological)
       Malathion (FAO/PL:1968/M/9/1)
       Malathion (FAO/PL:1969/M/17/1)
       Malathion (AGP:1970/M/12/1)
       Malathion (WHO Pesticide Residues Series 3)
       Malathion (WHO Pesticide Residues Series 5)
       Malathion (Pesticide residues in food: 1977 evaluations)
       Malathion (Pesticide residues in food: 1984 evaluations)
       Malathion (Pesticide residues in food: 1997 evaluations Part II Toxicological & Environmental)
       Malathion (IARC Summary & Evaluation, Volume 30, 1983)