METHAMIDOPHOS JMPR 1976 IDENTITY Chemical name OS-dimethyl phosphoramidothioate Synonyms Tamaron R, Monitor R, SRA 5172, Bayer 71 628, RE 9006 Structural formulaOther information on identity and properties Molecular weight: 141.1 State: yellowish to colourless crystals (technical) colourless crystals (pure) Melting point: 37 - 39°C (technical) 44.5°C (pure) Vapour pressure: 3 x 10-4 mm Hg at 30°C (pure) Density: 1.31 Solubility: Readily soluble in water, alcohols ketones, aliphatic chlorinated hydrocarbons; sparingly soluble in ether; practically insoluble in petroleum ether. Stability (alkaline Half-life of 120 hours at pH 9 and acidic conditions and 37°C Half-life of 140 hours at pH 2 and 40°C Impurities in the technical product Maximum level, % Total impurities 27 OO-Dimethyl phosphoramidothioate 12 Other methyl esters of phosphoramido and phosphoro-N-methylamidothioates 12 Methyl esters of phosphoric and phosphorothioic acids 8 Phosphate, sulphate, sulphonium ion, water 8 EVALUATION FOR ACCEPTABLE DAILY INTAKE BIOCHEMICAL ASPECTS Absorption, distribution and excretion Male and female rats were orally administered methamidophos (32p-labelled) following a two-week preconditioning period where a daily dose of 0.5 mg/kg was orally administered. Male rats only were administered 14C-methamidophos with no preconditioning at the same dose level. In the 14C study (label = S-CH3) absorption and a distribution was rapid. Elimination of 14C02 within the first 6 hours accounted for 32% of the recovered radioactivity with 9% of the dose in the urine. Over a 5-day period only 50% of the administered dose was recovered (39% as 14C02 and the rest predominantly in urine). The remainder of the 14C was probably present as residual radioactivity in the body, distributed randomly. The major tissue residue was found in the polar lipid fraction with lesser amounts as phospholipids and proteins (probably incorporated as natural products). Studies with 32p confirm the rapid absorption and distribution of radioactivity in rats. The major amount of material recovered following 32p labelled methamidophos dosing was observed in urine within 1 day. Feces contained small quantities that were continually excreted (<20% of the dose was excreted in feces over a 28-day test interval). Thus, methamidophos is rapidly absorbed and distributed in the body. Methamidophos and metabolites are excreted predominantly as urinary products (Crossley and Tutass, 1969). The metabolic fate of methamidophos in a lactating ruminant (goat) was essentially the same as reported for the rat. Small amounts of radioactivity were observed in milk, predominantly as the parent molecule shortly after dosing. About 1% of the radioactivity (<0.003 mg/kg) in the milk was methamidophos, which disappeared rapidly. Distribution of the applied radioactivity in the goat was rapid, predominantly as exhaled 14C02, in urine and as radioactivity incorporated into some tissues (muscle, liver, fat). 24% of the administered radioactivity was excreted within the dosing period. In tissues, the predominant residue was not the parent compound as evidenced by solvent partition characteristics (Lee and Crossley, 1972). Further details of the fate of metamidophos in the rat and lactating goat are given in the section "Fate of residues". Biotransformation Methamidophos is rapidly degraded primarily by cleavage of the P-N bond yielding dimethyl phosphoric acid derivatives. Microsomal preparations in vitro accelerated the hydrolysis of the phosphoramidate bond (Tutass, 1968a). A qualitative sequence of metabolism is as follows:
(Crossley and Tutass, 1969) A list of the identified metabolites of methamidophos is given in Figure 1. Effects on enzymes and other biochemical parameters Methamidophos is an active, direct cholinesterase inhibitor. In blood, cholinesterase depression was observed to be maximum within 3 hours following oral acute intoxication of male rats. Cholinesterase activity was maintained at a low level for at least 24 hours following acute poisoning. Three days after treatment, the enzyme activity of whole blood returned to a level that was almost normal (Lorke and Kimmerle, 1967).
RBC cholinesterase appears to be more sensitive than plasma cholinesterase to inhibition as noted by the differing I50 values. I50(M) Bovine RBC 3.1 x 10-5 Human Plasma 1.6 x 10-4 (Tucker, 1972) The sensitivity of RBC relative to plasma cholinesterase to inhibition by methamidophos was noted in a 3-week dietary study where methamidophos was fed at 0 and 10 ppm. No cumulative enzyme depression was observed and RBC was affected slightly more than plasma (Cavalli and Spence, 1970;) [see acephate - Cavalli and Spence, 1970f, summary of the data in addendum]. TOXICOLOGICAL STUDIES Special studies on delayed neurotoxicity Groups of hens (10 mature laying hens - 1-2 years old/group) were fed methamidophos in the diet (dosage levels were said to be 0, 0.3, and 3.0 mg/kg body weight) for four weeks. There was a control of TOCP fed at a dose of 50 mg/kg body weight. No mortality or severe weight loss was observed and clinical signs of ataxia were not noted. The TOCP-treated hens developed clinical signs of ataxia. Histological examination of nervous tissue showed no abnormalities, even in the TOCP control group (Fletcher et al., 1971). In two trials, groups of adult hens (varying in number from 2 to 10/group) were administered methamidophos once orally or by intraperitoneal injection in the presence or absence of 2-PAM and atropine. The test protocol used doses equal to and exceeding the LD50 values in an attempt to establish the neurotoxic potential for methamidophos. Although mortality was observed, no clinical signs of delayed neurotoxicity were evident. Histopathological examinations were not performed (Lorke and Kimmerle, 1967). A group of six hens each were administered methamidophos orally at an LD50 dose level of 27.5 mg/kg, observed for 21 days, re-treated with the same dose and maintained for an additional 21 days. Two separate control groups were used - an untreated control and a TOCP (500 mg/kg, oral dose) positive control. While some of the hens (2/6) died from acute toxicity, the survivors of the methamidophos treatment did not lose weight or show signs of delayed ataxia. The TOCP treatment group lost weight and showed positive signs of ataxia within 14 days of poisoning. Histopathology examination was not performed (Wolvin, et al., 1968). Special studies on reproduction Rat Groups of rats (8 male and 16 female/group) were fed methamidophos in the diet at dose levels of 0, 3, 10, and 30 ppm for 100 days and mated to begin a standard 3 generation, 2 litter/generation reproduction study. Body weights of parental males (but not females) were reduced but the reduction was slight and not dose-related. Cholinesterase activity values of the female animals were measured after the mating trials were concluded. RBC and plasma cholinesterase depression was evident at 30 ppm in males and females (male plasma was also depressed at 10 ppm). There were no significant effects noted on gross or microscopic examinations of tissues and organs. Reproductive indices were generally affected at the 30 ppm dose level. The incidence of pregnancy (fertility index) incidence of parturition (gestation index) and viability and lactation indices were reduced at 30 ppm. Numbers of stillborn and cannibalized pups were increased at the 30 ppm dose but no suggestions of terata were presented. Histopathologic examination of the progeny did not reveal any abnormalities. No effects on reproduction were observed at 10 ppm (Arnold et al., 1970). Special studies on teratogenesis Groups of pregnant rabbits (from 17-23 does/group) were administered methamidophos by gelatin capsule from day 6 through day 18 of gestation at dosage levels of 0, 0.1, and 0.3 mg/kg. A positive control (thalidomide, 37.5 mg/kg) was administered to a group of 27 pregnant does. All animals were sacrificed on day 29 of gestation. Positive teratogenic signs were observed as expected with thalidomide. No effects were noted as a result of administration of methamidophos on growth of does or on fetal mortality. Fetus weight was normal and skeletal development was unaffected. One of 63 pups at the 0.3 mg/kg dose level displayed talipomanus, club hand. This malformation was not observed in 75-79 pups at the low dose or in the controls, but it occurred in 8 of 85 in the positive controls (Ladd et al., 1971). There were no other obvious defects noted in the study with methamidophos. A larger number of defects were noted with thalidomide (umbilical hernia, harelip, cervical elongation, frontal fontanel) that were not seen with methamidophos. Skeletal examinations of the pups from methamidophos-treated does were normal. Special studies on mutagenicity Groups of mice (12 males/group) were administered methamidophos by intraperitoneal injection at dosage levels of 0, 1, and 2 mg/kg and mated to virgin females for 6 consecutive weeks in a dominant lethal test. A positive control group administered ethylmethane sulfonate (EMS, 400 mg/kg) was included in the study. Females sacrificed one week after breeding with the treated males were examined for pregnancy and resorption of fetuses. Mutagenicity, measured by comparing viable embryos in the test and control groups, was not evident with methamidophos treatment. A mutagenic response to EMS was indicated in the first two weeks of the test (Arnold et al., 1971). Results of in vitro tests using Salmonella and Escherichia Spp tester strains were negative when methamidophos was examined (Hanna and Dyer, 1975). Special studies on antidotes Antidotal effects were noted with atropine and atropine + 2-PAM combination administered following intoxication. BH6 and 2-PAM alone were not effective antidotes. Atropine + PAM was more effective than atropine alone but less effective than atropine + BH6 (Lorke and Kimmerle, 1967). The acute oral toxicity of methamidophos in male rats was reduced 3-fold when animals were administered atropine alone or in combination with 2-PAM. While 2-PAM alone was not as therapeutic as atropine it reduced the toxicity by more than two-fold (Schoenig et al., 1968). Special studies on potentiation Simultaneous administration of methamidophos to rats either by oral gavage or intraperitoneal injection in combination with malathion showed a significant potentiation (acute interaction in the toxicity as measured by reduction of the LD50 value). Graduated doses of malathion and methamidophos by oral administration resulted in a greater than two-fold potentiation with malathion. (Gröning, 1975). A slightly less than two-fold increase was observed when ip injection was the route of administration used (Crawford and Anderson, 1973). Oral administration of methamidophos in combination with parathion or EPN did not result in potentiation of the acute toxicity (Gröning, 1975). Acute toxicity TABLE 1. Acute toxicity of methamidophos Species Sex Route LD50 Reference (mg/kg) Rat M Oral 15.6-32.3 Lorke & Kimmerle, 1967; Cavalli et al., 1968a; 1968b; 1968g. F Oral 13.0-29.6 M ip 21.3 Lorke & Kimmerle. 1967 F ip 26.4 Lorke & Kimmerle, 1967 TABLE 1. (Cont'd.) Species Sex Route LD50 Reference (mg/kg) Rat M Dermal 4-hr exp. 110 Lorke & Kimmerle, 7-day exp. 50 1967 Inhalation 1-hr exp. 0.525 mg/L Lorke & Kimmerle, 4-hr exp. 0.162 mg/L 1967 Guinea Pig Oral 30-50 Lorke & Kimmerle, 1967 Mice Oral 16.2 Cavalli et al., (Technical) 1968c Oral (75% 18.0 Cavalli et al., Technical) 1968d Rabbit Oral 10-30 Lorke & Kimmerle, 1967 Dermal 118-125 Cavalli et al., 1968e; 1968f Cat Oral 10-30 Lorke & Kimmerle, 1967 Dog Oral 10-30 Lorke & Kimmerle, 1967 Chicken F Oral 25 Lorke & Kimmerle, 1967 F ip 10 Lorke & Kimmerle, 1967 The observed signs of poisoning were typical of anticholinesterase agents. The signs appeared rapidly following poisoning (5-20 minutes) and disappeared in 4-6 days. Typical parasympathomimetic signs of poisoning would be expected. TABLE 2. Acute toxicity of impurities LD50 Compound Species Sex Route (mg/kg) Reference (CH3O)2P(S) Rat M Oral 633 Cavalli et al., 1968h NH2 (RE9169) F Oral 549 Cavalli et al., 1968h Signs of poisoning were typical of CNS depression - paresis, hyporeflexia and anesthesia. Rabbit Dermal 2,500- Cavalli et al., 1969 5,000 (intact skin) 1,570 Cavalli et al., 1969 (abraded skin) See the monograph on acephate for the toxicity of other products. Short-term studies Rat Groups of rats (15 females/group) were administered methamidophos orally or by intraperitoneal injection, 5 days/week for 60 days to examine possible cumulative effects. Oral application consisted of daily doses of 0, 1.5, 3.0, 6.0, 10.0, and 15.0 mg/kg while ip application consisted of doses of 0, 1.3, 2.6, 5.3, 8.8, and 13.2 mg/kg. While slight mortality was observed (oral = 3/15, ip = 6/15) there was no evidence of cumulative action. The cumulative LD50 by either oral or ip administration was estimated to be > 15 mg/kg body weight (Lorke & Kimmerle, 1967). Groups of rats (30 females/treatment group - 90 females were used as controls) were fed methamidophos in the diet at dosage levels of 0, 0.25, 0.50, 1.00, 2.00 and 4.00 ppm for 13 weeks (a further 4-week recovery period using a control diet followed this test interval). There were no deaths or adverse behavioral reactions. Growth, as evidenced by body weight gain, was normal at all treatment levels. Cholinesterase depression of plasma, RBC and brain was reported only in the high dose group. At 4.0 ppm depression of enzyme activity was obvious in plasma and RBC, less so in brain. A comparison of the total depression of cholinesterase activity and recovery during the 4-week control diet period suggests a no-effect level of 2.0 ppm (Reyna et al., 1973). Groups of rats (35 males and 35 females/group) were fed methamidophos in the diet for 13 weeks at dietary levels of 0, 0.3, 1.0, 3.0 and 10.0 ppm. Cholinesterase activity was examined at periodic intervals using plasma, RBC, and brain tissue. Significant dose-related depression of plasma cholinesterase activity was noted in male rats at 3 and 10 ppm and in females at 10 ppm. Decreases in cholinesterase activity of RBC and brain were noted at 3 and 10 ppm in both sexes. RBC activity was depressed in both sexes at 10 ppm and in males only, marginally depressed at 1 and 3 ppm. Recovery of enzyme activity was noted after one week of control diets following 3 ppm and at 4 weeks of control diets following 10 ppm (Plank et al., 1969). Groups of rats (15 males and 15 females/group; 30 of each sex were used as controls) were fed methamidophos in the diet at dosage levels of 0, 2, 6, 20, and 60 ppm for 3 months. No mortality or behavior changes were observed although growth was depressed significantly at 60 ppm in both sexes. Blood chemistry, hematology, and urine analysis parameters were normal (bilirubin content of males only at 20 and 60 ppm was depressed at 90 days, and OCT (Ornithinecarbamoyl transferase) activity was depressed in females fed 60 ppm at 90 days. These unusual effects were reported to be within physiological limits). Cholinesterase activity was depressed at 6 ppm and above in both plasma and RBC (RBC was slightly more sensitive). No effects were noted at 2 ppm over the 13 week feeding interval. Gross, macroscopic examination of tissues and organs showed a reduction in both males and females in the size of several organs; thymus, liver, spleen, adrenals, and gonads only at 60 ppm. There were no effects noted at 20 ppm. Microscopic examination of the tissues did not show any adverse effects related to the presence of methamidophos in the diet. A no-effect level based on cholinesterase depression is 2 ppm in the diet (Löser and Lorke, 1970b; Gröning, 1976). Rabbit Groups of rabbits (5 males and 5 females/group) were administered methamidophos dermally to intact or abraded skin at dose levels of 0, 5, or 10 mg/kg (a 75% technical product). The application was made 6-7 hours/day, 5 days/ week for 3 weeks. Application was to a new site each day. Mortality occurred in the 10 mg/kg abraded skin group accompanied by signs of cholinergic stimulation. Hematologic studies, blood chemistry and urinalysis parameters were unaffected by dermal application of methamidophos. Gross and microscopic analyses of tissues and organs were uneventful. Dermatologic reactions, such as erythema, were minor. The affected skin returned to normal within 1-2 days. Growth was slightly retarded at the 10 mg/kg dose group but was not accompanied by other adverse signs of poisoning (Mastri et al., 1968). Dog Groups of dogs (3 male and 3 female/group) were administered methamidophos orally by capsule daily, seven days per week, for 90 days at dosage levels of 0, 0.025, 0.075, and 0.25 mg/kg body weight. There was no mortality. Growth and food consumption were unaffected. Red blood cell and plasma cholinesterase activity, examined at several intervals both before and during treatment, was normal. There was no depression of cholinesterase noted in this study (Carlson et al., 1969). Groups of dogs (3 male and 3 female/group) were used in a repeat of the above test to examine the 90-day subacute anticholinesterase effects of methamidophos. Growth was reduced in males at the highest dose level. In females, a reduced weight gain was observed at all dose levels. Food consumption was reduced at the highest dose level in both sexes. Depressed erythrocyte cholinesterase was observed at 0.25 mg/kg in both males and females. No effects were noted on plasma enzymes. A no-effect level based on RBC cholinesterase is 0.075 mg/kg (Lindberg et al., 1970). Groups of dogs (2 male and 2 female/group) were administered methamidophos by capsule for periods varying from 21-28 days, at doses ranging from 0 to 0.6 mg/kg/day. In this study each dog was examined for 34 weeks to determine a pre-treatment baseline for cholinesterase activity. Animals were then administered a dose of 0.025 mg/kg for 28 days after which the dose was increased to 0.05 mg/kg. The total treatment sequence was as follows: 0.025, 0.05, 0.075, and 0.10 mg/kg each administered for 28 days, followed by 0.125, 0.20, 0.30, 0.40, 0.50, and 0.60 mg/kg each administered for 21 days, after which a control diet was administered for 28 days. Cholinesterase activity was measured several times (generally at weekly intervals) during the course of treatment. In both males and females a dose level was reached where plasma and RBC cholinesterase activity was depressed. In males, plasma and RBC cholinesterase was depressed when the dose regimen reached 0.125 mg/kg. In females, plasma was depressed at a dose of 0.3 mg/kg while RBC was affected at approximately 0.2 mg/kg. Based on these data, no effects on cholinesterase would be expected to occur at levels around 0.1 mg/kg/day. In this study, depression was considered positive when the average value for cholinesterase activity declined below the lowest range of the pre-treatment mean (Greco et al., 1971). Groups of dogs (2 male and 2 female beagles/group), 3 of each sex were used as controls) were fed diets containing methamidophos at dosage levels of 0, 1.5, 5, and 15 ppm for 90 days. There was no mortality or behavioral change noted over the course of treatment. Growth and food consumption were normal. There were no effects noted in blood chemistry, hematology or urinalysis parameters. Cholinesterase depression (RBC and plasma) was noted at 5 ppm and above in both sexes (within the first week of dietary administration at the high dose and after 2 months at the intermediate dose). Gross macroscopic examination of tissues and calculation of relative organ weights did not indicate any effects at the highest treatment level. A dietary no-effect level in this study is 1.5 ppm (Löser and Lorke, 1970a) Histopathological examination of tissues and organs showed no effects of methamidophos (Gröning and Lorke, 1976). Groups of dogs (3 male and 3 female beagles/group) were administered methamidophos by capsule, orally, for two years at dosage levels of 0, 0.075, 0.25 and 0.75 mg/kg. There were no deaths or observable differences in behavior over the two-year study. Growth was unaffected by methamidophos. There were no effects noted on clinical chemistry, hematology, or urinalysis parameters recorded. Cholinesterase activity was not reported. Gross and microscopic examination of tissues and organs showed no abnormalities (Lindberg et al., 1969). Long-term studies Rat Groups of rats (45 males and 45 females/group) were fed methamidophos in the diet at dosage levels of 0, 3, 10, and 30 ppm for two years. Growth was reduced at the high dose level in males. Growth in females was unaffected in this study. A separate paired feeding study (21-day) was conducted where it was noted that growth was unaffected at 30 ppm. Examination of food consumption data suggested this effect to be due to diet rejection rather than a direct effect of methamidophos in the diet. (Plank et al., 1968). In the long-term study, there were no effects reported on behavior, hematologic blood chemistry or urinalysis parameters. Gross and microscopic examination of tissues and organs at 12 or 24 months showed no abnormalities attributed to methamidophos in the diet. The tumor incidence was unaffected and a definitive, no-effect level based on somatic changes would be greater than 30 ppm. Cholinesterase activity was not measured in the study (Plank et al., 1970). OBSERVATIONS IN MAN A study was conducted where controlled doses of combinations of acephate and methamidophos were orally administered to people for periods up to 21 days. A cohort of 14 people were divided into 3 groups containing either 4 or 6 people. The groups received either a combination mixture of 1:4 (methamidophos:acephate), 1:9 or a control dose, daily (divided into 3 equal doses), for periods of 21 days. Each group was administered the combination at a dose of 0.1 mg/kg/day for 21 days after which the dose was increased (to 0.2 mg/kg/day) and continued for another 21 days. The protocol called for a total of 4 dose levels (up to 0.4 mg/kg/day) each to be administered for 21 days. Blood cholinesterase (plasma and RBC) was examined periodically. No effects were noted on RBC cholinesterase at any treatment level. There was no substantial depression noted in the female group receiving the 1:9 combination tested at the highest dose (0.4 mg/kg/day). The male cohort was substantially affected at the 0.3 mg/kg dose and was not retested at the higher dose level. The group (both males and females) receiving the higher methamidophos:acephate ratio (1:4) showed reduction of plasma cholinesterase after two weeks of receiving a dose of 0.2 mg/kg/day. The 0.3 mg/kg regimen was not used in the 1:4 dose group. This study indicated the minimal dose necessary to decrease plasma cholinesterase for the combination of acephate and methamidophos was 0.2 mg/kg/day of a 1:4 combination of the two toxicants. No effects with this combination were noted at 0.1 mg/kg (Garofalo et al., 1973). See the monograph on acephate for a further amplification of these data. COMMENTS Methamidophos, an anticholinesterase organophosphorus ester, is also a metabolite of acephate. Methamidophos is rapidly absorbed, distributed, metabolised and excreted in mammals. Bioaccumulation, based on data on solubility properties, would not be expected to occur. Biotransformation in mammals results in metabolites of insignificant toxicological properties. Methamidophos is an acutely toxic pesticide. The acute signs of poisoning can be relieved with the aid of atropine (and reactivators, such as 2-PAM). In contrast to the innocuous metabolites, methamidophos is an active cholinesterase inhibitor, primarily affecting RBC (and presumably brain) cholinesterase in animal models. In humans, plasma cholinesterase appears to be the more sensitive enzyme parameter. Methamidophos does not induce delayed neurotoxicity although potentiation of the acute toxicity of malathion has been noted. In reproduction studies, several parameters were affected at relatively low levels (the levels were however sufficient to impart cholinesterase depression in the parents). No terata were induced in the reproduction study or in special teratological studies in rabbits, although the dose levels used in this latter study were extremely low. Mutagenicity tests were negative. In short and long term studies no significant somatic effects were noted. Cholinesterase depression was manifested in short term studies. In 90 day dog studies, cholinesterase depression was observed at 5 ppm while a dietary level of 1.5 ppm (equivalent to 0.04 mg/kg/day) caused no observable effects. In a 2 year dog study, somatic effects were not noted at levels of 0.75 mg/kg/day but cholinesterase activity was not determined. Depression of cholinesterase activity in rat feeding studies was observed at dietary levels of 3 ppm and above while 2 ppm was found to induce no depression of cholinesterase activity. In a controlled study where a combination dose of methamidophos and acephate was administered to a group of male and female volunteers, cholinesterase depression was observed predominantly when a higher methamidophos:acephate ratio was used (1:4 rather than 1:9). Based on the results of this study a no-effect level of 0.02 mg/kg was suggested. A higher total concentration (0.2 mg/kg), administered in a lower ratio (1:9) also gave no depression of enzyme activity. Based on the no-effect level of 0.04 mg/kg bw in dog an ADI was allocated. The no-effect level in human studies was further assurance of the adequacy of the numerical value of the ADI. TOXICOLOGICAL EVALUATION Level causing no toxicological effect Rat: 2 ppm in the diet equivalent to 0.1 mg/kg bw Dog: 1.5 ppm in the diet equivalent to 0.04 mg/kg bw Man: 0.02 mg/kg bw/day ESTIMATE OF ACCEPTABLE DAILY INTAKE FOR MAN 0 - 0.002 mg/kg bw RESIDUES IN FOOD AND THEIR EVALUATION USE PATTERN Methamidophos is an organophosphorus insecticide with systemic properties. It is a good stomach poison with further contact poison action and has good residual activity. It is claimed to be effective for the control of sucking pests such as aphids, whitefly and spider mites as well as biting pests such as Laphygma, Prodenia, Trichoplusia and others. Methamidophos is used world-wide, chiefly in regions with a subtropical climate. Formulations based on methamidophos are registered and sold in more than 40 different countries of the world, including Egypt, many countries of Latin America, Turkey, U.S.A., Canada and several countries of Europe. Methamidophos is marketed in different emulsifiable concentrate formulations. The major uses of methamidophos are pre-harvest treatments on cotton, vegetables, potatoes, maize, sugar beet, stone fruit, tobacco, hops and ornamentals. Further details on its uses and recommendations are given in Table 3. RESIDUES RESULTING FROM SUPERVISED TRIALS In vegetables Residue data are presented in Table 4 on a number of vegetable crops, namely broccoli, Brussels sprouts, cabbage, cauliflower, celery, cucumber, eggplant, lettuce, peaches, pepper, potatoes and tomatoes. Most of the data have been received in detailed as well as in summarised form from the manufacturers (Chevron Chemagro and Bayer-reports in file), although with some supplementary material from other sources (Davis et al., 1974). Generally vegetable crops are treated with 0.5 - 1 kg/ha of methamidophos, which leaves residues of a non-persistent nature and with a well-known distribution pattern, viz. the outer leaves, especially of leafy vegetables, carry considerably higher residues than the internal parts of the plants. Some of these outer leaves may or may not be discarded as "trimmings" when marketed. With only occasional high values, the data in Table 4 indicate that residues of methamidophos will rapidly dissipate to low levels. With appropriate pre-harvest intervals, from 1-3 weeks, the average residues in all vegetables are usually well below 1 mg/kg. TABLE 3. Recommended use patterns of methamidophos Crop Dose, Number Recommended kg a.i./ha of pre-harvest treatments interval, days Cotton 0.05-1.5 3-5 21 Cucumbers 0.4 -0.75 3-4 3 Tomatoes 0.4 -0.75 3-4 3 Cauliflower 0.4 -1.12 3-5 7 (U.S.A. 14) Brussels sprouts 0.56-1.12 3-5 7 Broccoli 0.56-1.12 3-5 7 Other brassicas 0.4 -1.12 3-5 7 Celery 0.56-1.12 5 21 Lettuce 0.56-1.12 3-4 21 Potatoes 0.5 -1.12 3 7 Maize 0.9 -1.5 3 21 Sugar beet 0.5 -0.6 3-5 28 Stone fruit 0.5 -1.0 2-3 21 (peach) Tobacco 0.3 -0.6 3-4 14 Hops 0.3 -2.0 4 21 Rape 0.28-0.56 2 10 TABLE 4. Residues of methamidophos in vegetable crops (supervised trials) Crop Dose, Number Time after Residues (mg/kg) Number Country kg/ha of final range average of treatments application values (w=weeks-d=days) Broccoli heads 0.56-1.12 3-6 2 w n.d. - 2.84 0.45 21 U.S.A. 3 w n.d. - 0.14 0.03 9 4 w n.d. - 0.09 0.02 7 leaves 1.12 3-6 2 w 0.22 - 7.62 2.50 7 U.S.A. 3 w n.d. - 0.88 0.28 7 4 w n.d. - 0.54 0.16 7 total 1.12 3-6 2 w 0.16 - 3.74 1.41 7 U.S.A. 3 w n.d. - 0.57 0.18 7 4 w n.d. - 0.44 0.13 6 Brussels sprouts heads 1.12 3-5 2 w 0.07 - 0.47 0.26 6 U.S.A. 3 w 0.08 - 0.64 0.27 6 4 w 0.05 - 0.22 0.12 6 leaves 1.12 3-5 2 w 0.16 - 0.17 0.17 2 U.S.A. 3 w 0.06 - 0.21 0.14 2 4 w n.d. - 0.10 0.05 2 total 1.12 3.5 2 w 0.10 - 0.11 0.11 2 U.S.A. 3 w 0.12 0.12 2 4 w 0.05 - 0.08 0.07 2 Cabbage heads 0.56 - 1.12 2-9 1 w n.d. - 0.7 0.11 11 U.S.A. 2 w n.d. - 0.35 0.04 11 GFR TABLE 4. (Cont'd.) Crop Dose, Number Time after Residues (mg/kg) Number Country kg/ha of final range average of treatments application values (w=weeks-d=days) Cabbage savoy 0.3 3 1 w 0.09 1 GFR 2 w 0.07 1 Cauliflower heads 0.56 - 1.12 3-12 2 w n.d. - 0.45 0.11 16 U.S.A. 3 w n.d. - 0.23 0.06 7 GFR 4 w n.d. - 0.12 0.03 7 leaves 0.56 - 1.12 3-12 2 w 0.1 - 6.28 1.85 8 3 w n.d. - 1.11 0.46 7 4 w n.d. - 0.62 0.24 7 Celery stalks 1.12 5-7 3 w 0 - 1.55 0.33 6 U.S.A. tops 1.12 5-7 3 w 0.08 - 6.0 2.59 6 whole plant 1.12 5-7 3 w 0.02 - 2.0 0.74 6 Cucumber 0.6 4-6 0 d 0.19 - 0.45 0.35 3 Mexico (dust treatment) 1 d 0.13 - 0.44 0.32 3 3 d 0.05 . 0.41 0.24 3 5 d 0.06 - 0.2 0.15 3 7 d 0.03 - 0.31 0.16 3 Eggplant 0.45 - 0.6 7-9 0 d 0.03 - 0.10 0.07 2 Mexico 1 d 0.02 - 0.05 0.04 2 3 d 0.05 - 0.06 0.06 2 5 d 0.03 - 0.04 0.04 2 TABLE 4. (Cont'd.) Crop Dose, Number Time after Residues (mg/kg) Number Country kg/ha of final range average of treatments application values (w=weeks-d=days) Lettuce 7 d 0.05 - 0.06 0.06 2 head (1) 0.56 - 1.12 3-8 2 w 0.01 - 0.68 0.24 12 U.S.A. 3 w n.d. - 0.02 n.d. 5 wrapper leaves (2) 1.12 3-4 3 w 0.1 - 0.77 0.19 5 U.S.A. (1) + (2) 1.12 3-4 3 w n.d. - 0.25 0.07 5 U.S.A. head 1.12 5 2 w n.d. - 1.40 0.21 9 U.S.A. wrapper 3 w n.d. n.d. 6 leaves (2) 1.12 5 2 w 0.01 - 2.42 0.78 5 U.S.A. 3 w 0.02 - 0.15 0.07 6 (1) + (2) 1.12 5 2 w 0.05 - 0.93 0.31 5 U.S.A. 3 w n.d. - 0.39 0.09 6 Peppers 0.34 - 1.12 4-20 1 w 0.04 - 2.3 1.00 7 U.S.A. 2 w 0.08 - 1.09 0.38 8 Mexico 3 w n.d. - 0.58 0.16 8 ) Australia 4 w n.d. n.d. 4 ) Potatoes 1.12 6-8 1 w n.d. - 0.11 0.04 12 U.S.A. 2 w n.d. n.d. 5 U.S.A. Canada Tomatoes 0.5 4 1 d - 0.50 1 Venezuela 3 d - 0.28 1 7 d - 0.22 1 10 d - 0.23 1 14 d - 0.20 1 TABLE 4. (Cont'd.) Crop Dose, Number Time after Residues (mg/kg) Number Country kg/ha of final range average of treatments application values (w=weeks-d=days) 18 d - 0.19 1 Tomatoes 0.6 - 1.33 4-10 0 w 0.10 - 5.07 1.05 17 Australia 1 W 0.08 - 1.83 0.55 18 U.S.A. 2 w 0.03 - 1.62 0.30 18 Venezuela 3 w 0.03 - 0.18 0.09 6 In oil seeds, hops and sugar beets Rape and cotton plants have been treated with varying amounts of methamidophos, ranging from 0.28 kg/ha up to an overdose of 2.24 kg/ha and with up to 4 applications. In these experiments residues in the harvested seeds ranged from undetectable to 0.06 mg/kg 21-26 days after the last treatment, and no residues could be detected at 32-72 days after application. Limited residue data for methamidophos in hops are available. After six applications with 0.66 kg/ha dried hops contained 3.0 mg/kg at 17 days. After 3 spray applications with 1.8 kg/ha, the residue levels of methamidophos amounted to 1.4-2.8 mg/kg at 20 days. Sugar beets were sprayed on the foliage 4 times with 0.48 kg/ha. The residue levels 23 days after the last application were 0.01 mg/kg of methamidophos in the roots and 0.6 mg/kg in the leaves. After 28 days the roots were free of residues, while the tops contained 0.9 mg/kg methamidophos. In peaches Peaches were sprayed with 1 kg/ha of methamidophos under German climatic conditions. 1 week after the second of 2 applications residues varied from 0.50-1.05 mg/kg (average, 0.73 mg/kg), and 1 week later they were 0.28-0.75 mg/kg (average 0.45 mg/kg). FATE OF RESIDUES The identified metabolites formed from methamidophos are shown in Figure 1. In animals Studies of the absorption, distribution and excretion of methamidophos in the rat (Crossley and Tutass, 1969), and lactating goat (Lee and Crossley, 1972) are described in the section "Biochemical aspects," p. ). Some features of these studies, particularly the distribution in tissues, are amplified below. Female and male rats each received a single dose of radiolabelled methamidophos, administered intragastrically as an aqueous solution (Crossley and Tutass, 1969). The doses received by each rat were 0.16-0.19 mg of 14C methylthio-labelled methamidophos and 0.21 mg of 32P-labelled methamidophos. In both experiments, the majority of the applied dose was excreted within 24 hours. This period of fast elimination in which over 50% of the activity was excreted was followed by a second phase during which only 1-2% of the applied dose was eliminated daily. In the experiments with 14C-labelled methamidophos, the major excretion route was via the breath (approx. 39% of the applied dose); urine and faeces contained 11.1% and 1.5% respectively. In the experiments with 32P-labelled methamidophos, on the other hand, the majority of radioactivity was eliminated in the urine (until day 14), and up to about 20% was eliminated in the faeces. There was no accumulation in tissues or organs (see Table 5). TABLE 5. Distribution of radioactivity in tissues and organs of rats after dosing with labelled methamidophos (Crossley and Tutass, 1969) Radioactivity, % of applied dose Organ/ 14C 32P tissue female rats male rats female rats Day 5 - 9 Day 1 Day 28 Day 1 Day 28 Carcass 21.9 9.9 6.7 11.3 4.4 Liver 0.4 6.9 0.1 5.6 0.1 Kidney 0.1 0.5 0.0 0.4 0.0 Heart 0.3 0.1 0.0 0.1 0.0 Lung 0.1 - - - - Fat - 0.1 0.0 0.2 0.0 Muscle - 0.2 0.1 0.3 0.1 Total 22.6 17.4 6.9 17.4 4.6 The degradation of methamidophos in the animal body is hydrolytic in nature (Crossley & Tutass, 1969; Tutass, 1968). The degradation sequence (Crossley & Tutass, 1969), is shown above ("Biotransformation"). Data on the quantitative distribution of residues 32P residues in the urine are presented in Table 6. In the faeces, the majority of radioactivity was not extractable with either water or acetone. That which was extractable showed the same degradation profile as in the urine. The non-extractable radioactivity was in all probability incorporated into natural biochemical constituents. TABLE 6. Distribution of 32P in urine of rats dosed with 32P-methamidophos (Crossley and Tutass, 1969) 32P, % of total in sample Metabolite* Male rats Female rats Day 1 Day 2 Day 1 Day 2 methamidophos (I) 40 10 40 trace OS-dimethyl phosphorothioate (II) 5 - 5 - methylphosphoric acid (III) 40 25 40 30 phosphoric acid (IV) 15 65 15 70 * Numbers in parenthesis refer to Figure 1. S-methyl-14C-methamidophos was given orally to a goat in order to assess its fate in a lactating ruminant (Lee and Crossley, 1972). The goat received 3.75 mg. of 14C methamidophos daily after the morning milking for 7 consecutive days. Methamidophos was rapidly metabolized and about half of the radioactivity rapidly excreted. This excreted radioactivity was distributed mainly between the breath and urine and, to a lesser extent, the faeces and milk. The activity that remained in the animal after the initial rapid excretion was eliminated at a slower but constant rate. None of the radioactivity remaining in the body was concentrated in any one organ, although slightly higher levels were found in the liver than elsewhere. The distribution of radioactivity in the goat is summarized in Table 7. The results are essentially the same as those found in the rat study (Table 5), although a somewhat higher proportion was excreted in the breath of the rats. TABLE 7. Distribution of radioactivity in goat dosed with 14C-methamidophos expressed as percentage of applied dose or mg/kg methamidophos equivalents (Lee and Crossley, 1972) Sample Dosing Period Recovery Period Total Blood and Tissues (days 0 - 8) (days 9 - 17) Excreted % % % % mg/kg Urine 17.24 0.63 17.87 Bloodx 5.87 - Faeces 4.00 0.98 4.98 Liver 3.93 0.22 Milk 2.49 0.74 3.23 Kidney 0.61 0.16 Total 23.73 2.35 26.08 Heart 0.61 0.16 Mammary 0.82 0.10 Brain 0.21 0.08 Fatxx 4.13 0.16 Musclexxx 33.10 0.16 Total recovered = 75% Total 49.28 1.04 x Assuming blood to be 10% of the body weight xx Assuming fat to be 5% of the body weight xxx Assuming muscle to be 40% of the body weight TLC of urinary samples revealed the presence of methamidophos and the first metabolite OS-dimethyl phosphorothioate (II in Figure 1). In the sample taken 12 hours after the third dose, 7% of the activity in the urine was found to be parent compound and 16% metabolite II. Extraction procedures applied to the samples of tissues, blood, milk, urine and faeces extracted only small proportions of their activity, indicating that the main source of activity was not methamidophos. The maximum possible level of methamidophos in milk estimated on this basis was 0.008 mg/kg. The maximum content in the milk found by direct analysis was 0.003 mg/kg (about 1% of the total activity in the milk). Since the data obtained from the goat are similar to those from the rats; it is believed that the non-extractable activity in the goat tissues, milk, etc. is due to the incorporation of 14C into the natural constituents of the body, as in rats. In plants Methamidophos is an insecticide with systemic activity (Werner, 1972). The physical and chemical basis of systemic movement in the cotton plant was investigated by Hussain et al. (1974). Root and leaf treatment of tomato, cabbage and bean plants demonstrated that 14C-methamidophos is readily taken up by roots and leaves and transported with the transpiration water towards the margins of leaves, while little is retained in the stem and petioles. The labelled compound accumulating at the margins of leaves is essentially only methamidophos, while in the centre of the leaf some more polar compounds are present, apparently as the result of metabolic degradation. The uptake of methamidophos by roots appears to be independent of pH within the physiological range of 3.5-7.5 (Tutass, 1968b). Horler et al., (1975) showed that in tomato plants there is only slight movement of the intact insecticide from treated leaves to the fruit. After rapid penetration of 14C-methamidophos or/and 32P-methamidophos into tomato leaves, the residue in leaves and fruit in the initial phase has a biological half-life of 7-10 days. However, especially in the fruit, after this initial period the rate slows to give a half-life of about 6 weeks (Horler et al., 1975). The authors assume that intact insecticide is lost from fruits and leaves by a mechanism associated with transpiration, rather than by metabolism and translocation. Werner (1973) also reports rapid absorption of methylthio-14C-methamidophos by the roots of Loblolly pine seedlings. After 96 hours exposure, an accumulation of radioactivity was evident in the needle tips. Besides unchanged methamidophos, 3 compounds were isolated from the seedlings. One was identified as the major metabolite, OS-dimethyl phosphorothioate (II, Figure 1), and in addition two unidentified compounds were detected, of which one (Unknown 1) was probably phosphoric acid (VI, Figure 1). Methamidophos, II and Unknown 1 were detected in the roots, stems and needles, but Unknown 2 only in the needles, of seedlings treated for 72 and 96 hours (see Table 8). TABLE 8. Percentage distribution of radioactive methamidophos and 14C-compounds from TLC and GLC of pine seedling extracts (Werner, 1973) Seedling exposure period (h) Compound* Plant part 6 12 24 48 72 96 I (methamidophos) Roots 80.0 46.0 40.0 35.7 3.3 0.9 Stem 0 0 1.6 1.0 1.1 1.7 Needles 0 31.0 27.9 26.3 22.0 30.5 II (OS-dimethyl Roots 20.0 3.8 1.6 1.0 3.3 3.5 phosphorothioate) Stem 0 3.8 13.1 6.1 11.0 10.4 Needles 0 46.0 26.2 18.4 16.5 17.4 Unknown 1 Roots 0 0 1.6 1.0 2.2 1.7 Stem 0 3.8 4.9 3.1 4.4 2.6 Needles 0 19.2 18.1 17.4 19.7 10.4 Unknown 2 Roots 0 0 0 0 0 0 Stem 0 0 0 0 0 0 Needles 0 0 0 0 16.5 20.9 * Roman numerals refer to Figure 1. Experiments with methylthio-14C-methamidophos on cabbages and tomatoes as well as on plant tissue cultures of sweet potatoes and tobacco have shown that degradation occurs chiefly by hydrolysis (Chevron, 1968). Hydrolysis first occurs at the P-N bond to yield OS-dimethyl phosphorothioate (II, Figure 1), which was consistently detected as a metabolite in all tissues investigated. Continued, though somewhat slower, hydrolysis of II apparently leads to cleavage of CH3S-P and CH3O-P bonds yielding VII and III as major products. Quantitative data on the residues of these metabolites are not given. The final fate of the radiolabel is its incorporation into plant constituents such as pigments, amino acids, carbohydrates and structural material (up to 19% of the activity in the sample of cabbage plants). In soil Methamidophos is degraded at a relatively fast rate in soils. In laboratory experiments, Leary and Tutass (1968) found half-lives of 1.9 days in silt soil, 4.8 days in loam soil and 6.1 days in sandy soil. Sterilization of silt soil by autoclaving resulted in a considerable increase in the half-life, to about 6 weeks. In laboratory experiments conducted by Horler et al. (1975), methamidophos was also found to undergo rapid degradation. In basic soils (pH 7.40 and 7.75), only 3-7% of the original activity was found after 4 days; in an acid soil (pH 4.30), 25% was present after 9 days. In the basic soils, there were differences in the degradation of the metabolites. In clay-loam soil (pH 7.40), the metabolite degraded at a slower rate than in loam soil (pH 7.75). Since both soils had a similar pH, such differences may be connected with the fact that microorganisms play a role in the decomposition. In degradation experiments with field soils sampled from depths of 0-10 cm, and from depths of 0-15 cm and 15-30 cm, it was noted that at sampling on the day of application to the sail, only a fraction of the applied methamidophos was still present (Bayer, 1972; Chemagro, 1974). Within one week, the residue dropped further to 10% of the level measured on the day of application. At depths of 15-30 cm, only very small amounts of residue (maximum of 0.04 mg/kg) were found in the soil, and no accumulation was noted after several applications. Leary and Tutass (1968) found that metabolic degradation of S-methyl-14-methamidophos in silt soil under aerobic conditions resulted after 3 days in the evolution of 70% of the applied dose as 14CO2. On the other hand, only 7.6% was evolved as 14CO2 under aerobic conditions, with 84.6% of the applied activity remaining in the soil. Up to about 30% of the activity that remained in the soil was not acetone-extractable. Most of the extractable radioactivity was in the form of OS-dimethyl phosphorothioate (II, Figure 1), while minor amounts were in the form of methamidophos, amino acids and carbohydrates. Quantitative data on these constituents were not given. Horler et al (1975) also identified OS-dimethylphosphorothioate (II) as the major metabolite in soil. However, they also found very small amounts of V, VI and III in clay loam soil. In loam soil, VI was not found. It is stated that the loss of the methoxy group results in the evolution of CO2. Quantitative data are not given. In water The hydrolysis of methamidophos was examined at elevated temperatures in aqueous solutions as a function of pH (Leary, 1968; Magee, 1966). In solutions with a pH below 2, the half-life was a matter of hours. At pH 9, the half-life was 2.6 days at 25°C and 1.5 days at 37°C. In mildly acidic or neutral solutions, methamidophos showed remarkable stability at temperatures up to 80°C (less than 5% hydrolysis in 25 hours). The P-NH2 bond is broken first, followed by hydrolysis of the CH3SP grouping. Leaching experiments with methamidophos and three soil types (muck, sandy loam and loam) (Tutass, 1968c), showed little retention of the compound. Soil type apparently had only a minor effect upon retention. The compound was least retained in loam. Experiments to study the leaching behaviour of methamidophos with a standardized soil column technique revealed that about two-thirds of the compound was leached out of sandy and sandy loam soils while only about one-fifth was leached out of a soil with an increased content of organic matter (Bayer, 1972). Flint and Shaw (1972) also studied the absorption of methamidophos by and its leaching from three different soil types. The absorption by soil was very slight, and it was further decreased by increasing temperature and decreasing pH. Methamidophos was not retarded by the soil when passed through soil columns in water. The chemical was found in soil run-off water only in the first two of five irrigations. It was degraded in a natural water system outdoors with half-lives of 15.9 days in water and 7.5 days in silt. It was concluded that although it was leached, methamidophos was degraded rapidly in natural water systems. In fish Bass fingerlings which had been in water containing 0.01 mg/1 of methamidophos for eight days, were analyzed. Less than 0.02 mg/kg of methamidophos was found in all samples (Stanley, 1971 b). In processing and storage The effect of simulated commercial processing on methamidophos residues in tomatoes was investigated by Morris and Olson (1974). The tomatoes were processed to juice (pressed, pasteurized), to canned tomatoes (sterilized), to ketchup. (sterilized, pasteurized), to dry tomato pomace and to dry tomato pulp. The residues in juice, canned tomatoes and ketchup were smaller than those found in whole tomatoes, amounting to 89.5, 73.0 and 68.4% respectively, of the residue levels prior to processing. On the other hand, the residues in dry tomato pomace and dry tomato pulp were 2.0 and 2.5 times the original levels respectively. The effect on residues of washing in a manner simulating commercial packing practice depended upon the treated crop. After treatment, the fruits were allowed to stand outdoors for 24 hours. Whereas washing green peppers and eggplants did not remove any significant residue, washing tomatoes removed approximately 15% and washing cucumbers approximately 20% of the methamidophos residue (Thornton, 1973). Dehydrated celery flakes were prepared commercially by placing celery leaves on trays and drying at temperatures of up to 60°C (Morris, 1975). Dehydration removed 80% of the weight as water, and it was concluded that the dehydration process did not destroy any of the methamidophos residue present in fresh celery leaves. Cottonseed was processed into delinted seed, meal, hulls. and oil. No residue was found in any of the seed samples or in any of the fractions from the processed seed. The residue found in the trash ranged from 0.1 to 2.7 mg/kg (Chevron, 1976). Rapeseed was processed into oil and meal (Chemagro, 1976). The following residue levels were measured: rapeseed, 0.02-0.25 mg/kg, rapeseed oil, n.d.-0.02 mg/kg and rapeseed meal 0.01-0.30 mg/kg. Leary (1968) conducted a study to determine the stability of weathered residues of methamidophos on chopped crop samples (broccoli, lettuce, cabbage, cauliflower, Brussels sprouts) stored deep frozen for several months. No significant evidence of decomposition was obtained and there was no indication of any trend towards decomposition with increased time of storage. Möllhoff (1971), however, noted that methamidophos residues decreased by 10-40% in cauliflower samples stored for three months at -20°C, and by up to 10% in tomatoes stored under the same conditions. In further studies conducted by Chemagro (1976), to investigate the effect of frozen storage at about -20°C on methamidophos residues in peanut meat, peanut forage, sorghum grain, rapeseed, celery and peppers, decomposition ranged from 0 to 22%. The effect of frozen storage at about -20°C on methamidophos residues in cattle tissues and milk and in poultry tissues and eggs has also been studied (Chemagro, 1976). It was found that decomposition in poultry liver amounted to 33% after 60 days and in cattle liver to 85% after 150 days. On the other hand, the residues remained completely stable in cattle steak, fat and milk, and decomposition amounted to a maximum of only 13% in eggs, heart and gizzard of poultry. METHODS OF RESIDUE ANALYSIS Methods for the gas-chromatographic determination of methamidophos have been developed and published in several modifications, but in practically all cases are based on clean-up procedures with silica gel columns and GLC with thermionic detection. Möllhoff (1971) developed a method for determination in plants, soil and water. Maceration with acetone or an acetone/water mixture is used for extraction. The limit of determination is generally about 0.01 mg/kg, but for oil seeds only 0.05 mg/kg and for hops and tobacco 0.1 mg/ kg. Recoveries in the 0.01-1.0 mg/kg range vary between 80% and 100%. Leary (1971) also determined methamidophos in crops, but with the use of ethyl acetate for extraction. The GLC instrument in this case was equipped with a caesium bromide thermionic detector and the method is claimed to have a limit of determination of 0.003-0.005 mg/kg. At levels of 0.5-0.1 mg/kg the majority of recovery values were from 90-100%. Both these methods have been tested for specificity in conjunction with other organophosphorus insecticides. They were found to be adequately specific and without interferences from extractives with a number of different crops (Olson, 1973). Stanley and co-workers have developed methods for residues in different forage crops such as hay, peanut meal, sorghum grain and rape seed (McNamara & Stanley, 1975) and in animal tissues and milk (Stanley, 1971c). Extractions were with either methanol/chloroform or acetonitrile and Skellysolve B. With minor amendments these methods have been found suitable also for the determination of methamidophos in fish and water with limits of determination down to 0.02 mg/kg and 0.004 mg/kg respectively (Stanley 1971 a). These methods for the determination of methamidophos follow closely the general pattern for organophosphorus insecticides in current multi-residue procedures. Undoubtedly, therefore, they will be suitable for regulatory purposes. NATIONAL TOLERANCES REPORTED TO THE MEETING TABLE 9. National tolerances reported to the meeting Country Crop Tolerance, Pre-harvest mg/kg interval Australia Capsicums 0.25 14 days Peaches 0.25 21 days Tomatoes 0.25 21 days TABLE 9. (Cont'd.) Country Crop Tolerance, Pre-harvest mg/kg interval Potatoes 0.05 7 days Canada Broccoli, Brussels sprouts, lettuce, peppers 1.0 Cabbage, cauliflower, cucumbers, eggplant, tomatoes 0.5 Potatoes, melons negligible residue Netherlands Brassica 0.1 Potatoes 0.01x New Zealand Brassicas, potatoes 0.1 Tomatoes 0.1 3 days South Africa Peaches 1.0 21 days Cabbage 1.0 21 days Potatoes 1.0 14 days U.S.A. Cauliflower, lettuce tomatoes 2.0 Broccoli, Brussels sprouts, cabbage, peppers, cucumbers, eggplant 1.0 Melons 0.5 Cottonseed, potatoes 0.1xx x = limit of determination xx = negligible residues APPRAISAL Methamidophos is a chemically simple organophosphorus insecticide with broad spectrum effects used to control caterpillars, aphids, larvae etc. It is active by contact as well as systemically. Since 1969 methamidophos has been registered in a number of countries around the world, chiefly in regions with a sub-tropical climate. Its major uses are pre-harvest treatments of industrial crops, fruit and vegetables, mainly with spray solutions at rates from 0.5-1.5 kg/ha. Detailed studies on the metabolic fate of methamidophos indicate that the compound is readily taken up by plant roots and translocated with the transpiration stream within the plants, i.e. towards the edges of the foliage. In studies with the radio-labelled compound, the further fate has been followed and a complete degradation pathway with final incorporation into plant constituents is suggested, mainly through steps involving hydrolytic breakdown. The degradation of methamidophos in the animal body is also hydrolytic and there are no signs of accumulation in tissues or organs. Most of the phosphorus-containing moiety of methamidophos (up to about 70%) is eliminated via the urine, while a major part of the carbon (about 40% in the rat) is excreted as CO2 with the breath, the remainder being found in the urine and faeces. The results of extensive residue trials on a number of vegetable and agricultural crops are available. The data confirm methamidophos as a systemic compound of relatively low persistence. Most of the data are from trials under practical conditions which are likely to represent good agricultural practice. Methamidophos penetrates into the flesh of the fruit in, e.g., tomatoes, peppers, eggplants, etc. Washing or peeling such products therefore does not significantly remove residues. In the frozen storage of crops treated with methamidophos, residues are found to be stable for several months. Methamidophos has been found to be degraded fairly rapidly in soils and in natural water systems without any signs of accumulation or concentration in living organisms. Gas-chromatographic methods making use of thermionic detectors are available for the determination of methamidopho in various crops and animal products. They are combined with appropriate extraction and clean-up procedures and are likely to be suitable for regulatory purposes. RECOMMENDATIONS Maximum residue limits are recommended for methamidophos in the following commodities at the levels indicated. They refer to the parent compound. Limit, Limit, Commodity mg/kg Commodity mg/kg Hops 5 Peppers 1 Broccoli 2 Sugar beet leaves 1 Cauliflower 2 Cottonseed 0.1 Celery 2 Potatoes 0.1 Lettuce 2 Rapeseed 0.1 Limit, Limit, Commodity mg/kg Commodity mg/kg Tomatoes 2 Sugar beets 0.1 Brussels Meat and fat of sprouts 1 cattle, goats Cabbage 1 and sheep 0.01* Cucumber 1 Milk 0.01* Eggplant 1 Peaches 1 * At or about the limit of determination FURTHER WORK OR INFORMATION DESIRABLE 1. Further studies an the teratogenic potential in a suitable species. 2. Further studies to elucidate the contribution of acephate and metamidophos alone or in combination in depressing cholinesterase activity. 3. 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Acute Oral Toxicity of 1968a Monitor Technical in Rats. Unpublished report from Industrial Hygiene and Toxicology, Standard Oil of California, submitted to the WHO by Chevron Chemical Company. Cavalli, R.D., Hallesy, D.W. and Spence, J.A. Acute Oral Toxicity 1908b of Monitor 6S in Rats. Unpublished report from Industrial Hygiene and Toxicology, Standard Oil of California, submitted to the WHO by Chevron Chemical Company. Cavalli, R.D., Hallesy, D.W. and Spence, D.H. Acute Oral Toxicity 1968c of RE 9006 (95%) in Mice. Unpublished report from Industrial Hygiene and Toxicology, Standard Oil of California, submitted to the WHO by Bayer, A.G. Cavalli, R.D., Hallesy, D.W. and Spence, J.A. Acute Oral Toxicity 1968d of RE 9006 (75%) in Mice. Unpublished report from Industrial Hygiene and Toxicology, Standard Oil of California, submitted to the WHO by Chevron Chemical Company. Cavalli, R.D., Hallesy, D.W. and Spence, J.D. Acute Dermal Toxicity 1968e of Monitor 6S. Unpublished report from Industrial Hygiene and Toxicology, Standard Oil of California, submitted to the WHO by Chevron Chemical Company. Cavalli, R.D., Hallesy, D.W. and Spence, J.A. Acute Dermal Toxicity 1968f of Monitor Technical. Unpublished report from Industrial Hygiene and Toxicology, Standard Oil of California, submitted to the WHO by Bayer, A.G. Cavalli, R.D., Hallesy, D.W. and Spence, J.A. Acute Oral Toxicity 1968g of RE 9006 (95%) in Rats. Unpublished report from Industrial Hygiene and Toxicology, Standard Oil of California, submitted to the WHO by Chevron Chemical Company. Cavalli, R.D., Hallesy, D.W. and Spence, J.A. Acute Oral Toxicity 1968h of RE 9169 (SX-196) in Rats. Unpublished report from Industrial Hygiene and Toxicology, Standard Oil Company of California, submitted to the WHO by the Chevron Chemical Company. Cavalli, R.E., Hallesy, D.W. and Spence, J.A. Acute Dermal Toxicity 1969 RE 9169 (SX 198) in Rabbits. Unpublished report from Industrial Hygiene and Toxicology, Standard Oil Company of California, submitted to the WHO by Chevron Chemical Company. Cavalli, R.E. and Spence, J.A. Effect of Repeated Doses of RE 12420 1970 on Cholinesterase Activity in Rats. Unpublished report from Standard Oil of California, submitted to the WHO by Chevron Chemical Company. Chemagro Reports nos. 44249, 44250 on methamidophos in 1974 soil. Submitted to the Joint Meeting. Chevron Metabolism of Monitor insecticide by plants. 1968 Chevron report submitted to the Joint Meeting. In file at FAO, Plant Protection Division. Crawford, C.R. and Anderson, R.H. The Acute Toxicity of Monitor 1973 Technical in Combination with Marathion to Female Rats. Unpublished report from Chemagro Division of Baychem Corporation, submitted to the WHO by Bayer, A.G. Crossley, J. and Tutass, H.O. Metabolism of Monitor Insecticide 1969 by rats. Unpublished report no. 721.14 from the Ortho Division, Chevron Chemical Company, submitted to the WHO by Bayer, A.G. Davis, A.C., Bourke, J.B. and Kuhr, R.J. Disappearance of Monitor 1974 Residues from Cole crops. J. Econ. Entomol. 67, 766-768. Fletcher, D., Jenkins, D.H., Keplinger, M.L. and Fancher, O.E. 1971 Demyelination Study with Monitor (RE 9006) Technical in Mature Laying Hens. Unpublished study from Industrial Bio-Test Laboratories, Inc., submitted to the WHO by Bayer, A.G. Flint, D.R. and Shaw II, H.R. The Mobility and persistence of 1972 MONITOR in soil and water. Report No. 34 483 submitted by Chemagro. Garofalo, M., Palazzolo, R. and Sanders, R. A Study on the Effect 1973 of Orthene and Monitor on Plasma and Erythrocyte Cholinesterase Activity in Human Subjects During Subacute Oral Administration. Unpublished report from Industrial Bio-Test Laboratories, Inc., submitted to the WHO by Chevron Chemical Company. Greco, R.A., Lindberg, D.C., Keplinger, M.L. and Fancher, O.E. 1971 Effect of RE 9006 on Plasma and Erythrocyte Cholinesterase Activity in Beagle Dogs. Unpublished report from Industrial Bio-Test Laboratories, Inc., submitted to the WHO by Bayer, A.G. Gröning, P. Tamaron. Special Toxicity Studies (Potentiation). 1975 Unpublished report from Institut für Toxikologie, submitted to the WHO by Bayer, A.G. Gröning, P. Bayer 17628 (Tamaron) Untersuchung Der Subchronischen Toxizitat an ratten (3-monate-futterungversuch). Unpublished report from the Institut für Toxikologie, submitted to the WHO by Bayer, A.G. Gröning, P. and Lorke, D. Untersuchung der subchronischen toxizitat 1976 an hunden. Unpublished report from the Institut für Toxikologie, submitted to the WHO by Bayer, A.G. Hanna, P.J. and Dyer, K.F. Mutagenicity of Organophosphorus Compounds 1975 in Bacteria and Drosophila. Mut. Res. 28: 405-20. Horler, D.F., Lubkowitz, J.A., Revilla, A.P., Baruel, J. and Cermeli, 1975 M.M. Uptake and Degradation of Monitor by Tomato Plants. Pesticides, IUPAC 3rd Int. Congr. Pest. Chem., Helsinki, 151-156. Hussain, M., Fukuto, T.R., and Reynolds, H.T. Physical and Chemical 1974 Basis for Systemic Movement of Organophosphorus Esters in the Cotton Plant. J. Agr. Fd Chem. 22 (2): 225-230. Ladd, R., Jenkins, D.H., Wright, P.L. and Keplinger, M.L. 1971 Teratogenic Study with Monitor Technical in Albino Rabbits. Unpublished report from Industrial Bio Test Laboratories, Inc. submitted to the WHO by Bayer, A.G. Leary, J.B. Stability of Monitor insecticide residues in frozen crops and in aqueous solutions. Chevron report files No. 740.01 and No. 721.2. Leary, J.B. and Tutass, H.O. Degradation of Monitor insecticide 1968 in soil. Chevron report file No. 721.2. Leary, J.B. Gas-chromatographic Determination of Monitor 1971 (OS-Dimethyl Phosphoramidothioate) Residues in Crops. J. Ass. Off. Analyt. Chem. 54 (6): 1396-98. Lee, H. and Crossley, J. The Fate of Monitor in a Lactating 1972 Ruminant (Goat). Unpublished report from Ortho Division, Chevron Chemical Company, submitted to the WHO by Bayer, A.G. Lindberg, D., Vondruska, J.F. and Fancher, O.E. Two Year Chronic 1969 Oral Toxicity of RE 9006-III, SX-116 in Beagle Dogs. Unpublished study from Industrial Bio-Test Laboratories, Inc., submitted to the WHO, by Bayer, A.G. Lindberg, D., Keplinger, M.L. and Fancher, O.E. RE 9006 (70%) 1970 SX 171 on the Cholinesterase in the Beagle Dog. Unpublished study from Industrial Bio-Test Laboratories Inc., submitted to the WHO by Chevron Chemical Company. Lorke, D. and Kimmerle, G. Toxicological Studies on the Active 1967 Ingredient Bayer 71 628. Unpublished report from the Institut für Toxikologie, submitted to the WHO by Bayer, A.G. Löser, E. and Lorke, D. Subchronic Toxicological Studies on Dogs 1970a (Three Month Feeding Experiment). Unpublished report from Institut für Toxikologie, submitted to the WHO by Bayer, A.G. Löser, E. and Lorke, D. Subchronic Toxicological Studies on rats. 1970b Unpublished report from the Institut für Toxikologie, submitted to the WHO by Bayer, A.G. Magee, P.S. Hydrolysis of Monitor insecticide. Chevron Report 1966 File No. 721.2. Mastri, C., Keplinger, M.L. and Fancher, O.E. Twenty-one Day 1968 Subacute Dermal Toxicity of Monitor (RE 9006) 75% Technical SX 171. Unpublished report from Industrial Bio-Test Laboratories, Inc., submitted to the WHO by Chevron Chemical Company. McNamara, F.T., and Stanley, C.W. Gas-Chromatographic Method for 1975 the Determination of Residues of MONITOR in Peanut Meat and Hay, Sorghum Grain, and Rapeseed. Chemagro Report No. 45.439. Möllhoff, E. Metode zur gaschromatographischen Bestimmung 1971 von Tamaron-Rückständen in Pflanzen. Pflanzenschutz-Nachr. 24 (2): 256-262. Morris, R.A. Effect of Dehydration on Residues of MONITOR 1975 in Celery Leaves. Chemagro Report No. 45.586. Morris, R.A., and Olson, T.J. The Effect of Processing on MONITOR 1974 Residues in Tomatoes. Chemagro Report No. 41.107. Olson, T.J. Interference Study on MONITOR (TAMARON) in Various 1973 Crops. Chemagro Reports No. 37.380 and No. 37.392. Plank, J., Keplinger, M.L. and Fancher, O.E. Paired Feeding Study 1968 of RE 9006 - III SX-116 Albino Rats. Unpublished report from Industrial Bio-Test Laboratories. Inc., submitted to the WHO by Chevron Chemical Company. Plank, J., Keplinger, M.L. and Fancher, O.E. Ninety-Day 1969 Cholinesterase Study of SX-171 Monitor (RE 9006) - Albino Rats. Unpublished report from Industrial Bio-Test Laboratories, Inc., submitted to the WHO by Bayer, A.G. Plank, J., Keplinger, M.L. and Fancher, O.E. Two Year Chronic 1970 Oral Toxicity of RE 9006 - III, SX-116 in Albino Rats. Unpublished report from Industrial Bio-Test Laboratories, Inc., submitted to the WHO by Bayer, A.G. Reyna, M.S., Kennedy, G.L. and Keplinger, M.L. Ninety-Day Subacute 1973 Oral Cholinesterase Study with Monitor Technical in Female Albino Rats. Unpublished report from Industrial Bio-Test Laboratories, Inc., submitted to the WHO by Bayer, A.G. Schoenig, G., Keplinger, M.L. and Fancher, O.E. Study on the Efficacy 1968 of Atropine Sulfate and 2-PAM C1 as Antidotes for Monitor (RE 9006) 75% Technical SX-171. Report from Industrial Bio-Test Laboratories, Inc., submitted to the WHO by Bayer, A.G. Stanley, C.W. A Gas Chromatographic Method for the Determination 1971a of MONITOR in Fish and Water. Chemagro Report No. 30.975. Stanley, C.W. Analysis of Bass and Water for MONITOR. Chemagro 1971b Report No. 30.979. Stanley, C.W. A Gas Chromatographic Method for the Determination 1971c of Residues of MONITOR in Animal Tissues and Milk. Chemagro Report No. 31.093. Thornton, J.S. Effect of Washing on Residues in Tomatoes, Cucumbers, 1973 Eggplants and Peppers. Chemagro Reports Nos. 37322, 37539, 37540 and 37541 submitted to the Joint Meeting. In File at FAO, Plant Protection Division. Tucker, B. Acetylcholinesterase Inhibition of Orthene and Ortho 1972 9006. Unpublished report from Ortho Division of Chevron Chemical Company, submitted to the WHO by Chevron Chemical Company. Tutass, H.O. Microsomal Oxidation of Monitor. Unpublished report 1968a no. 721.2 from the Ortho Division of Chevron Chemical Company, submitted to the WHO by Bayer, A.G. Tutass, H.O. Uptake and Translocation of Monitor Insecticide 1968b by tomato, cabbage and bean plants. Chevron Report File No. 721.2. Tutass, H.O. Leaching of Monitor insecticide in soils. Chevron 1968c Report File No. 721.2 Werner, R.A. Systemic insecticidal action of disulfoton and 1972 Monitor in Loblolly pine seedlings. J. Georgia Entomol. Soc. 7 (1): 64-67. Werner, R.A. Absorption, Translocation and Metabolism of Root- 1973 Absorbed 14C Monitor in Loblolly Pine Seedlings. J. Econ. Entomol. 66 (4): 867-872. Wolvin, A.A., Palazzolo, R.J. and Fancher, O.E. Neurotoxicity 1968 Study - Chickens Monitor RE 9006, 75% Technical. Unpublished report from Industrial Bio-Test Laboratories, Inc., submitted to the WHO by Bayer, A.G.
See Also: Toxicological Abbreviations Methamidophos (HSG 79, 1993) Methamidophos (ICSC) Methamidophos (JMPR Evaluations 2002 Part II Toxicological) Methamidophos (Pesticide residues in food: 1979 evaluations) Methamidophos (Pesticide residues in food: 1981 evaluations) Methamidophos (Pesticide residues in food: 1982 evaluations) Methamidophos (Pesticide residues in food: 1984 evaluations) Methamidophos (Pesticide residues in food: 1985 evaluations Part II Toxicology) Methamidophos (Pesticide residues in food: 1990 evaluations Toxicology)