PESTICIDE RESIDUES IN FOOD - 1983 Sponsored jointly by FAO and WHO EVALUATIONS 1983 Data and recommendations of the joint meeting of the FAO Panel of Experts on Pesticide Residues in Food and the Environment and the WHO Expert Group on Pesticide Residues Geneva, 5 - 14 December 1983 Food and Agriculture Organization of the United Nations Rome 1985 METHIOCARB TOXICOLOGY Explanation Methiocarb was evaluated by the 1981 Meeting and a full acceptable daily intake (ADI) was allocated.1/ Observations in humans, considered by the previous Meeting to be desirable information, are still not available. Other additional toxicological data have been submitted and are summarized and discussed in the following monograph addendum. In 1981, recommendations were made for maximum residue levels (MRLs) (and temporary MRLs pending further information on GAP) in a range of commodities on which methiocarb was used for snail and slug control, spray application as a bird repellent or as a seed treatment. Information on good agricultural practice for commodities on which bait applications are recommended or used, was required, and it was noted that additional metabolism and residue data would be necessary if future uses of the compound on animal feeds could result in potential residues in meat, milk, poultry and eggs. Information on levels of methiocarb residues in foods in commerce or at consumption was also listed as desirable. A considerable amount of additional information, not all relevant to the above requests was evaluated by the meeting and is presented in this monograph addendum. EVALUATION FOR ACCEPTABLE DAILY INTAKE BIOCHEMICAL ASPECTS Absorption, Distribution, Elimination and Biotransformation In male Wistar rats given a single oral dose of approximately 5 mg/rat (body weight of the animals not given) of methiocarb, only a small amount of the administered dose was eliminated in the urine, mainly within 48 hours of treatment, as the unchanged parent compound (<2.3 percent) and its metabolites (3.3 percent) (van Hoof & Heyndrickx 1975). 1/ See Annex 2 for FAO and WHO documentation. Under aerobic conditions and in the presence of NADPH, methiocarb was oxidized by the flavin-adenine dinucleotide-dependent monoxygenase of pig liver microsomes. The rate of sulphoxidation of methiocarb was low as compared to other thioether-containing pesticides such as disulfoton, an organophosphate, and thiofanox, a carbamate (Hajjar & Hodgson 1982). Effects on Enzymes and Other Biochemical Parameters Rat Groups of 15 female Sprague-Dawley-derived rats were intubated with technical methiocarb (97 percent pure) or methiocarb sulphoxide (95.2 percent pure) in Carbowax at 0, 0.5 or 2 mg/kg b.w./day five times weekly for four weeks. Determination of cholinesterase activity 30 minutes after dosing on days 7, 14, 21 and 28 (i.e. following the two- day withdrawal period) showed a significant dose-related inhibition of plasma cholinesterase (21-61 percent) and erythrocyte cholinesterase (22-46 percent) at practically all intervals following methiocarb sulphoxide dosing. Treatment with technical methiocarb resulted in plasma cholinesterase inhibition on days 0, 7 and 14 (23-41 percent) following the 2 mg/kg b.w. dose and on days 0 (21 percent) following the 0.5 mg/kg b.w. dose. Erythrocyte cholinesterase was depressed by 29 percent on day 0 with 2 mg/kg b.w. of technical methiocarb. Four hours after dosing on days 4, 11 and 18, only rats treated with 2 mg/kg b.w. of methiocarb sulphoxide showed a decrease (>20 percent) in activity of plasma cholinesterase (days 11 and 18) and erythrocyte cholinesterase (day 11) (Hixson 1981). Dog Groups of two male and two female beagles were given daily 0.05 or 0.5 mg/kg b.w. of technical methiocarb (97 percent pure) or methiocarb sulphoxide (95.2 percent pure) in gelatin capsules for 29 days. Control groups comprised two males and two females. Slight to heavy salivation or vomiting was observed in animals (both sexes) given either test material at 0.5 mg/kg b.w. One female treated with 0.05 mg/kg b.w. of the sulphoxide also exhibited slight salivation on one occasion. Assay of plasma and erythrocyte cholinesterase from unfasted animals, using an unspecified method, at various intervals after the first and second or third dosing each week showed inhibition (> 20 percent) of the enzyme in plasma or erythrocytes by both test compounds at 0.5 mg/kg b.w. Depression of cholinesterase peaked at 1-2 h and was essentially reversible at 6 h, post-treatment. In general, the extent of inhibition, which was greater with plasma cholinesterase than with erythrocyte cholinesterase, did not increase with time with either compound. Data indicated 0.05 mg/kg b.w. to be a marginal no-effect level (NOEL) on cholinesterase for technical methiocarb. A no-effect level for methiocarb sulphoxide was, however, not demonstrated (Hayes 1981). TOXICOLOGICAL STUDIES Special Study on Teratogenicity Rabbit Groups of 17-19 female rabbits (New Zealand White), artificially inseminated, were intubated with methiocarb (a mixture of five batches, 97.3 percent pure) as a suspension (in distilled water containing 0.5 percent carboxymethyl cellulose and 0.5 percent Tween 80) at 0, 1, 3 or 10 mg/kg b.w./day from day 6 to 18 inclusive of gestation (day 0 = day of insemination). The does were sacrificed on day 29 of gestation and uterine contents were examined. Foetuses were examined for external, skeletal and internal abnormalities. Three to six females per group, including the control, died or were sacrificed in extremis 6 to 25 days after insemination, mainly owing to respiratory tract infection and/or gastrointestinal disorder or accidental tracheal intubation. The number of does found to be pregnant with viable young on day 29 was only 11 in control group, 13 at 1 mg/kg b.w., 12 at 3 mg/kg b.w. and 10 at 10 mg/kg b.w. Toxic signs such as increased respiratory rate, muscular tremors, pupillary constriction, incoordination and prostration were noted at 10 mg/kg b.w. in maternal animals. Increased respiratory rate was also seen, although infrequently, at 3 mg/kg b.w. Does of the top-dosage group exhibited actual weight loss during the first two days of treatment and growth depression thereafter throughout the dosing period. Premature parturition occurred in one doe at 3 mg/kg b.w. on day 28 and two does at 10 mg/kg b.w. aborted on day 25. The incidence of pregnant does with early resorptions was elevated at 10 mg/kg b.w. but the mean number of early resorptions per litter in this dosage group was within the ranges of background data submitted. There were no treatment-related effects on the number of corpora lutea, implantations, viable young or late resorptions or on pre- and post- implantation loss, foetal weight and placental weight. An increase in incidence of foetuses with pale areas in the liver was noted at 10 mg/kg b.w. but frequency of foetal malformations was not affected by treatment. The study appeared to give no evidence suggestive of teratogenic activity of methiocarb under the conditions of the experiment. Maternal and/or foetal toxicity, however, occurred at 3 mg/kg b.w. and above (Tesh et al 1981). Acute Toxicity The four-hour inhalation LC50 of an aerosol of technical methiocarb (97.9 percent pure) in male and female Wistar rats (Bor: WISW (Spf (Cpb)) was greater than 322 mg/cu m air, in terms of analytical concentration of the test material in the inhalation chamber. No information was given on the particle size of the aerosol (Thyssen 1982). Short-Term Studies In Wistar rats (Bor: WISW (Spf (Cpb)) exposed to an aerosol (particle size not specified) of technical methiocarb (97.9 percent pure) at chamber (analytical) concentrations of 0, 27, 92 or 298 mg cu m air, 6 h/day for-five days, the LC50 was greater than 298 mg/cu m air in males and approximately 300 mg/cu m air in females (Thyssen 1982). COMMENTS The cholinesterase studies in both rats and dogs indicated methiocarb sulphoxide to be a more potent inhibitor than methiocarb. No evidence of teratogenicity of methiocarb was observed in the rabbit teratology study. The Meeting confirmed the ADI estimated at the 1981 Meeting and further desirable work was recommended. TOXICOLOGICAL EVALUATION Level Causing no Toxicological Effect Rat: 25 ppm in the diet, equivalent to 1.3 mg/kg b.w./day. Dog: 5 ppm in the diet, equivalent to 0.125 mg/kg b.w./day. Estimation of Acceptable Daily Intake for Man 0 - 0.001 mg/kg b.w. FURTHER WORK OR INFORMATION Desirable 1. Data to clarify the significance of the inhibition of plasma cholinesterase noted in the dog study which led to the estimation of the no-effect level by the 1981 Meeting, bearing in mind the views of the 1982 Meeting on the function of plasma cholinesterase. 2. Information on the method of analysis used to estimate plasma and erythrocyte cholinesterase activities in vivo in the above dog study. 3. Observations in humans. REFERENCES-TOXICOLOGY Hajjar, N.P. & Hodgson, E. Sulfoxidation of thioether-containing 1982 pesticides by the flavin-adeninine dinucleotide-dependent monooxygenase of pig liver microsomes. Biochem. Pharmacol 31: 745-752. Hayes, R.H. Cholinesterase evaluation study of 1981 methiocarb technical and methiocarb sulphoxide in dogs. Report from Mobay Chemical Corp. submitted to WHO by Bayer AG. (Unpublished) Hixson, E.J. Cholinesterase no-effect level of 1981 RMesurol and RMesurol sulphoxide in female rats. Report from Mobay Chemical Corp. submitted to WHO by Bayer AG. (Unpublished) Tesh, J.M., Rose, F.W., H321: effects of oral administration Secker, R.C. & Wilby, O.K. upon pregnancy in the rabbit. 2. Main 1981 study. Report from Life Science Research, England, submitted to WHO by Bayer AG. (Unpublished) Thyssen, J. H321 (Mesurol active ingredient). Acute 1982 inhalation toxicity. Report from Bayer AG, submitted to WHO by Bayer AG. (Unpublished) van Hoof, F. & Heyndrickx. The excretion in urine of four 1975 insecticidal carbamates and their phenolic metabolites after oral administration to rats. Arch. Toxicol. 34: 81-88. RESIDUES RESIDUES IN FOOD AND THEIR EVALUATION USE PATTERN Further detailed information on use patterns in various countries only served to confirm those identified in 1981. The only additional uses brought to light were on sorghum in Africa and South America, where a single application of 1 - 1.5 kg/ha is made at the dough stage and the uses in Eastern Africa mentioned below. Bruggers et al. (1981) summarized the results of field evaluations, during several years, of methiocarb when used to protect ripening crops of rice, wheat, sorghum and sunflowers in Eastern Africa from losses due to birds. The positive results and favourable cost/effectiveness obtained in nearly all situations justifies the use of methiocarb under typical farming conditions in Africa. Treatment rates are in the range of 1-2 kg/ha and application is made at the dough stage when the crop first becomes vulnerable to damage by birds. There is world-wide acceptance of methiocarb for use as a bait against slugs and snails. The baits contain either 2 percent or 4 percent methiocarb, together with wheat bran or a similar attractant, formulated into small pellets designed to be scattered by hand or machine. The treatment is designed to deposit the pellets on the soil between the crop plants so as to be readily accessible to foraging molluscs. It is recognized that some pellets will impact upon the leaves and stems of crop plants and that some may lodge between leaves of crops such as leafy vegetables. The structure of the pellets is sufficiently strong that they do not disintegrate and thus contaminate plant parts to a significant extent. The pellets are generally coloured distinctively and can be dislodged in preparing crops for market and cooking. When methiocarb is used as bait, the rate of application is in the range of 120-240 g a.i./ha. One application is generally sufficient but a second may be required. Treatment is usually made early in the crop cycle to prevent damage to young seedlings or emerging plants. In the event of snails and slugs invading from adjacent fields, perimeter baiting is usually effective. In 1981, the United Stated Environmental Protection Agency granted authority to use methiocarb to repel birds depredating grapes. The rate of application is 4 kg/ha with a maximum of four applications per season. Maximum residue limits (MRLs) were established for grapes, raisins, raisin waste and grape pomace together with secondary MRLs in foods of animal origin. This use was extended in 1982. In 1983, almost 900 tonnes of methiocarb active ingredient were used on grapes in 15 states, in addition to quantities used to repel birds from blueberries, cherries, peaches and cereal grains (Anonymous 1983). RESIDUES RESULTING FROM SUPERVISED TRIALS Extensive data from supervised trials were evaluated in 1981. The following additional reports were considered by the Meeting. Cauliflower A trial carried out in Germany in 1981 involved the application of 4 percent methiocarb granular snail bait, on two occasions 16 days apart, at the rate of 0.12 kg a.i./ha. Samples of leaves, flowers and stalks taken 0, 4, 7, 14 and 28 days after the second treatment were analysed by a method sensitive to 0.05 mg/kg of methiocarb, its sulphoxide and sulphone. Though the deposit on the day of treatment ranged up to 2 mg/kg on stalks, by the fourth day the residue was at or slightly above the limit of determination. By the seventh day there was no detectable residue in flowers, leaves or stalks (Bayer 1983). Maize Methiocarb is used as a seed treatment against fruit flies and as a bird repellent. The treatment involves the application of 300 ml of formulation (500 g/l) per 100 kg of seed. The treated seed is distributed at a rate equivalent to 150 g of methiocarb/ha. Green maize plants cut for silage approximately 90 days after planting were analysed, as was the ripe grain harvested approximately 180 days after planting. In none of six trials conducted in various regions of Germany or of two similar trials in the United Kingdom was methiocarb or its metabolites detected when samples were analysed by methods sensitive to 0.05 mg/kg or 0.01 mg/kg (Bayer 1983). Sorghum Pelissier (1978) working in Senegal showed that when methiocarb was applied to ripening sorghum at the rate of 2 kg/ha, together with an adhesive, the pesticide had a half-life of six to seven days. Residue levels in grain were less than 3.6 mg/kg 20 days after application. Gras et al. (1981) conducted trials with methiocarb to determine the level and fate of residues when the pesticide was applied to ripening sorghum as a bird repellent. Such treatments are finding favour in West Africa to reduce the depredations wrought by the red-billed Quelea. Methiocarb was applied at the rate of 2 kg/ha in aqueous suspension when the sorghym grain was at the "milk" stage. The crop was subjected to overhead irrigation. Samples (heads on stalk) were collected 12 and 24 hours, and 5, 10, 15, 20, 25 and 30 days after treatment. Residues were determined in the entire head (grain and glume) and in the grain only. The results are given in Table 1. The normal harvest date is approximately 21 days after treatment (milk stage). The glumes are usually removed during threshing or prior to and during milling. A mathematical analysis of these data indicates that the residual half-life is 6 days on grain and 7.5 days on grain plus glumes. The authors drew attention to the fact that sorghum is always cooked prior to eating. In view of the sensitivity of methiocarb to heat it is entirely likely that any residues remaining on the sorghum grain would be partially or wholly degraded during preparing and cooking. However, the data on residues in grain plus glumes leads to the conclusion that the residue level on the stubble after harvest would be high. The consequences of feeding such stubble to livestock have not been evaluated. Cherries Workers in Belgium studied the use of methiocarb sprays for the protection of cherries against damage by starlings. The concentration of the residues was determined on cherries treated with methiocarb spray once, 23 days before harvest and twice, 23 and 6 days before harvest. The maximum concentrations of methiocarb (13.4 mg/kg) and methiocarb sulphoxide (1.25 mg/kg) were recorded on cherries sprayed twice. No methiocarb sulphone was detected in any samples (Hoyoux & Zenon-Roland, 1979). Information concerning the use of methiocarb and the residues resulting from such use was received from the Government of Thailand. This information is summarized in Table 2. With the exception of Chinese radish the results were within the MRLs recommended in 1981, notwithstanding the fact that the treatments had been made by spraying. In the case of Chinese radish the residue reported (4.25 mg/kg) appears to be high for a root crop but there may be special reasons for this result. The Meeting proposed an MRL for Chinese radish. FATE OF RESIDUES General The 1981 evaluation of methiocarb (FAO/WHO 1982b) included a review of the fate of its residues, which is supplemented below. Revised versions of the list of identified metabolites and of the diagram of metabolic pathways given in 1981 are reproduced here for convenience (Table 3 and Fig. 1, respectively). In Animals Some studies on the fate of methiocarb in rats, dogs, cattle, poultry and fish were evaluated in 1981. It should be noted that the last sentence describing the study on dogs in FAO/WHO 1982 (p.318) should end "... tissue residues were below 0.4 mg/kg." Additional studies are reviewed below.Table 1. Persistence of methiocarb on sorghum, Bambey, Senegal, 1978. Sample Interval Methiocarb residue (mg/kg) No. after Thin-layer Gas application1 chromatography chromatography (days) Seed only 1 5 20 36 2 10 17 13 3 15 3.8 8.8 4 20 3.5 4.3 5 25 3.0 4.0 6 30 0.9 2.2 Seed and glumes 7 0 180 200 8 0.5 140 150 9 1 150 170 10 2 100 140 11 5 92 140 12 10 100 140 13 15 11 76 14 20 24 56 15 25 17 24 16 30 8 11 1 Application rate was 2 kg/ha spray. Table 2. Residues Resulting from Supervised Trials of Methiocarb, Thailand Application Crop Pest Rate No. Pre-harvest Residue (kg/ha) Interval (mg/kg) (days) Cabbage Diamond back moth 1.0 1 7 0.16 Aphis Chinese Kale Aphis 1.0 2 5 N.D.2 Phyllotreta sp. Edible Rape Thrips 1.0 1 5 0.14 Phyllotreta sp. Chinese Thrips 1.0 2 7 4.25 Radish Phyllotreta sp. Broccoli Aphis 1.0 1 7 0.13 Yard long bean Aphis 1.45 1 7 N.D. Thrips Multiplier Thrips 2.5 1 7 0.02 onion Mung bean Aphis 1.0 2 12 N.D. 1 50% wettable powder used in all treatments. 2 N.D. = not detectable Table 3. Chemical Names, Structures and Designations of Methiocarb and its Metabolites Identified in Animals Designation Chemical name Structure
O " Methiocarb 3,5-dimethyl-4-(methylthio)=phenyl R1 = -O-C-NH-CH3 methylcarbamate R2 = -SCH3 O " Methiocarb sulphoxide 3,5-dimethyl-4-(methylsul=phenyl)phenyl R1 = -O-C-NH-CH3 methylcarbamate O ' R2 = -SCH3 O " Methiocarb sulphone 3,5-dimethyl-4-(methylsul=phonyl)phenyl R1 = -O-C-NH-CH3 methylcarbamate O ' R2 = -SCH3 ' O O " N-hydroxymethyl-methiocarb 3,5-dimethyl-4-(methylthio)=phenyl R1 = -O-C-NH-CH2OH N-hydroxymethyl)carbamate R2 = -SCH3 Table 3 (continued) Designation Chemical name Structure O " N-hydroxymethyl-methiocarb 3,5-dimethyl-4-(mathylsul=phinyl)phenyl R1 = -O-C-NH-CH2OH sulphoxide N-hydroxymethyl)carbamate O ' R2 = -SCH3 Methiocarb phenol 3,5-dimethyl-4-(methythio)=phenol R1 = -OH R2 = -SCH3 Methiocarb sulphoxide phenol 3,5-dimethyl-4-(methylsul=phinyl)phenol R1 = OH O ' R2 = -SCH3 Methiocarb sulphone phenol 3,5-dimethyl-4-(methylsul=phonyl)phenol R1 = -OH O ' R2 = -SCH3 ' O Rats Krishna & Casida (1966) studied the elimination of carbonyl-14C methiocarb after intraperitoneal (i.p.) administration to rats. Within 48 hours, 66.1 percent of the applied radioactivity was expired as 14CO2, 22.3 percent excreted in the urine and 2.5 percent eliminated in the faeces. Only 8.9 percent remained in the body. The excretion and distribution of radioactivity were examined in foetal and maternal tissues of rats after i.p. injection of carbonyl-14C methiocarb (Wheeler & Strother 1974). The behaviour was characterized by rapid maternal distribution and placental transfer. Foetal kidney and heart contained the most radioactivity. Liver was the maternal tissue with the highest 14C content. From the total radioactivity recovered in the urine, faeces and exhaled air after 8 h, it appeared that the pregnant rat retained about 10 percent more radioactivity than the nonpregnant animal. Following a single oral application of unlabelled methiocarb to rats, only minor quantities of the unchanged product and its phenol were excreted in the urine during the 48 hours following administration (van Hoof & Hendrickx 1975). In Vitro Studies The in vitro metabolism of methiocarb by liver, kidney and blood fractions was studied in man, dog and rat (Strother 1970, 1972; Wheeler & Strother 1971). Methiocarb was degraded in all preparations. All species produced essentially the same major metabolites (methiocarb sulphoxide and N-hydroxymethyl-methiocarb) although quantitative differences were observed. In studies with microsomal-NADPH2 systems of houseflies (Tsukamoto & Casida 1967) and rats (Oonnithan & Casida 1968), sulphoxidation of methiocarb was also found to be the main degradation pathway. In vitro studies on houseflies revealed the formation of N-hydroxymethyl-methiocarb as a further metabolite (Metcalf et al 19670 The request of the 1981 Meeting for analysis of ruminant liver and kidney for N-hydroxy-methyl-methiocarb was reconsidered since, in one of the studies cited by that Meeting, essentially all of the radioactive residue in those tissues was accounted for and none of it was that metabolite. Even if N-hydroxymethyl-methiocarb were present, it would be measured by the recommended residue method for meat and milk. In view of the need to apply methiocarb to grain crops to prevent destruction by plagues of birds, consideration needs to be given to the consequences of feeding the straw to livestock or grazing livestock on the stubbles. Such limited information as is available suggests that the residues on the waste plant material would be quite substantial. The animal transfer studies evaluated by the Meeting in 1981 were not adequate to demonstrate the fate of such residues. The Meeting therefore was unable to make recommendations for MRLs in meat, milk and eggs to deal with such practices. Further studies are needed. In Plants Beans Following both foliar application and stem injection, carbonyl -14C methiocarb was rapidly converted to the sulphoxide and sulphone (Abdel-Wahab et al 1966). Six days after stem injection, a 64 percent loss of radioactivity was measured, which the authors considered possibly resulted from expiration as 14CO2 (Table 4). In Soil Methiocarb is readily absorbed by soil but not desorbed. The parent compound is classified as having only low mobility in leaching studies. The degradation products of methiocarb also appear in only very small amounts in the leachate. In laboratory studies, which were conducted in accordance with BBA-Merkblatt No. 37 (Biologische Bundesanstalt für Land- und Forstwirtschaft 1980), the leaching behaviour of methiocarb, formulated as 50 percent WP and as slug bait, methiocarb sulphoxide and methiocarb sulphone was investigated. After the application of methiocarb or its sulphone and addition of water equivalent to about 200 mm rainfall in 60 h, residues found in the leachate amounted to less than 2.5 percent of the applied dose (Bayer 1974a, 1978a). In the studies with methiocarb sulphoxide, the residues in the leachate amounted to 3.8 percent (Bayer 1978G). Laboratory degradation studies with the two standard soils specified in BBA-Merkblatt No. 36 (Biologische Bundesanstalt für Land- und Forstwirtschaft, 1976) gave half-lives of about four and six weeks for methiocarb (Bayer 1974b). A study of the adsorption of methiocarb sulphoxide from water by sandy loam (Strankowski & Murphy 1982) was complicated by rapid degradation. It was not feasible to measure the adsorption because the methiocarb sulphoxide was rapidly hydrolysed to its phenol. Within half an hour, 24 percent decomposition had occurred in the soil/water system. A maximum of 4 percent of the methiocarb sulphoxide was adsorbed, and this decreased to 1 percent during the four-hour study. In Rotational Crops Wheat, sugarbeet and spinach were planted as rotational crops one year after bare soil (sandy loam) had been treated with ring-1-14C methiocarb at a field application rate of 5.6 kg a.i./ha (Strankowski & Kottman 1979). Maize was used as the original crop in this soil. At the time of planting the rotational crops, the soil contained total radioactivity of 2.62 mg/kg methiocarb equivalents. The rotational crops were grown to maturity and, at the time of harvest, each crop matrix contained an average total radioactivity of slightly less than 0.1 mg/kg methiocarb equivalents. Samples of the immature crops usually contained radioactivity equivalent to more than 0.1 mg/kg. The carbamate residues found in each of the rotational crops by thin-layer chromatography were <0.08 mg/kg (wheat heads, 0.022 mg/kg; wheat stalks, 0.025 mg/kg; wheat forage, 0.080 mg/kg; spinach, 0.016 mg/kg; sugarbeet roots, <0.014 mg/kg). Methiocarb metabolites identified in these crops included N-hydroxymethyl-methiocarb, methiocarb sulphoxide, N-hydroxymethyl-methiocarb sulphoxide, methiocarb sulphoxide phenol, methiocarb sulphone and methiocarb sulphone phenol. No methiocarb was detected. The radioactivity in the soil at the final harvest was 1.57 mg/kg methiocarb equivalents. In Water Studies of methiocarb hydrolysis and its degradation in a pond water/sediment combination were evaluated in 1981 (FAO/WHO 1982). A further hydrolysis study was carried out according to the OECD Test Method for determining hydrolysis as a function of pH in compliance with the requirements of the German Biologische Bundesanstalt für Land- und Forstwirtschaft (Wilmes 1983). The half-lives at different temperatures and pH values were as follows: pH 7 7 9 9 Temperature (°C) 40 50 20 30 Half-life (hours) 26.5 4.6 8.5 1.5 Photodecomposition Methiocarb in solution and in the adsorbed state is degradable by light in the laboratory. Owing to the ultraviolet adsorption properties of the pesticide, direct photodegradation should also be possible under environmental conditions. The most important primary degradation reactions are hydrolysis to the phenol and oxidation to the sulphoxide and sulphone. Kumar et al (1974) examined the photolysis of methiocarb in aerated and degassed ethanol and cyclohexane solutions. The exciting wavelength was above 300 mm. Photolysis yielded only one major product, methiocarb phenol. Degradation by light on glass, silica gel and soil surfaces (Houseworth & Tweedy 1974) was mentioned in the 1981 evaluation. Crosby et al (1965) irradiated ethanolic solutions of methiocarb in sunlight and found a number of cholinesterase-inhibiting products, which were not identified. Table 4. Distributien of Radioactivity among Methiocarb and its Metabolites in Bean Plants % of applied label In organic extract Treatment Time after Methiocarb Methiocarb methiocarb In Unextracted Loss last sulphoxide sulphone water application phase Foliage Treatment 0 h 99.3 0.4 0.1 14CO 2 h 93.8 4.6 0.2 80 µg - 190 mg/kg 4 h 84.9 8.7 0.5 8 h 70.8 9.3 0.8 1 day 68.8 9.9 1.1 2 days 60.0 8.3 2.2 3 days 55.4 6.7 1.9 Stem Injection 0 h 98.0 1.3 <0.1 0.3 0.4 14CO 12 h 49.0 19.8 2.3 4.2 11.5 13.2 25 µg = 12 mg/kg 1 day 19.8 23.3 8.2 7.2 18.0 23.5 2 days 2.2 5.5 5.5 8.2 25.3 53.3 3 days <0.1 4.7 1.8 8.7 24.0 60.8 4 days <0.1 3.21 8.1 26.1 62.6 6 days <0.1 1.31 7.7 27.5 63.5 1 Total found in chloroform extract after column clean-up. Thin-layer chromatography omitted. METHODS OF RESIDUE ANALYSIS In addition to the methods reviewed in 1981 (FAO/WHO 1982) the Meeting was aware of several other developments that were considered useful. Blass (1974) described a multi-residue method for the determination of methiocarb and five other carbamate insecticides in plant material. This method offers the possibility of determining the unchanged parent compound following acylation or the total residue after phosporylation of the phenols. Recoveries from lettuce, potatoes, apples and carrots in the 0.5-5 mg/kg range were 70-104 percent. Muth & Erro (1980) described a rapid multi-residue procedure for determining carbamate residues in vegetable crops, using water as the extracting solvent and adsorbing the proteinaceous material and particulate matter on Filter-Cel before filtration. The clarified extract was passed through a reverse-phase Sep-pak cartridge from which the residue was eluted with methanol and determined by high precision liquid chromatography (HPLC). Fifteen crops spiked at a level of 5 mg/kg were analysed. Interferences were minimal and the recovery of methiocarb was generally in the range 80-107 percent, although parsley, artichokes and onions showed lower recoveries. The method of Strankowski & Stanley, which was reviewed in FAO/WHO 1983, has since been published (Strankowski & Stanley 1981). A gas-chromatographic method for the determination of carbaryl, propoxur and methiocarb in vegetables and fruit has been elaborated at the National Health Institute of The Netherlands (Anon. 1976). It involves extraction with dichloromethane, followed by hydrolysis with sodium hydroxide, steam distillation, derivatization with 1-chloro-2,6-dinitro-4-trifluoromethylbenzene and measurement by gas-liquid chromatography (GLC). The lower limit of determination is 0.1 mg/kg. The sulphoxide and sulphone metabolites are not determined by this method. APPRAISAL A considerable amount of information has been received since the original evaluation was made in 1981. Some of this information was directly related to the requests made on that occasion. It is clear that many countries recognize the use of methiocarb bait (4 percent) against slugs and snails in vegetables, grains, fruit and sugarbeets. Some delegations to the 15th Session of the Codex Committee on Pesticide Residues (CCPR) questioned whether it was good agricultural practice to use methiocarb as a bird repellent to prevent destruction of such crops as blueberries, cherries, currants and grapes. There can be no doubt that numerous countries have accepted such uses as Being essential to the protection of these crops. Methiocarb is also used to protect cereal grain crops from attack by birds in Africa, New Zealand, the United States and elsewhere. Studies made in Belgium indicated that when methiocarb is used to protect cherries from birds, the residues can be up to 15 mg/kg. The Meeting gave special consideration to the toxicological significance of the residues on apples, blueberries, cherries, currants, grapes and peaches arising from the use of methiocarb as a bird repellent. While recognizing that the majority of the fruits would normally contain methiocarb residues at concentrations considerably lower than the MRLs, the Meeting considered the possibility that these commodities, consumed fresh, might contain residues that could lead to an intake of methiocarb that exceeded the demonstrated NOEL for plasma cholinesterase in dogs noted by the 1981 Meeting. The Meeting did not have available for review the details of this dog experiment and therefore was unable to evaluate fully the significance of this demonstrated inhibition of plasma cholinesterase. Attention was drawn to para 3.2 of the report of the 1982 Meeting dealing with the function of plasma cholinesterase. The Meeting agreed to request that the relevant information should be made available to it in 1984 to allow full consideration of the safety of the observed residue levels. The Meeting recommended that the MRLs for the commodities in question should be withdrawn pending this further review. Information concerning the use of methiocarb and the resulting residues was received from Thailand. This supported several of the recommendations already made for MRLs but also allowed the Meeting to estimate a maximum residue level for Chinese radish. Further information was available on the fate of methiocarb in animals, plants, soil and water and the effect of light on deposits and solutions. This only served to confirm the evaluations made in 1981. It is clear that methiocarb is rapidly degraded in plants, water and soil so that there is little possibility of carry-over in crop rotation. Similarly, because it is rapidly metabolized in animals, there is unlikely to be any transfer of residues into foods of animal origin following the feeding of forage or food offals to livestock. However, this applies only to current practices and does not allow for the possibility of livestock being grazed or fed on stubbles or other plant parts from grain crops sprayed with methiocarb to repel birds. Further methods of residue analysis have been noted. It was suggested at the 15th Session of the CCPR that the limit of determination in regulatory practice should be 0.1 mg/kg, not 0.02 mg/kg as proposed in 1981. The Meeting reconsidered this question and noted that in the case of several commodities the apparent residue on untreated control samples was frequently higher than that reported on the treated samples. It was reasonable to expect that food control officials would find difficulty in working to a limit of determination as low as 0.02 mg/kg. Bearing in mind the decision published in para. 2.4 of the 1981 Meeting report, the Meeting agreed not to recommend MRLs lower than 0.05 mg/kg. RECOMMENDATIONS The further information evaluated enabled the Meeting to recommend additional MRLs. The previously recommended MRLs for strawberries, sugarbeets and citrus fruit are increased to reflect the re-evaluated limit of determination. Some MRLs are withdrawn pending toxicological re-evaluation. MRLs refer to the sum of methiocarb, its sulphoxide and its sulphone. Commodity MRL Pre-harvest intervals mg/kg on which recommendations are based, days *Chinese radishes 5 7 * Sorghum 5 21 Strawberries 0.05** 14 Sugarbeets 0.05** 7 Citrus fruit 0.05** 30 *Meat 0.05**1 *Milk 0.05**1 *Eggs 0.05**1 *Poultry 0.05**1 Apples Blueberries Cherries MRLs withdrawn pending Currants(red) toxicological re-evaluation Grapes by JMPR. Peaches * new recommendations ** at or about the limit of determination 1 These recommendations do not allow for the possible feeding of straw and similar fodder from crops sprayed with methiocarb to repel birds. REFERENCES-RESIDUES Abdel-Wahah, A.M., Kuhr, R.J. & Fate of C14 -carbonyl-labelled Casida, J.E. aryl methyl-carbamate insecticide 1966 chemicals in and on bean plants, J. Agric. Food Chem., 14: 290-298. Anonymous. The gas-chromatographic 1976 determination of carbaryl, propoxur and methiocarb in vegetables and fruit (In Dutch). Report 39/76 TOX-ROB, February 1976. Rijksinstituut Voor de Volksgezondheid, Utrecht, The Netherlands. Anonymous. Pesticide and Toxic Chemical News, 1983 17 August, p. 20-21. Bayer. Reports Nos. 2154-2159/74. 1974a Bayer. Report No. 2153/74. 1974b Bayer. Reports Nos. 2113-2115/78. 1978a Bayer. Reports Nos. 2110-2112/78. 1978b Bayer. Data on methiocarb residues from 1983 supervised trials on cauliflower and maize. 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Mesurol and its sulphoxide and 1981 sulphone in plant, animal and soil samples. J. Agric. Food Chem. 29, 1034-1037. Strother, A.: Comparative metabolism of selected 1970 N-methylcarbamates by human and rat liver fractions. Biochem. Pharmacol. 19, 2525-2529, Strother, A.: In vitro metabolism of 1972 methylcarbamate insecticides by human and rat liver fractions. Toxicol. appl. Pharmacol. 21, 112-129. Tsukamoto, M., Casida, J.E.: Metabolism of methylcarbamate 1967 insecticides by the NADPH2 - requiring enzyme system from houseflies. Nature 7, 49-51. van Hoof, F. & Henydrickx, A. The excretion in urine of four 1975 insecticidal carbamates and their phenolic metabolites after oral administration to rats. Arch. Toxicol., 34: 81-88. Wheeler, L., Strother, A.: In vitro metabolism of the N- 1971 methylcarbamates, ZECTRAN and MESUROL, by liver, kidney and blood of dogs and rats. J. Pharmacol. exp. Ther. 178, 371-382. Wilmes, R.: Hydrolytic stability of methiocarb 1983 as a function of pH. 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See Also: Toxicological Abbreviations Methiocarb (ICSC) Methiocarb (Pesticide residues in food: 1981 evaluations) Methiocarb (JMPR Evaluations 1998 Part II Toxicological)