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 PIRIMIPHOS-METHYL RESIDUES Explanation Pirimiphos-methyl was evaluated in 1974, 1976, 1977 and 1979.1 An acceptable daily intake (ADI) was established and maximum residue limits (MRLs) were recommended in a range of commodities. A number of items of information considered desirable by these Meetings still appear to be outstanding: 1. Information on residues in fruit and vegetables following approved uses (1974). 2. Further information on the level and fate of residues in food at the point of consumption following the use of primiphos-methyl for the control of various stored product pests (1974 and 1977). 3. Results of studies now in progress on the residues in peanuts and peanut products (1976). 4. Results from commercial trials in other commodities (1976). Pirimiphos-methyl has been approved for use in the control of vectors of human disease and an interim specification has been issued by the Vector Biology and Control Division of WHO (WHO 1982). RESIDUES IN FOOD AND THEIR EVALUATION USE PATTERN A limited amount of new information has been received. From the results of monitoring of imported fruits and vegetables provided by Sweden (1983) it is apparent there must be significant uses on citrus, sweet peppers and tomatoes, at least. Barry et al. (1981) report measurable residues of pirimiphos-methyl in chickpeas (Australia) - pigeon peas (Kenya), Moong dall (Tanzania), peanuts (South Africa), split peas (Kenya) and Sardo cheese (Argentina). The Spanish Ministry of Agriculture (Spain 1983) advised that pirimiphos-methyl is registered for use on fruit including citrus and grapes, olives, beets, potatoes and other vegetables, maize, sorghum, sugarcane and stored grain. 1 See Annex 2 for FAO and WHO documentation. The use against mosquito vectors of malaria was described by Rishikesh et al. (1977) and Shaw et al. (1979) and against the blackfly vector of onchocerciasis (river blindness) by Le Berre et al. (1972). McCallum Deighton (1978) reported that Soderstrom & Armstrong (1973) showed that raisins treated with pirimiphos-methyl at the rate of 4 mg/kg gave complete kill of several stored product pests 12 months after treatment. An application of 2 mg/kg killed more than 95 percent under similar conditions. The same author reports that Spitler & Hartsell (1975) found that almonds in the shell treated with an initial deposit of 1.6 mg/kg pirimiphos-methyl showed little damage after 10 months of storage. When treated at the rate of 3.6 mg/kg, the protection was excellent and little damage occurred at the end of 12 months. Similarly, it was reported that trials during 1974-75 demonstrated that packages containing dates treated on the outside, with pirimiphos-methyl at the rate of 0.5 g/sq m remained free of insects for at least 6 mo. irrespective of the nature of the packages (cardboard, wood, palm leaf). The dates remained free of any detectable residues. RESIDUES RESULTING FROM SUPERVISED TRIALS Postharvest Cereal Grains The extensive world literature on the usefulness, effect and fate of pirimiphos-methyl when applied to cereal grains for the control of the whole spectrum of stored-product pests was reviewed. The application rate necessary to control most species adequately is of the order of 4 mg/kg but depends on the insect species, the temperature and humidity of the grain, type of storage structure, and anticipated period of storage. The application rate ranges from 2-6 mg/kg. Treatment may be confined to pirimiphos-methyl alone but where tolerant species are a problem it is more practical to combine another insecticide that is specifically effective against the pirimiphos-methyl tolerant species than to increase the level of pirimiphos-methyl sufficient to control the tolerant pests. In India, Chawla, & Bindra (1971) reported that the rate of dissipation of pirimiphos-methyl residues from grain was significantly slower than that of malathion, bromophos or iodofenphos. The Cyprus Agricultural Research Institute observed (Anonymous 1972), when testing a range of insecticides as grain treatments against a variety of stored-grain insects, that by the third mouth most of the chemicals had lost their effectiveness, whereas pirimiphos-methyl was still effective four months after application. In the United States, La Hue (1974) studied the patterns of degradation of a number of potential grain protectants compared with malathion, the standard treatment. Pirimiphos-methyl residues were influenced less by high moisture content of the grain at time of treatment than those of malathion and fenitrothion. Bowker (1974) studied the degradation of radio-labelled pirimiphos-methyl during eight months storage, when applied as a 2 percent dust to wheat at 4 mg/kg. In grain with a moisture content below 14 percent, only 20 percent degradation was observed during storage, whereas 70 to 80 percent degradation was recorded, in grain with about 18 percent moisture. He found no significant levels of degradation products, other than the hydrolysis product 2-diethylamino-4-hydroxy-6-methyl pyrimidine. In particular, levels of the very unstable "oxon" and of the N-de-ethylated thionate were all less than 0.05 mg/kg total residue. Rowlands (1975) reported results of studies on the breakdown and recovery of radio-labelled pirimiphos-methyl in wheat treated at 4 mg/kg in hexane solution and stored under laboratory conditions at 21°C. After eight months, 78 percent of the radioactivity was still recoverable as intact parent compound and 9-10 percent was bound to lipid and proteinaceous matter in the grain. The latter could only be recovered by digestion and was thought to be associated chiefly with the protein. The only metabolite detected was the hydroxy-pyrimidine. Rowlands & Wilkin (1975) applied solutions of radiolabelled pirimiphos-methyl and also a 2 percent dust to wheat with a moisture content of 14 and 18 percent to give 4 mg/kg final treatment. The samples were then stored in a laboratory at 20°C for six months. They also found that approximately 10 percent of the radioactivity was bound to lipoprotein material and was unextractable except by digesting the aleurone regions of the grain. Such labelled material, as recovered by this means, appeared to be unchanged pirimiphos-methyl, but degradation during this extraction and subsequent purification hampered identification. During the storage period, the breakdown rates were approximately the same for both solvent and dust treatments. After six months, 15 and 50 percent degradation had occurred at the 14 and 18 percent moisture levels, respectively. Bengston et al. (1975), in studying the effect of pirimiphos-methyl against malathion-resistant insects and its fate in bulk wheat under typical semi-tropical conditions found that the deposit degraded rather slowly. Deposits of 2 and 4 mg/kg degraded to 1.62 and 2.94 mg/kg after 25 weeks, respectively. The authors estimated the half-life of such deposits to be of the order of 45 weeks (temperature and moisture conditions not stated). Weaving (1975) found pirimiphos-methyl, applied to maize and sorghum so as to give a deposit of 5 and 10 mg/kg, gave complete kill of Sitophilus zeamais for 12 months under tribal storage conditions in Rhodesia (now Zimbabwe). La Hue & Dicke (1976) found that pirimiphos-methyl applied at the rate of 8.4 mg/kg to high-moisture sorghum grain gave excellent protection against damage by six different insect pests for a period of 12 months. Morallo-Rejesus & Carino (1976) reported pirimiphos-methyl was more persistent on maize than on sorghum but apparently little allowance was made in their studies for any difference in temperature or moisture content. Desmarchelier (1977) reported that the degradation of pirimiphos-methyl over a 35-week period was too slow to allow accurate measurements of temperature effects (experiments conducted over the range 20 to 30°C and 11.5 to 12.5 percent moisture content). La Hue (1977) noted that pirimiphos-methyl degraded much more slowly than did malathion under similar conditions; 83 percent of the original deposit of pirimiphos-methyl remained on wheat 12 months after treatment, whereas only 16 percent of the malathion deposit remained under identical conditions. Cerna & Benes (1977) provided results of a study carried out in Czechoslovakia where 400 tonnes of wheat, containing 12.6-14.2 percent moisture at a temperature of 8-10°C, was treated with pirimiphos-methyl at a rate to provide 4 mg/kg. The wheat was analysed at the time of treatment and at intervals thereafter until discharged for milling at the end of 286 days. Over this period, the residue levels declined from 3.7 to 1.62 mg/kg. Bengston et al. (1978) carried out duplicate field experiments on the control of insect infestation in stored sorghum. Residues of pirimiphos-methyl declined from 3.9 to 3.0 mg/kg over a period of 24 weeks; the average temperature was 26°C and average moisture content was 12.4 percent. Banks & Desmarchelier (1978) discussed the finding that the loss of insecticide residues from stored grain follows pseudo first-order reaction kinetics and that this model applies equally well to pirimiphos-methyl (Desmarchelier 1977; Desmarchelier 1978). They noted the influence of water vapour and temperature on changes in pesticide residue levels with time and drew attention to the errors introduced in calculating the rate of degradation from a "linear" or a semi-logarithmic model. Desmarchelier & Bengston (1979) further developed this concept and explained how the mathematical models are developed and used. They compared the rate of degradation of 12 grain protectants. The half-life of pirimiphos-methyl at 30°C and 50 percent relative humidity was given as 70 weeks. This compared with 12 weeks for malathion under similar conditions. The interesting point is that the co-efficient of variation with respect to temperature is much smaller for pirimiphos-methyl than for any of the other compounds considered. Desmarchelier et al. (in press) provided extensive information from 21 commercial storages treated with pirimiphos-methyl at 6 mg/kg. Grain condition and protectant residue levels were regularly monitored. The residue level declined from 6 to 4 mg/kg over ten months on grain that remained at 30°C for seven months and then cooled gradually to 20°C. The moisture content of this grain was between 11 and 12 percent. The mean observed and predicted residue levels of pirimiphos-methyl were plotted at intervals after application. Figure 1 shows the high level of agreement with the model, based on pseudo first-order kinetics. Bengston et al. (1980b) reported the results of duplicate field trials carried out on bulk wheat in commercial silos, where pirimiphos-methyl was applied at the rate of 6 mg/kg in conjunction with phenothrin (2 mg/kg). The effectiveness against a range of stored-product pests and the residue levels were monitored throughout a period of nine months, during which the pirimiphos-methyl residue level did not change appreciably. The moisture content of the grain ranged from 10.7 to 11.1 percent and the temperature from 26 to 29°C. Bengston et al. (1980a) reported the results of field trials with various pesticide combinations carried out on bulk wheat in commercial silos in Queensland, South Australia and Western Australia. Once again, the concentration of pirimiphos-methyl declined only slightly during the eight months storage period. Desmarchelier et al. (1980b) reported the results of an extensive collaborative study of residues on wheat of methacrifos, chlorpyrifos-methyl, fenitrothion, malathion and pirimiphos-methyl. Pirimiphos-methyl degraded more slowly and to a lesser extent than the other compounds. The measured values of residues of pirimiphos-methyl on wheat at the experimental sites agreed with the values predicted from the calculation using first-order kinetics, grain temperature and interstitial relative humidity. The rate of degradation was surprisingly slow. Desmarchelier et al. (1980a) reported studies on the fate of pirimiphos-methyl and five other grain protectants or rice and barley after storage and during processing. The level of residues were determined on unhusked rice, husked rice, polished rice and barley over a storage period of six months. The observed levels were close to those predicted from use of a model relating rate of loss of residue levels to a rate constant and only two variables, temperature and equilibrium relative humidity. The difference between predicted and observed values of residues of pirimiphos-methyl on the four commodities was 8 percent. Residue data were obtained over a year from hard winter wheat, shelled corn and sorghum grain treated with pirimiphos-methyl stored in bins in a laboratory (La Hue 1974). Table 1 shows the average residues of pirimiphos-methyl in the stored grain over the twelve-month period at 27°C. It is noticeable that the degradation appears to be independent of the moisture content of the grain under these conditions. Malathion, included in the same trial, degraded to about one tenth of the initial dosage within the same period. In the case of the high moisture sorghum, it was almost entirely destroyed within one month. Seth (1974) reported trials to evaluate pirimiphos-methyl for the control of pests in stored rice in South-east Asia. Rice in husk and polished rice were treated with emulsifiable concentrate (E.C.) and dust formulations at rates equivalent to 2, 4 and 8 mg/kg. The rice was analysed at monthly intervals throughout the four-month trial. In polished grains, the residue at the end of four months was approximately 25 percent of the residue found on the first day of the trial. In rice in husk, it was not possible to deduce the degradation, since the residue data were expressed on hull and milled grain separately. However, there appeared to be relatively little loss of pirimiphos-methyl over five months. Relatively little of the insecticide (generally less than 0.5 mg/kg) transferred to milled grain. There was no significant difference between the dust and E.C. formulations. FATE OF RESIDUES In Grain Residues of pirimiphos-methyl on wheat grain are degraded and detoxified by hydrolysis of the phosphorus-ester side chain to give principally the parent hydroxypyrimidine (Figure 2, IV), and the related compounds (V and VI). At a given temperature, the rate of breakdown increases with increasing moisture content of the grain. Levels of the N-desethyl phosphorus compound (II) were always extremely low (approximately 0.05 mg/kg over a period of 32 weeks in wheat grain treated at 4 mg/kg). No residue of the chemically-unstable oxygen analogue (III) was detected. The limit of detection was 0.01 mg/kg (Bowker 1973). Radio-autograms of grain sectioned after four months showed that the insecticide and its degradation products were concentrated in the seed coat, so that residues in white flour and bread are likely to be lower than in bran and wholemeal products. The general pattern of breakdown on stored rice was similar to that found on wheat grain. The insecticide and its degradation products were concentrated in the husk, in which the rate of breakdown appeared to be unaffected by the moisture content of the rice (Bowker 1973; Bullock 1973). Table 1. Average Residues of Pirimiphos-methyl in Stored Grain1 Post-treatment residues (mg/kg) Grain Moisture Intended months Content dosage (%) (mg/kg) 24 h 1 3 6 9 12 Wheat 12.5 7.8 6.5 6.1 5.6 6.2 4.9 5.4 Maize 12.5 8.4 7.9 6.3 4.5 4.1 3.4 3.0 Sorghum 17.6 8.4 7.5 6.5 4.3 3.8 3.8 3.7 1 Over 12 months at 27°C. Degradation was marginally more rapid in contact with the grain than in the isolated formulation but whether this additional breakdown was caused by factors within the grain, or by the associated microflora, was not known. At a higher moisture content (ca. 18 percent) less pirimiphos-methyl was recovered on analysis but increased levels of the hydrolysis product (IV) were obtained, suggesting that a more rapid degradation of the insecticide occurred. Bowker (1973) concluded that grain may lack the enzymatic activity, believed to be present in plants and soil, to cleave the pyrimidine N-ethyl bonds. It is likely, therefore, that following the treatment of wheat and brown rice with pirimiphos-methyl, the major residues during storage will be the parent compound and the simple hydrolysis product (IV). Under optimum conditions, the maximum level of compound (IV) following treatment at 4 mg/kg was found to be 0.17 mg/kg; under poor storage conditions, with high moisture content grain it was 0.62 mg/kg. Solutions and also a 2 percent dust of radio-labelled pirimiphos-methyl were applied in the laboratory to wheat grain of 14 and 18 percent moisture to give levels of 4 mg/kg. Throughout a storage period of six months, the residues of pirimiphos-methyl were found almost entirely in the seed coat and aleurone layer, with only traces present in the germ or endosperm. It was also clear that, as with malathion but to a greater extent, there was transfer of insecticide between grains, possibly in the vapour phase. About 10 percent of the total residual radioactivity was bound to lipoprotein material in the aleurone region of the grain and could be extracted only by digestion of the aleurone protein. In contrast with other organophosphates studied, the bound material appeared to be unchanged pesticide rather than a metabolite. It was undoubtedly a P=S compound, but degradation during the liberation of the bound material complicated identification (Rowlands et al. 1974). In Animals The metabolism of pirimiphos-methyl was studied in the rat and dog. Twelve metabolites were detected, none of which showed anticholinesterase activity. No parent pirimiphos-methyl was detected in the urine. In both rats and dogs, 2-ethylamino-4-hydroxy-6-methyl pyrimidine was the major urinary metabolite (Bratt & Jones 1973). When rats were given a single oral dose of radio-labelled pirimiphos-methyl at 7.5 mg/kg, the uptake of radioactivity into blood and its subsequent disappearance from the bloodstream were both very rapid. More than 50 percent of the radioactivity present in the blood 30 min. after dosing had disappeared at one hour after dosing. Unchanged pirimiphos-methyl usually represented less than 10 percent of the total residue in the blood 24 h after dosing. When radio-labelled pirimiphos-methyl was administered orally to rats at 7.5 mg/kg per day for four days, total radioactive residues in the liver, kidney and fat did not usually exceed 2 mg/kg pirimiphos-methyl equivalents. There was no evidence to show that either pirimiphos-methyl or its metabolites accumulated in the liver, kidney or fat of rats following daily dosing with the insecticide over four days (Mills 1976). The rodioactive residues in the liver and kidney of a goat, dosed daily for seven consecutive days with 14C-pirimiphos-methyl at 30 mg/kg in the diet, were examined. (Curl and Leahy 1980). A radioactive residue of 0.25-0.30 mg/kg of 14C-pirimiphos-methyl equivalents was detected in the liver, and 89 percent of this was extracted and analysed by thin layer chromatography. 5.9 percent of the total radioactive residue in the liver was unchanged pirimiphos-methyl. The hydroxypyrimidines, compound (II) (2-diethylamino-6-methyl-pyrimidin-4-ol), compound (III) (2-ethylamino-6-methyl-pyrimidin-4-ol) and compound (IV) (2-amino-6-methyl-pyrimidin-4-ol) accounted for 3.7 percent, 21.8 percent and 17.5 percent respectively, of the total radioactive residue in the liver. Compounds (II), (III) and (IV) were present mainly as "free" metabolites. However, 13.7 percent of the hydroxypyrimidines detected were released from the liver by acid reflux. The remainder of the radioactivity extracted from the liver consisted of at least five compounds. A radioactive residue of 0.59-0.70 mg/kg of 14C-pirimiphos-methyl equivalents was found in the kidney; 91 percent of this was extracted and analysed by thin-layer chromatography. Compound (II), compound (III) and compound (IV) accounted for 7.9 percent, 35.3 percent and 17.0 percent respectively, of the total radioactive residue in the kidney. Compounds (II), (III) and (IV) were present mainly as "free" metabolites, however, 9.0 percent of the hydroxypyrimidines detected were released from the kidney by acid reflux. The remainder of the radioactivity extracted from the kidney consisted of at least six compounds. In Storage and Processing Bullock (1973, 1974) reported several individual experiments which demonstrated that residue levels of pirimiphos-methyl are significantly reduced during the milling and baking processes. Table 2 summarizes the results of residue trials carried out in the United Kingdom on wheat that had been treated to contain nominally 4 mg/kg pirimiphos-methyl. Table 3 summarizes results of a residue trial (Bullock 1973) carried out in the United Kingdom with wheat nominally treated to contain 8 mg/kg pirimiphos-methyl. These data are substantially in agreement with those of Bengston et al. (1975) and show that there is relatively little penetration beyond the seed coat, even throughout a storage period of nine weeks. Table 2. Effect of Milling and Baking on Residue in Wheat Admixed with Pirimiphos-Methyl1 Interval between Grain treatment and Residues (mg/kg) fraction sampling (months) Highest Lowest Mean Whole grain 0 4.2 1.9 2.9 (9) 3.0 -* 3.0* 1 4.1 1.5 2.8 (9) 2 4.1 1.6 2.6 (8) 2.5* 2.4* 2.5* (3) 3 3.6 1.3 2.3 (7) Whole meal 0 1.3 0.94 1.1 (3) flour 1 2.1 1.1 1.7 (4) 2 2.2 1.2 1.7 (3) 1.5* 1.4* 1.5* (3) 3 2.1 1.0 1.5 (4) White flour 0 0.88 0.30 0.52 (6) 0.56* 0.53* 0.55*(3) 1 0.77 0.44 0.59 (5) 2 0.64 0.24 0.56 (6) 0.29* 0.24* 0.27*(3) 3 0.67 0.38 0.56 (3) Wholemeal 0 0.72 0.53 0.64 (4) bread 1 0.91 0.55 0.79 (4) 2 1.1 0.65 0.93 (4) 0.97* 0.82* 0.88* (3) 3 0.54 0.21 0.49 (3) White bread 0 0.28 0.19 0.23 (6) 0.26* 0.24* 0.25* (3) 1 0.36 0.22 0.30 (5) 2 0.45 0.31 0.36 (8) 0.15* 0.13* 0.14* (3) 3 0.54 0.21 0.43 (3) 1 Dosage of pesticide was 4 mg/kg. All results are from field trials except those marked *, which are from a small-scale trial. Figures in parentheses are the numbers of results on which the means are based. Table 3. Residues of Pirimiphos-methyl in Whole Grains and in Milling and Baking Fractions Treated in a Laboratory Trial1 Interval between Residues (mg/kg) treatment and Whole White White Wholemeal Wholemeal sampling grain flour bread flour bread 0 days 6.0 0.86 0.52 6.0 0.91 0.56 6.0 1.0 0.57 9 weeks 4.8 0.47 0.28 3.2 1.7 5.2 0.59 0.33 3.0 1.6 5.2 0.60 0.30 3.2 1.5 1 Dosage of pesticide was 8 mg/kg. In all cases, no residues of the phosphorus-containing compounds (II) or (III) (Figure 1) were detected. (Limit of detection: 0.01 mg/kg in each case.) Seth (1974) discussed experiments on the admixture of pirimiphos-methyl with paddy rice (rice in husk) in Southeast Asia. Both emulsion and dust formulations were used and were applied at rates of 2, 4 and 8 mg/kg. A portion of the rice was milled immediately after treatment and further portions at the end of one, two, three and five months. It was found that the deposit of pirimiphos-methyl on both the husk and the milled grain was proportional to the amount applied, but the concentration on the husk was always about 25 times that on the milled grain. The residue on the husk and milled grain declined steadily over the five-month storage period. Although the concentration on the husk initially was as high as 12 mg/kg, the residue on the milled rice barely exceeded 0.5 mg/kg and even at the highest rate of application was always below 1 mg/kg. These experiments clearly showed that there is minimum transfer of pirimiphos-methyl from the husk to the kernel of rice, even during prolonged storage. The degree of penetration is comparable to that reported by Kadoum & La Hue (1974) for malathion on wheat, maize and sorghum. Residues of pirimiphos-methyl in flour are relatively stable during baking to bread and biscuits. However, because of the dilution which the flour undergoes during these processes, flour initially containing 1 mg/kg pirimiphos-methyl is likely to yield bread/ biscuits containing residues of ca. 0.5 mg/kg. Bullock et al. (1976) studied the fate of pirimiphos-methyl during processing of flour into bread and biscuits. In studies with the radio-labelled compound, flour was dosed with 2-14C-labelled pirimiphos-methyl and baked to produce white bread, wholemeal bread and biscuits. Although pirimiphos-methyl is known to be a relatively volative compound, there was no significant loss of radioactivity by volatilization. Distribution of radioactivity throughout the bread was fairly uniform. Unchanged pirimiphos-methyl accounted for 75-90 percent of the radioactivity in the baked product. The major degradation product formed during the baking was hydroxypyrimidine, which accounted for 3-10 percent of the radioactivity in the final product. Similar results were obtained by residue analysis in a second set of studies. After correcting for the weight increase when flour is converted to bread, residues of pirimiphos-methyl fell by 11-18 percent. Likewise, residue analysis of biscuits showed average losses of 8 percent. However, owing to the dilution of the flour that occurs during baking, the residue of pirimiphos-methyl in bread will be lower than that in the corresponding flour. Bullock et al. (1976) found levels of pirimiphos-methyl in bread and biscuits to be about 50 percent of those in the flour from which they were derived. These findings agreed well with the earlier work of Bullock (1973, 1974). No residues of the phosphorus-containing compounds (II) and (III) were detected in bread baked from flour treated with pirimiphos-methyl at 1 mg/kg or in biscuits baked from flour containing up to 5 mg/kg (Bullock et al. 1976) The hydroxypyrimidine (IV) also undergoes only slight degradation during the baking process. This compound constitutes only a minor part of the residue in stored grains (FAO/,WHO 1975). Bullock et al (1976) used radio-labelled compound (IV) and found that it degraded by less than 10 percent during the baking of bread. It can, therefore, be concluded that residues of compound (IV) in baked products will never exceed 0.2 mg/kg and will normally be considerably lower. Bullock & May (1976) studied the fate of residues in wheat during processing to semolina and pasta. White semolina, prepared from durum wheat treated at 10 mg/kg, contained only 1.6 mg/kg pirimiphos-methyl. Pirimiphos-methyl levels in both white and wholemeal pasta were approximately 85-90 percent of those in the corresponding semolina; 70 percent of the pirimiphos-methyl residue in semolina was transferred unchanged to cooked pasta. However, the weight of pasta increased by 100 percent on cooking, so that the concentration of pirimiphos-methyl in cooked pasta is likely to be approximately 35 percent of that in the corresponding semolina. Results of experiments in Czechoslovakia (Cerna & Benes 1977) indicated that residues of pirimiphos-methyl in grain, which had been in store for nine months, were substantially removed by the milling process. The bulk of the residue was removed in the bran; there was no substantial difference in the concentration in the different bran fractions. When white flour was made into white bread, there was a further loss of approximately 50 percent so that the concentration of the residue in the bread was only 10-15 percent of its concentration in raw grain. Some residue was destroyed during milling. Tempone (1979) reported a series of studies on the effects of insecticides on barley malting and the resulting residues. In the first of these trials, barley treated with pirimiphos-methyl at the rate of 6 mg/kg or 18 mg/kg was converted into malt but no residues were detected in the wort (unfermented extracts from the malted barley), the limit of determination being 0.004 mg/kg. Wort, prepared from barley malted after being in store for three, six and nine months, likewise showed no residues. In the second series of similar trials, the residue was determined in the malt (germinated grain that had been calcined). Residues could be found in the malt, but at a level of from 10 to 20 percent of that in the barley before malting. Barley kept in storage for three months after being treated with pirimiphos-methyl transferred significantly less residue to the malt than did the barley malted immediately after treatment, no doubt because there was significantly less residue on the outside of the kernel, where it would be protected from the enzymatic activity within the barley grain during germination. Bengston et al. (1980 a, b) arranged for wheat from bulk grain treated with pirimiphos-methyl and held in commercial silos to be processed through to wholemeal bread and white bread. The results obtained in these trials are reported in Tables 4 and 5. During processing from wheat to white bread, residues were reduced by 85-91 percent. Table 4. Mean Residues of Pirimiphos-methyl Following Milling and Baking of Stored Wheat Site Storage Residues (mg/kg) (weeks) Wheat Bran Pollard Wholemeal White Wholemeal White flour flour bread bread Site B 13 5-7 12-16 2-4 2.6 0.1-0.2 1.0-2.0 0.05-0.1 Site D 19 2.5-3.0 6-8 1-2 1.2 0.05-0.1 0.5-1.0 0.05 Table 5. Reduction in Pirimiphos-methyl Residues Following Milling and Baking of Stored Wheat Site Reduction in residues(%) Wheat to Wheat to Wholemeal flour White Flour Wheat to Wheat to Wholemeal White to Wholemeal to White Wholemeal White Flour Flour Bread Bread Bread Bread Site B 5 68 47 54 50 85 Site D 0 78 55 58 56 91 Desmarchelier et al. (1980b), after treating bulk wheat in commercial silos with pirimiphos-methyl, arranged for a portion to be milled and for the white flour to be converted into white bread. The results are given in Table 6. It is of interest to note that the residues in flour and bread were higher in the hard wheat which was held for 22 weeks before milling, than in the soft wheat which was held of 11 weeks before milling. Desmarchelier et al. (1980a) studied the fate of primiphos-methyl, and a number of other grain protectant insecticides, applied to unhusked rice, husked rice, polished rice and barley over a storage period of six months and subsequently during the processing and cooking of these grains. The results of these trials are given in Table 7, and show that only about 10-15 percent of the residue present on husked rice or polished rice was destroyed in the cooking process. However, if the treatment was applied to unhusked rice, approximately 70 percent of the residue was removed with the husk and a great deal more when the husked rice was milled for the removal of the bran. Subsequent cooking of the husked rice or polished rice brought about a further substantial reduction in the residue level. These same workers found that pirimiphos-methyl, applied to barley destined for malting, was substantially lost during the malting process. Barley treated at the rate of 6 mg/kg and held in storage for six months was found to contain 4.9 mg/kg pirimiphos-methyl. When this grain was malted, the pirimiphos-methyl residue in the prepared malt was only 0.9 mg/kg. The work of Tempone (1979) showed that little, if any, of the residue in the malt is extracted into the wort. There have been no reports of pirimiphos-methyl being metabolized by attack at either of the O,O-dimethyl groups (to give a desmethyl derivative) as is common with certain other organophosphorus triesters. Morallo-Rejesus & Carino (1976) noted, but did not identify, breakdown products in shelled maize stored in pirimiphos-methyl treated bags. Rowlands (1981), reporting work carried out some years previously using radio-labelled compound, treated small quantities (10 g) of wheat that were held in store for up to 6 months. Aliquots were removed at intervals and, after crushing, were extracted first with hexane, followed in turn by chloroform, methanol and acetonitrile and then, after sequential digestion by three enzymes, further extraction by chloroform. The results are recorded in Table 8. It is clear from Table 8 that very soon after treatment, a portion of the aged pirimiphos-methyl residue is not extractable from wheat unless it is digested out enzymatically. It is thought that this is due to a complex formed by intact pirimiphos-methyl within the aleurone layer (Rowlands 1975). Table 6. Residues of Primiphos-methyl in Wheat, Its Milling Products and Bread Residues (mg/kg) Type of Storage Wheat (weeks) Initial deposit Whole grain bran pollard flour bread Soft 11 5.2 4.2 9.1 8.3 0.2 0.1 Hard 22 6.1 4.5 12.4 10.4 1.3 0.35 Table 7. Residues of Pirimiphos-methyl on Husked, Polished and Unhusked Rice After Storage and Processing Grain Application Residues (mg/kg) level (mg/kg) After After After After 3 6 Cooking at Cooking at Months Months 3 Months 6 Months Husked rice 6 6.5 4.9 4.9 4.0 Polished rice 6 6.0 6.0 4.9 4.9 Residues (mg/kg) after 6 months storage and Milling Milling & Cooking Husked Polished Husked Polished rice rice rice rice Unhusked rice 6.0 6.0 4.6 1.4 0.3 0.6 0.1 Table 8. Recovery of Pirimiphos-methyl From Crushed Wheat (18% mo) By Sequential extraction Extraction by 14C activity recovered %1 at time of extraction after application (0 h) 0 h 1 h 7 days 14 days 1 month 6 months Hexane 97 89 84 78 72 67 Chloroform nil 1 4 7 8 8 Methanol trace 1 2 2 6 7 Acetonitrile trace 2 1 4 2 2 Digest Sequence2 (1) nil 2 1 4 2 2 (2) 4 5 7 7 9 12 (3) nil 2 2 1 2 1 Total extracted 101 102 101 103 101 99 1 Average of three results ± 3 percent. 2 Digest sequence: (1) = lipase pH 7.4; (2) = papain pH 4,5; (3) - cellulase pH 4.5, All at 37°C for 24 h Studies reported by Rowlands (1981) on the uptake from dust or solvent application as determined by the separated tissue of the wheat grains after storage, showed that intact pirimiphos-methyl and free pyrimidinols occur chiefly in the seedcoat. These findings received some confirmation in the work of Mensah et al. (1979), who found that pirimiphos-methyl emulsion, applied to wheat that was then milled six months later, had accumulated in the bran and middlings fractions, irrespective of the moisture content of the stored wheat (12 percent and 16 percent). They found little or no residue in white flour. Rowlands (1981) found only pirimiphos-methyl (I) and the corresponding pirimidinol (IV) as residues in pirimiphos-methyl treated wheat with 12 percent moisture that was kept in sealed jars for up to eight months. However, in wheat with 18 percent moisture, a small (less than 10 percent of the total residue) quantity of the N-des-ethyl pirimidinol (V) was found after six and eight months. (See Figure 2 for conformations of these compounds). Further to the work of Thomas & Rowlands (1975) on the uptake and degradation of pirimiphos-methyl by Cheshire cheese, similar studies were carried out using Stilton cheeses, which differ in physical characteristics and are stored differently. In these studies a wooden plank was treated with 14C-ring-labelled pirimiphos-methyl and allowed to dry for 24 h before two young Stilton cheeses were placed on the treated surface. One of the cheeses was first covered by a layer of cheesecloth. The cheeses were turned every two days. Replicate core samples were taken with a cork borer from each face of both cheeses. The core was analysed for pirimiphos-methyl. Some of the cores were sectioned with a microtome and the concentration of pirimiphos-methyl in the cheese at various depths was determined by means of a scintillation counter. The findings are reported in Table 9. This experiment has shown that only a small amount of pirimiphos-methyl penetrated into the cheeses under the various storage techniques used with Stilton cheese. No breakdown of the pirimiphos-methyl could be detected in the cheese or the cheesecloth throughout the seven weeks of storage. These results indicate that the MRL of 0.5 mg/kg for pirimiphos-methyl in cheese would not be exceeded when the insecticide was used for the control of cheese mites during the making and storage of Stilton cheese. Mensah et al. (1979) reported results of small-scale trials, in which water-based emulsions of malathion, bromophos, iodofenphos and pirimiphos-methyl were applied at two dosage rates to spring wheat of 12 percent and 15 percent moisture content, to compare the fate of the residues. Pirimiphos-methyl degraded at a comparatively slower rate than the other three compounds, but the rate of degradation was significantly higher on the high moisture grain. The results are given in Table 10. The residue levels were concentrated in the seed coat, resulting in high levels in bran and middlings. When applied at the rate of 6 mg/kg, the residues in these fractions exceeded the MRL recommended by the Meeting. Results were reported of small-scale trials in which the efficacy and fate of pirimiphos-methyl residue on wheat and milling fractions were assessed. Two rates (7.3 mg/kg and 14.6 mg/kg) were applied and the wheat was stored for up to 12 months. The results are reported in Table 11. These show that the residues in bran and middlings (shorts) exceeded the MRL following application at the lower rate and storage for less than six months. At the higher rate (which is well in excess of the recommended rate) the residues greatly exceeded the MRL. RESIDUES IN FOOD IN COMMERCE OR AT CONSUMPTION Residues of pirimiphos-methyl were detected in the United States in several samples of imported foods (Barry et al. 1981) and the identification of these residues by GC-MS and GLC was reported. The level and source of residues found are reported in Table 12. The Swedish National Food Institute (Sweden 1983) indicated that during the period 1-1-80 to 30-4-83, 8 654 samples of fruit and vegetables were analysed for pesticide residues. From among these, eight samples were found to contain pirimiphos-methyl at levels up to 1.4 mg/kg, as indicated in Table 13. METHODS OF RESIDUE ANALYSIS Barry et al. (1981) have provided details of GC-MS identification and analytical behaviour of pirimiphos-methyl in a variety of foods as part of an investigation of unknown residues detected in imported foods. Zakitis & McCray (1982), having reviewed the analytical methodology for residues of pirimiphos-methyl in a variety of substrates, concluded that none of the eight known methods met all the desired criteria for analysis of residues in water, fish and snails, viz. specificity, simplicity of sample preparation and recovery after clean-up. Water was simply extracted with hexane prior to determination by GC. Fish and snails were extracted with acetone in the presence of sufficient anhydrous sodium sulphate to form a dry powder. The acetone was filtered and the solids were extracted three more times with acetone. The combined, filtered extracts were evaporated to low volume before being transferred to de-ionized water for extraction with hexane as for analysis of water. Recoveries ranged from 93 to 101 percent. The method is being used for environmental studies involving the use of pirimiphos-methyl for disease vector control. The limit of determination in snails is 0.5 mg/kg. Below this concentration, some naturally occurring substance interferes with the determination of pirimiphos-methyl. Good recoveries were obtained from dechlorinated water at concentrations as low as 0.005 mg/l and in fish down to 0.05 mg/kg. Table 9. Pirimiphos-Methyl Residues From Microtomed Sections of Two Cheeses1 Residues (mg/kg) Storage period End Depth (mm) into cheese (days) sampled 0-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 Cheese in direct contact with treated surface 31 1 41.15 7.48 4.03 1.73 1.25 0.90 0.65 0.55 0.40 0.10 2 27.50 7.70 2.32 1.09 0.71 0.50 0.28 0.20 0.16 - 49 1 29.57 9.26 2.87 2.04 1.52 1.13 0.82 0.71 0.51 0.27 2 23.73 6.54 2.82 2.71 1.52 1.07 0.74 0.55 0.51 0.37 Cheese on cloth barrier 31 1 18.04 3.40 0.65 0.30 0.18 0.08 - - - - 2 35.28 11.48 4.19 0.62 0.36 0.27 0.22 0.16 - - 49 1 19.58 5.69 1.72 0.72 0.45 0.28 - - - - 2 12.26 7.13 1.84 1.11 0.84 0.66 0.46 0.31 - - 1 Dashes indicate that no pirimiphos-methyl was detected. Table 10. Mean1 Pirimiphos-methyl Residues (mg/kg) on Dry and Tough Wheat and Milled Fractions After Storage of Treated Wheat 12% moisture content (dry) 16% moisture content (tough) Storage Dosage period Whole Whole (mg/kg) (months) wheat2 Bran Middling3 Flour wheat2 Bran Middling3 Flour 4 1 3.29±0.07 15.66±0.79 21.44±1.96 1.94±0.25 3.24±0.03 19.81±1.63 15.35±0.58 1.57±0.03 3 2.98±0.10 13.93±0.26 15.10±1.00 1.93±0.12 3.04±0.01 13.45±0.80 13.51±0.36 1.16±0.06 6 2.54±0.07 13.16±0.51 13.80±1.36 1.56±0.06 2.26±0.18 11.98±0.67 10.68±0.60 0.81±0.23 6 1 4.93±0.13 20.87±0.82 23.15±1.47 2.86±0.14 4.92±0.04 27.33±1.41 21.83±1.73 1.91±0.07 3 4.54±0.10 20.54±1.38 19.34±0.74 2.42±0.22 4.44±0.04 20.90±2.92 17.89±0.59 1.50±0.08 6 3.66±0.23 19.32±0.73 16.05±1.35 2.23±0.13 3.86±0.26 18.60±0.73 16.89±0.56 1.33±0.14 1 Mean of 3 replicates ± SE. 2 Ground wheat. 3 Shorts, wheat germ and coarse particles of flour. Table 11. Average Pirimiphos-methyl Residue (mg/kg) on Hard Winter Wheat and Wheat Fractions Stored up to 12 Months Rate of application 7.3 mg/kg 14.6 mg/kg Storage Whole Whole Period wheat1 Bran Shorts Flour wheat1 Bran Shorts Flour 24 h 7.15 25.50 18.29 0.83 14.75 50.41 36.01 2.01 Months 1 6.88 21.45 17.89 1.38 13.13 41.43 31.01 2.94 3 6.60 20.11 20.82 1.47 12.98 39.97 42.97 3.15 6 5.78 18.06 20.46 1.47 11.17 30.52 46.58 2.91 9 5.43 17.34 17.90 1.17 10.39 34.52 31.47 2.44 12 5.27 13.47 15.35 1.05 9.72 32.29 26.98 2.14 1 Before tempering to 15% for milling. Table 12. Pirimiphos-methyl residues in imported foods Food Country of Origin Residue (mg/kg) Chickpeas Australia 0.07 Dried green peas Australia 3.0 Pigeon peas Kenya 0.19 Moong dall Tanzania 0.03 Peanut butter South Africa 0.23 Split peas Kenya 0.1-0.171 Sardo cheese Argentina 0.21-1.41,2 1 Represents range found in multiple samples of the same commodity. 2 Determined on fat basis. Table 13. Primiphos-methyl Residues Detected in Sweden1 Food Origin No. of No. of samples with residues within Maximum samples given ranges (mg/kg) residue analysed <0.21 0.21-0.53 0.54-1.05 >1.05 (mg/kg) Mandarin Import 292 291 1 0.21 Orange Import 622 621 1 0.41 Pepper Import 206 201 3 1 1 1.4 (sweet) Tomato Sweden 205 205 Import 500 499 1 0.30 1 Total number of samples analysed:8 654 Period: 81-01-01 -- 83-04-30 A number of the methods reviewed by Zakitis & McCray (1982) have not been considered previously by the Meeting. They are briefly mentioned for information. A general review of analytical methods was published by Bullock (1976) including procedures suitable for animal tissues. There is also a TLC-UV method for residues in water, soil and plants (Krasnykh, 1978), an HPLC method for determining pirimiphos-methyl and five metabolites in samples of plasma and urine (Beasley & Lawrence, 1979), and gas chromatographic methods for residues in stored grain (Varca et al. 1975), peanuts (Redlinger & Simonaitis 1977) and milled fractions of wheat (Mensah et al. 1979). In addition, a gas-chromatographic method has been proposed for the simultaneous determination of residues of pirimiphos-methyl and malathion in peanuts (Simonaitis et al. 1981), and GC-MS was used to identify residues of pirimiphos-methyl in imported foods (Barry et al. 1981). The residue in stored grains (barley, maize, oats, rice in husk and wheat) consisted mainly of the parent compound. Neither the oxygen analogue (O,O-dimethyl-O(2-diethylamino-6-methyl-pirimidine-4yl) phosphate), (metabolite III) nor the desethyl analogue (O, O-dimethyl-O(2-ethylamino-6-methyl-pyrimidin-4-yl) phosphorothionate), (metabolite II) was detectable. No metabolite was detectable in milled products or in white and wholemeal breads. A large number of plant samples (leafy vegetables, carrots, celery, spring onions, potatoes, sugarbeets, cucumber, tomatoes, peppers, mushrooms, lettuce, blackcurrants, apples, pears, plums, lemons, oranges, olives) were analysed for residues of pirimiphos-methyl and the phosphorous-containing metabolites. However, significant amounts of the metabolites did not appear and so were not reported. The residues listed in various tables of the 1976 Evaluations are of the parent compound only. Analyses of wheat grains treated with labelled compounds confirmed the low level of both metabolites. No oxygen analogue was detected. The level of desethyl analogue was below 0.05 mg/kg over a period of 32 weeks, while the parent compound was present at 4 mg/kg. The oxygen analogue was not detectable in whole rice seedlings. The levels of parent compound in mg/kg and desethyl analogue (expressed as parent equivalent) were as follows: Compound Residue (mg/kg at intervals (days) after treatment 1 3 6 8 10 13 pirimiphos-methyl 0.5 0.51 12 0.6 0.51 0.32 metabolite (II) and unknown compound 0.17 0.11 0.14 0.12 0.15 0.14 The oxygen analogue reached its maximum concentration on plant leaves approximately 1 to 3 days after treatment. The ratio of maximum levels of oxygen analogue and the parent compound were about 0.3 and 3 on cotton and citrus leaves, respectively, while the two compounds were present in nearly equal concentration on bean leaves. The disappearance of pirimiphos-methyl residues was rapid on the leaf surfaces of all plants due to evaporation. The total 14C represented 10-12 percent of the applied material after three days and the phosphorous containing metabolites were below 10 percent (Bowker 1973). The Meeting concluded that the metabolites can be excluded from the definition of the residue. The MRLs need not be changed because the metabolites represent such a minor proportion of the total residue. The MRLs refer to the present compound alone. APPRAISAL Pirimiphos-methyl was evaluated in 1974, 1976, 1977 and 1979 and a number of items of information previously requested appear to be still outstanding. It would appear that there are a number of uses that give rise to residues in food moving in international trade, which have not yet been considered by the Meeting and about which relevant information will be available in 1984. An extensive amount of information about the use and fate of pirimiphos-methyl when applied after harvest to cereal grains has appeared in open scientific literature and this was reviewed by the Meeting. The review only serves to confirm the evaluations made previously. All authors have drawn attention to the stability of pirimiphos-methyl deposits on treated grain and the retention of the bulk of the deposit in the seed coat. This is reflected in the low transfer of residue into white flour and milled rice and the relatively higher residues in wheat bran. This review has confirmed that the degradation and metabolism of pirimiphos-methyl on grain leads only to the production of 2-diethylamino-4-hydroxy-6-methyl pyrimidine. No cholinesterase-inhibiting metabolites occur. However, between 4 percent and 15 percent of the residue of the parent compound forms a complex with the aleurone layer of the grain and resists extraction by a variety of solvents. It may be dislodged by digesting the substrate with proteolytic enzymes. The Meeting, having reviewed all available data, concluded that the metabolites can be excluded from the definition of the residue. The MRLs, which remain the same, refer to the parent compound alone. A further study of the use of pirimiphos-methyl for the control of cheese mites in cheese stores confirmed that the MRL previously recommended adequately covers the resulting residues. Several reviews have been made of analytical methods suitable for dealing with various substrates. The Meeting considered that the information evaluated in 1983 did not indicate any need to amend or modify recommendations previously made for MRLs in a range of commodities. The Meeting reviewed the requests made by earlier Meetings for additional information in the light of knowledge about the current use of this insecticide and considered that they had been satisfied. FURTHER WORK OR INFORMATION Desirable Further information about residues in peanuts, oil seeds, lentils and citrus. REFERENCES - RESIDUES Anonymous. Chemical control of stored grain insects 1972. Cyprus 1972 Agric. Res. Inst. Ann. Rep., p. 69. Banks, H.J. & Desmarchelier, J.M. New chemical approaches to pest 1978 control in stored grain. Chem. Aust. 45(6): 276-281. Barry, T.L., Petzinger, G. & Gretch, F.M. GC-MS identification and 1981 analytical behaviour of pirimiphos-methyl in imported foods. Bull. Environ. Contam. Toxicol., 27: 524-528. Beasley, C.J. & Lawrence, D.K.: J. Chromatog., 168: 461. 1979 Bengston, M., Cooper, L.M. & Grant-Taylor, F.J. A comparison of 1975 bioresmethrin, chloropyrifos-methyl and pirimiphos-methyl as grain protectants against malathion-resistant insects in wheat. J. Agric. Anim. Sci., 32: 51-78. Bengston, M., Cooper, L.M., Davies, R., Desmarchelier, J., Hart R. & 1978 Phillips, M. 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See Also: Toxicological Abbreviations Pirimiphos-methyl (WHO Pesticide Residues Series 4) Pirimiphos-methyl (Pesticide residues in food: 1976 evaluations) Pirimiphos-methyl (Pesticide residues in food: 1977 evaluations) Pirimiphos-methyl (Pesticide residues in food: 1979 evaluations) Pirimiphos-methyl (Pesticide residues in food: 1992 evaluations Part II Toxicology)