RESIDUE AND ANALYTICAL ASPECTS
Phosmet has been evaluated at several Joint Meetings between 1976 and 1988. MRLs were recommended for a number of commodities of plant and animal origin. Updated information on GAP, and reports of supervised trials and studies of processing, metabolism and the stability of residues in stored analytical samples have been made available for evaluation within the CCPR periodic review programme.
Phosmet is a broad-spectrum organophosphorus insecticide used to control a variety of insect and mite pests which attack pome, stone and citrus fruit. It is also used on field, pasture and forage crops. Phosmet is non-systemic and acts by contact and ingestion as a cholinesterase inhibitor. It is registered in a number of countries, mainly for protecting fruits and vegetables. The direct use of phosmet on livestock for the control of warble fly, ticks and lice of cattle, resulting in residues in animal commodities, was not reported to the Meeting.
Carbonyl-labelled [14C] phosmet was used in studies of metabolism and environmental fate.
The absorption, distribution, metabolism and excretion of [14C] phosmet has been studied in rats, goats and hens. The chemical is rapidly absorbed, distributed and excreted, predominantly in the urine, in all three species. Biotransformation also appeared to be similar in the species studied. Hydrolysis of the phosphorus-containing moiety to yield N-mercaptomethylphthalimide is followed by methylation and oxidation at the sulfur atom to give sulfoxides and sulfones. These metabolites, together with N-mercaptomethylphthalimide, are hydrolysed to generate a series of phthalamic acids and finally phthalic acid.
The principal metabolites in tissues and milk reflect a single metabolic sequence: hydrolytic displacement of the phosphorus-containing moiety to yield N-mercaptomethylphthalimide, followed by methylation and oxidation of the thiol group. Hydrolytic degradation via N-hydroxymethylphthalimide also occurred. These reactions generated a series of phthalimide derivatives, which were hydrolysed to the analogous phthalamic acids. Treatment of extracted samples with hydrazine solubilized more than half of the bound residues. Solubilized products of hydrazinolysis consisted mostly of phthalohydrazide. The results indicate that bound residues in tissues and milk contain the N-substituted phthalimide moiety, with little or no chemical modification. Residues of phosmet do not accumulate significantly in edible tissues or eggs. Although the rat liver microsomal NADPH enzyme system readily converts phosmet to phosmet oxon, neither phosmet nor its oxygen analogue could be detected in the tissues of the goats or hens.
Lactating goats were dosed with [14C] phosmet at the equivalent of 8-8.8 ppm in the diet for four days. Most of each day's dose was recovered in the urine within the following 24 hours. In total, urinary excretion accounted for 60% of the cumulative dose. Less than 6% remained in the edible tissues at slaughter, 13-14 hours after the final dose. The total radioactivity ranged from 0.006 mg/kg phosmet equivalent in the fat to 0.24 mg/kg in the kidneys. Nine metabolites containing the phthalimide moiety were identified. Neither phosmet nor phosmet oxon was detected in the edible tissues (<0.002-0.003 mg/kg) or milk (<0.0004 mg/kg).
Laying hens dosed for seven days at a level equivalent to 10.5 ppm in the diet excreted 89.6% of the cumulative dose. Edible tissues collected at slaughter and eggs accounted for only 0.3% of the cumulative dose. In egg yolks the highest level of 14C (as phosmet equivalents) was 0.040 mg/kg on day 7, and in whites 0.007 mg/kg on day 4. At slaughter the levels of total radioactivity expressed as phosmet were 0.24 mg/kg in liver, 0.21 mg/kg in kidney, 0.021 mg/kg in breast muscle, 0.015 mg/kg in thigh muscle, 0.005 mg/kg in fat and 0.068 mg/kg in blood. Phosmet itself was not detected (<0.005 mg/kg) in any of the edible tissues, but 0.001 mg/kg was found in egg yolks. None of the metabolites exceeded 0.005 mg/kg in the edible tissues or eggs. The metabolites identified in the edible tissues and egg yolks were phthalimide and phthalic acid.
Plant metabolism studies on sour cherries, cotton, maize and potatoes were reported. Forty four per cent of the applied radioactivity was absorbed by sour cherries within 4 hours. The main surface residue was the parent compound, while 16 or 17 metabolites occurred in the fruit. Phthalic acid was the major metabolite and accounted for 17-21% of the total radioactivity. Several other metabolites accounting for a small fraction of the radioactivity were identified. These included phosmet oxon, phthalimide, and phthalamic acid derivatives. No benzoic acid or ring-hydroxylated products were detected. Related conjugates of N-glycosylphthalimide accounted for 27-32% of the total radioactivity, but phthalic acid accounted for 85-90% of the extractable radioactivity after acid hydrolysis.
In maize the major part of the total residue was present in the maize fodder (267 mg/kg expressed as phosmet equivalent) and forage (31 mg/kg). Cobs (5 mg/kg) and grain (3 mg/kg) contained much lower residues. The metabolism of phosmet in maize involves various hydroxylation (oxidation), hydrolysis and conjugation reactions, giving products that are distinctly more polar than phosmet. The pattern of metabolites was similar in all parts of the plant, but their ratios varied. The parent phosmet amounted to 53% of the total residue in fodder, with the oxon (1.2%) and derivatives of phthalimide and phthalic acid present in small amounts, whereas in the grain phthalic acid was the single identified residue (61%) and the parent compound was not detectable. Most of the radiocarbon in the unidentified metabolites (32.7%) was accounted for as phthalic acid after acid hydrolysis.
In potatoes the foliage contained most of the residue (14-109 mg/kg), and translocation to tubers (1.4-2.1 mg/kg) was limited. Phthalic acid and phthalamic acid were the major metabolites. Phosmet, its oxygen analogue and hydroxylated phthalic acids were not observed in any of the extracts.
The environmental fate of phosmet was studied in soil and water. Degradation in soil was studied under aerobic followed by anaerobic conditions. Under anaerobic conditions the degradation continued, but at a slower rate. The main components of the residue, expressed as phosmet equivalent, found in aerobic soil were phosmet (36.6%), phosmet oxon (0.5%), N-methoxymethylphthalimide (5.68%), N-methylsulfinylmethylphthalimide (2.59%), N-hydroxymethylphthalimic acid (2.44%) and phthalimide (1.53%). In addition, 7 identified metabolites containing the phthalimide moiety (each <1%) and some unidentified intermediate products were also detected. Hydrolysis was shown to be an important factor in limiting the persistence of phosmet in soils, and the initial degradation products were metabolized by soil micro-organisms. After hydrolysis the aryl moiety, with or without a mercapto group depending on the point of cleavage, was further degraded through a variety of reactions including oxidation of the mercapto group to sulfonic acid, its methylation followed by oxidation to the sulfoxide, and imide bond cleavage. Ultimately, mineralization to carbon dioxide occurred. The products under aerobic and anaerobic conditions were largely the same.
Phosmet did not undergo significant photodegradation when exposed on thin layer plates of soil to natural sunlight for a period of 30 days.
Phosmet undergoes fairly rapid hydrolysis at ambient temperatures, with half-lives in water at 25°C of 7.5-9.7 days at pH 5, 9.4 hours at pH 7 and 5.5 minutes at pH 9. Degradation is enhanced by light.
The major hydrolysis products formed at pH 5 in the dark were O,O-dimethyl O-hydrogen phosphorodithioate (79.4 mol %), O-methyl O,O-dihydrogen phosphorodithioate, phthalamic acid, phthalimide, and phthalic acid. Following irradiation with a xenon lamp at pH 5, dimethyl hydrogen phosphate, (72.3 mol %), phosphoric acid, methyl dihydrogen phosphate, phthalimide, phthalamic acid and phthalic acid were detected. Other minor products were also detected but not identified.
Residues in rotational crops were studied in radishes, lettuce and wheat which were planted in the soil 30, 120 and 365 days after treatment with [carbonyl-14C] phosmet at a rate equivalent to 5.6 kg ai/ha. The total radioactive residue taken up by the plants varied from about 2% to 64% depending on the plant and the time between soil treatment and harvest. Neither phosmet nor its oxygen analogue were detected in the plant extracts. The radioactive residue consisted of a number of polar metabolites, most of which were characterized by chemical and enzymatic hydrolysis as esters or conjugates of phthalic acid.
The current analytical methods for residues are based on extraction with acetone or ethyl acetate, clean-up on charcoal, silica gel or SX-3 gel columns, and gas-chromatographic determination. Phosmet and its oxon are determined simultaneously. Recoveries are above 70%. The typical limits of determination in plant materials, milk and animal tissues are 0.01-0.05 mg/kg. In most of the supervised trials the LOD reported was 0.05 mg/kg.
Storage stability studies showed that phosmet is stable at -20 ± 10°C in almonds, apples, soya beans, and wheat grain and straw for a minimum of 2 1/2 years and in alfalfa, maize, oranges, peppers and potatoes for a minimum of 2 years.
Definition of the residue
Phosmet is the major residue component; the oxon is either not detected or is less than 10% of the parent compound in most cases. In addition, the other metabolites are water-soluble compounds without the phosphorodithioate group and are less toxic than the parent compound. The significant residue for both regulatory control and dietary intake purposes is therefore the parent compound.
The Meeting noted that phosmet was previously classified as fat-soluble. On the basis of its octanol/water partition coefficient and the distribution of residues between fat and meat, the Meeting concluded that the compound is not fat-soluble.
Definition of the residue for compliance with MRLs and for the estimation of dietary intake: phosmet
Supervised trials
Supervised trials were conducted on oranges in Argentina and Brazil. In the Argentine trials residues were determined in whole fruit, peel and pulp, but in Brazil only the pulp was analysed and the results cannot be used to estimate maximum residue levels. The application rate in the three Argentine orange trials corresponded with the current use pattern and resulted in residues in the whole fruits of 0.07, 0.13 and 0.32 mg/kg. The pulp did not contain detectable residues (<0.05 mg/kg) in any of the trials.
The data were too limited to estimate a maximum residue level for oranges, and since no residue data were provided for other citrus commodities, the Meeting recommended the withdrawal of the existing CXL for citrus fruits (5 mg/kg).
A number of trials were carried out on apples and pears in Brazil, Canada, Germany, The Netherlands, the UK and the USA. No GAP was reported for Germany, The Netherlands or the UK. Trials were according to current GAP in Canada (1.9 kg ai/ha) and the USA (1.7-4.1 kg ai/ha for apples; 1.7-5.6 kg ai/ha for pears) or at somewhat higher rates. The residues in the fruit were generally below 5 mg/kg at 7 days PHI. The residues in pears (1.7, 1.3 and 0.85 mg/kg) were lower than in apples. The Brazilian trials resulted in residues below 0.05 mg/kg in apples 14 days after application at single or double GAP rates. The residues in apples from the Canadian and US trials at approximately maximum GAP rates in rank order were 1.8, 1.8, 2.8, 3.3, 3.4, 3.4, 3.7, 4.2, 4.3 and 7.3 mg/kg.
The Meeting estimated a maximum residue level of 10 mg/kg, and an STMR level of 3.4 mg/kg for apples. Owing to the lack of sufficient data, the Meeting concluded that no maximum residue level could be estimated for pears and recommended the withdrawal of the existing CXL (10 mg/kg).
Field trials on apricots, nectarines and peaches treated at rates up to 1.3 times the US GAP rate resulted in residues up to 6.8 mg/kg at 14 days PHI. The residues in apricots (*) and peaches treated at 0.7-1.3 times the maximum rates according to Canadian and US GAP in rank order were 0.87, 1.2, 1.5, 1.6, 2.9, 4.2, 4.7*, 6.4 and 6.8 mg/kg. The residues in nectarines were lower, 0.45 and 0.55 mg/kg, and could not be combined with those of apricots and peaches.
The Meeting estimated maximum residue levels of 10 mg/kg and STMR levels of 2.9 mg/kg for apricots and peaches, and recommended the withdrawal of the existing CXL for nectarines (5 mg/kg).
Following treatments at about 1-1.3 times current GAP rates, residues in plums of 0.41, 0.55 and 0.48 mg/kg, and in fresh and dried prunes of 2.3 and 2.2 mg/kg were reported. The information was insufficient to estimate a maximum residue level for plums (including prunes).
Grapes were treated at rates of 1.4-2.2 kg ai/ha which accord with GAP for the eastern states of the USA (1.5-2.5 kg ai/ha). Residues up to about 10.2 mg/kg were found 7 days after the last application and up to 9.2 mg/kg after 14 days. The residues from treatments according to GAP in rank order were 0.17, 0.24, 0.61, 2.8, 3.3, 4.0, 4.2 and 9.2 mg/kg.
The Meeting estimated a maximum residue level of 10 mg/kg and an STMR of 3.1 mg/kg for grapes.
In supervised trials on olives in France, Italy and Spain the residues declined to 0.02-0.34 mg/kg after PHIs of 28-30 days. The trials in France were evaluated against Spanish and
Italian GAP. The residues from GAP applications in rank order were 0.02, 0.09, 0.12. 0.16. 0.24 and 0.34 mg/kg.
The available information indicates that a maximum residue level of 0.5 mg/kg and an STMR of 0.14 mg/kg for olives would be appropriate, but because there was no suitable supporting processing study the Meeting could not make any recommendation.
Of the supervised residue trials on kiwifruit carried out in New Zealand during 1974-76 only one complied with current GAP. The Meeting recommended the withdrawal of the CXL for kiwifruit (15 mg/kg).
In two supervised trials on peas carried out in two states of the USA, phosmet residues were below the limit of determination (<0.05 mg/kg) in succulent peas, <0.05-0.08 mg/kg in dried peas, 0.15-0.51 mg/kg in succulent pods, 2.7-5.6 mg/kg in succulent pea forage and 2.5-17 mg/kg in dry pea hay. Phosmet oxon residues were <0.05 mg/kg in peas and green forage, and 0.06-0.28 mg/kg in hay. The oxon residue was less than 10% of that of the parent compound.
The Meeting concluded that the data were not sufficient to estimate maximum residue levels, and recommended the withdrawal of the existing CXLs for peas (pods and immature seeds), peas (dry), pea hay or fodder (dry) and pea vines (green).
Numerous trials on potatoes in Canada, The Netherlands and the USA indicated that the translocation of the compound to the tuber was limited, and residues in the tubers following applications at recommended and double rates were <0.05 mg/kg. Residues up to 0.11 mg/kg were detected in trials at fivefold rates however, which indicates that this is not a nil residue situation.
The Meeting estimated a maximum residue level of 0.05* mg/kg and an STMR of 0.05 mg/kg for potatoes. This is the level of the current CXL.
Residues from six supervised trials on cotton in Brazil at 1.5-4.5 times the GAP rate were all below the limit of determination (0.05 mg/kg).
The Meeting concluded that no detectable residue is likely to occur in cotton seed if GAP is followed, and estimated a maximum residue level of 0.05 mg/kg and an STMR level of 0 mg/kg.
Supervised trials were reported on alfalfa, Bermuda grass, lupins, maize forage, peas, rape and soya bean plants used for animal feed. Most of the trials were on alfalfa.
The residue data on forage and fodder crops showed that residues were generally high (commonly 40-80 mg/kg) immediately after application to alfalfa, but declined fairly rapidly. After 14 days they were mainly in the range 0.2-2 mg/kg. The residues of phosmet on lupins, maize, peas and rape were lower and generally below 2.0 mg/kg 7 days after the last application. The residues in fresh alfalfa from applications according to GAP in rank order were 0.13, 0.21, 0.24, 0.26, 0.3, 0.4, 0.77, 0.84, 0.84, 1.2, 1.6, 2.1, 2.1, 2.24 and 3.5 mg/kg. The Meeting did not estimate any maximum residue levels for animal feed items (see "Animal products" below).
The data, if any, were insufficient to estimate maximum residue levels in blueberries, feijoa, maize, maize fodder and forage, pea hay or fodder, sweet corn, sweet potatoes and tree nuts. The Meeting therefore recommended the withdrawal of the existing CXLs for these commodities.
Animal products. Although no detectable residues of phosmet or its oxon occurred in edible animal products in metabolism studies, the Meeting was not able to estimate any maximum residue levels for animal feeds or animal products because of the high residues in animal feed items and the lack of animal transfer studies. Consequently, the Meeting recommended the withdrawal of the existing CXLs for alfalfa fodder and forage, cattle meat and milks.
Processing
Studies have been carried out to determine the effect of processing on residues of phosmet in apples, grapes, peaches, olives, potatoes and prunes.
Field-treated apples containing 12-14 mg/kg phosmet residues were processed to unclarified and clarified juice and wet and dry pomace, which contained 5.3, 1.4, 29 and 89 mg/kg respectively. The oxon residue was less than 1% of the phosmet residue in all samples. Most of the phosmet residue is evidently in or on the peel, since processing decreased residues about 2.5-10 times in the products which were separated from the peel. Fractions which are normally processed with the peel, such as wet and dry pomace and the combined peels and cores, showed about a 2-6-fold concentration of the residues. The Meeting therefore concluded that maximum residues up to 60 mg/kg might occur in dry apple pomace.
Field-treated grapes were processed to raisins and raisin waste by sun-drying, and into wet and dry pomace. There was no concentration of the residue in the raisins but concentration occurred by factors of 12 in raisin waste, 3 in wet pomace, and about 6 in dry pomace.
Potatoes, treated with excessive amounts of phosmet to obtain detectable residues (0.1 mg/kg), were processed to yield potato chips, potato granules, wet peel and dry peel. There was no detectable residue in potato chips or granules (LOD £ 0.05 mg/kg). Residues in the wet peel were at the same level as in the washed potatoes, but were concentrated about threefold in the dry peel. This was accounted for by an 85% loss of moisture partly offset by the loss of some phosmet during drying (the theoretical residue would be 0.72 mg/kg).
Olives were processed to crude oil and neutralized oil. The residue in the crude oil was about four times that in the original olives, and purification ("neutralization") of the crude oil reduced the residues about threefold. The process used for neutralization was not reported, so the residues in the oil could not be used to estimate those likely to result from industrial processing. The Meeting concluded that the database was not sufficient to estimate maximum residue levels in crude or refined olive oil.
Fresh prunes were processed into dried prunes and both commodities were analysed for phosmet and phosmet oxon. The average phosmet residue in fresh prunes was 2.63 mg/kg, and in dried prunes 0.82 mg/kg. Phosmet oxon was not detectable in any of the samples. The decrease in dried prunes was attributed to the loss of residues during the drying process at 54-60°C, which more than offset the loss of moisture. Since it had not been possible to estimate a maximum residue level for fresh prunes the Meeting could not estimate one for dried prunes.
