Thiram was originally evaluated in 1965 (toxicology) and 1967 (toxicology and residues) and is included in the dithiocarbamate group of compounds. It was evaluated at the present Meeting within the CCPR periodic review programme.
Thiram is a protective dithiocarbamate fungicide used as a foliar treatment on fruits, vegetables and ornamentals to control Botrytis species, rust, scab and storage diseases, and as a seed treatment to control seedling blights and a number of fungi that cause "damping off in seedlings. Thiram formulations are registered for use in many countries. The Meeting was provided with information on registered uses on fruits, vegetables and other crops.
The Meeting received extensive information on the metabolism of thiram in rats, farm animals, apples, grapes, soya beans, cotton, wheat and sugar beet, its environmental fate in soil and water/sediment systems, methods of residue analysis, the stability of residues in stored analytical samples, approved use patterns, supervised residue trials and the fate of residues during processing.
When rats were dosed orally with [thiocarbonyl-14C]thiram much (40-60%) of the 14C was eliminated as volatile compounds in exhaled air, 25-35% was excreted in the urine and 2-5% in the faeces. The volatiles were collected in traps suggesting the presence of CS2, CO2 and COS. Five polar metabolites and conjugates were identified in the urine: 2-thioxo-4-thiazolidinecarboxylic acid, dimethyldithiocarbamoyl glucuronide, dimethyldithiocarbamoylsulfenic acid, methyl dimethyldithiocarbamate, and dimethyldithiocarbamoylalanine.
A major part of the 14C was eliminated in respiration gases from lactating goats dosed with [thiocarbonyl-14C]thiram equivalent to 2.5, 3.3 and 23 ppm in the feed for 4 consecutive days. Most (90% or more) of the 14C in the expired air was present in CO2, with the remainder in CS2 and COS. The levels of C in the milk were quite low and reached their plateaux within 1.5 to 3 days of the first dose. The total 14C in the milk constituted 1.0-1.8% of the administered dose. The levels of 14C were much higher in the liver than in the other tissues.
The metabolism of thiram in goats was quite extensive and much of the 14C in the milk and tissues was present as very polar extractable material or as bound residues. It is likely that thiram is rapidly converted to dimethyldithiocarbamic acid and then to dimethylamine and CS2. CS2 is converted to COS and carbonate, [14C]carbonate then enters fat, protein and carbohydrates.
When laving hens were dosed with [thiocarbonyl-14C]thiram equivalent to 0.6 and 6.0 ppm in the feed for 4 consecutive days approximately 1 % of the dose was present in the liver, which had higher levels than the other tissues. Levels of 14C in egg white and egg yolk were quite low throughout the 4 days, with approximately 0.15% of the administered 14C appearing in the eggs.
About half of the 14C in the liver was incorporated into natural products such as acids, amino acids, peptides and proteins. Three metabolites constituting only a small percentage of the 14C were identified as dimethyldithiocarbamoylomithine, 2-thioxo-4-thiazolidinecarboxylic acid and dimethyldithiocarbamoyl glucuronide.
Apples and leaves on apple trees were treated with [thiocarbonyl-14C]thiram and examined for 14C periodically after treatment. Initially most of the 14C was on the fruit and leaf surfaces, but by day 14 only half of the remaining residue was on the surface. By day 101 only 2.7% of the residue was on the surface with 38% in the peel and 60% in the pulp. Thiram itself was not detected within the fruit except on day 0. The residue incorporated in the fruit contained only a small percentage of the dimethyldithiocarbamoyl moiety as demonstrated by the release of small amounts of CS2 by acid digestion, equivalent to 1-3.5% of the incorporated C.
When grapes and vine leaves were treated with [thiocarbonyl-14C]thiram the 14C residues on and within the fruit were quite persistent. The residues on the leaf surfaces disappeared more quickly. In grapes harvested 27 days after the final thiram application 35% of the remaining 14C was in surface washings, 44% in juice and 21% in press cake.
Most of the 14C in the surface washings from grapes after 27 days was in thiram itself, but HPLC showed the presence of two metabolites both more polar than thiram. Approximately 5% of the 14C residue within the grapes (harvested 0, 14 or 27 days after the final treatment) liberated CS2 on acid digestion, which demonstrated that very little of it contained the dimethyldithiocarbamoyl moiety. Most of the 14C residue in the juice was shown to have a molecular weight below 500, but could not be positively identified. Much of the 14C in the grapes was very polar or unextractable and had probably become incorporated into natural products.
When soya bean. cotton and wheat plants were grown from [thiocarbonyl-14C]thiram-treated seed, the 14C levels in the cotyledons and roots of the seedlings were higher than in the leaves and stems. In mature plants the highest 14C levels were in the roots and the lowest in the seeds.
The major metabolites in soya bean, cotton and wheat seedlings were identified as dimethyldithiocarbamoyl and dimethylthiocarbamoyl glycosides. When an aqueous wheat extract was treated with hot acid only 3.4% of the 14C was liberated as CS2, suggesting that if any remaining metabolites contained the dithiocarbamoyl moiety they were largely unextractable.
In further studies on soya beans and wheat produced from thiram-treated seed it was shown that much of the 14C had been incorporated into endogenous natural products such as sugars, fatty acids and citric acid, but some of the dimethyldithiocarbamoyl moiety had conjugated with amino acids and sugars. Thiram itself was not detected.
The main metabolite identified was 2-dimethylamino-4-thiazolinecarboxylic acid, apparently produced from dimethyldithiocarbamoylalanine. When homogenized soya bean tissue (forage, straw, pod and seed) was digested with acid 3-9% of the 14C in each tissue were released as CS2 and 3-24% as CO2. In straw, chaff and wheat grain the corresponding figures were 2-9% and 8-21%.
14C levels in sugar beet tops and roots were generally very low in plants produced from [thiocarbonyl-14C]thiram-treated seed. As in the soya bean and wheat studies 14C was detected in control plants growing nearby, suggesting that 14CO2 had been produced. Only 3.2% of the '14C in the roots was released as CS2 when the homogenized tissue was digested with acid, demonstrating that very little of the incorporated 14C contained the dithiocarbamoyl moiety.
Thiram, [thiocarbonyl-14C]-labelled, disappeared rapidly when incubated in a sandy loam soil under aerobic conditions at 20°C and 75% of field moisture capacity, with an initial half-life of about 2 days and 85% disappearance in 7 days. Mineralization was also rapid with 9% of the applied 14C evolved as 14CO2 in 2 days and 50% within 21 days. The major metabolite was identified as dimethylcarbamoperoxothioic acid, which reached its maximum concentration on day 4 of the incubation. There were a number of other minor metabolites, three of which were identified.
Labelled thiram disappeared with a half-life of 3.7 days when exposed on a thin layer of sandy loam soil to simulated sunlight. After 21 days the volatile 14C amounted to 57%, 37% as CO2 and 20% corresponding to CS2 in an HPLC system, but not fully identified.
The adsorption and desorption properties of thiram were measured on four soils, a sandy loam, a loamy sand, a silt loam and a loam. Thiram was judged to be slightly mobile to immobile in the soils tested.
When [thiocarbonyl-14C]thiram was incubated in aquatic systems of river or pond water and sediment in the dark at 20°C under aerobic conditions for 101 days the initial half-life of thiram was about 2 days with more than 90% disappearance within 7 days. CS2, CO2 and methyl dimethyldithiocarbamate were identified as metabolites. CS2 and the ester reached their peak concentrations in the water on day 4.
The analytical methods for dithiocarbamates which rely on CS2 evolution may be used for the determination of thiram residues. Such methods have been reviewed previously for mancozeb, maneb and propineb (1993 JMPR) and metiram (1995 JMPR). Methods where the generated CS2 is measured by colorimetry or by head-space GLC have been shown to be suitable for thiram, as for the other dithiocarbamates. Limits of determination in various commodities are usually 0.05-0.1 mg/kg (as CS2).
An HPLC method has been developed for thiram residues in crops that measures thiram as the intact molecule and distinguishes it from other dithiocarbamates. The residue is extracted with solvent from crop samples and, after clean-up on a C18 microcolumn, determined on a C18 reversed-phase HPLC column with UV detection at 280 nm. Quantitative recoveries were achieved from fruit and processed fruit fractions down to 0.1-0.2 mg/kg. Low recoveries occurred in wine fortified at the 0.1 mg/kg level because thiram was being degraded.
Data were available on the frozen storage stability of thiram residues on plums and in apple juice and apple pomace.
Thiram residues were shown to be stable on frozen whole plums stored at a freezer temperature below -20°C for 500 days. At three of the samplings the plums were analysed by a CS2 evolution method and by an HPLC method specific for thiram. The results were in good agreement. If plums were macerated in a blender thiram was decomposed by exposure to the macerate. It was recommended that fruit for residue analysis should be stored whole, subjected to a minimum of cutting (into halves and quarters) while still frozen, and analysed immediately.
Thiram residues in apple juice, wet pomace and dry pomace fortified at 1 mg/kg were stable at -20 ± 5°C for the intervals tested, 35 to 49 weeks.
The Meeting considered the definition of thiram residues in terms of the crop metabolism studies, the supervised trials where residues had been determined by both a CS2 evolution method and an HPLC method, and the needs of enforcement agencies.
The metabolism studies suggest that thiram is the major part of the CS2-evolving residue, particularly when the residue is reasonably fresh and at the higher levels. Analyses of samples in supervised trials by the HPLC and CS2 methods are usually in good agreement, which also suggests that thiram itself is the main residue.
The 1995 JMPR (Report, Section 2.8.1), in explaining the current basis for the definition of residues, stated "Preferably no compound, metabolite or analyte should appear in more than one residue definition."
The Meeting agreed that thiram should be included in the definition of dithiocarbamate residues:
The MRLs refer to total dithiocarbamates, determined as CS
2 evolved during acid digestion and expressed as mg CS2/kg.
For dietary intake estimations the supervised trials median residue (STMR) will be expressed as thiram because intakes need to be in terms of thiram itself for comparison with its ADI. For estimates of acute intake a residue such as an MRL, which is expressed in terms of CS2, must be multiplied by a factor of 1.58 for comparison with an acute reference dose expressed in terms of thiram.
The Meeting received data on thiram residues from supervised trials on apples (Belgium, France, Germany, Italy, The Netherlands, Poland), pears (Belgium, Germany, Italy, Spain), peaches (France, Italy, Spain), plums (France, Italy), cherries (Italy, Spain), grapes (France, Germany), strawberries (Belgium, France, Germany, Poland), dwarf French beans (Germany), French beans (France), Savoy cabbage (Germany), green peas (France), head lettuce (Germany, The Netherlands), spinach (Germany, The Netherlands), and tomatoes (France).
In the trials thiram was determined by CS2 evolution methods or by HPLC, and in some trials by both methods. Residues are expressed as thiram in the following discussion.
In some trials other dithiocarbamates had been used on the crop during the growing season. Samples from such trials were considered valid for estimating thiram residue levels only if they had been analysed specifically for thiram by an HPLC method.
Thiram is registered for application up to 12 times on pome fruit in Germany at a spray concentration of 0.1-0.16 kg ai/hl or 1.5-2.4 kg ai/ha, with a 10-day PHI. Decline curves from pome fruit trials suggest that thiram residues decrease with a typical half-life of about 10 days and that data from 7-15 days, equivalent to a concentration range of ±30%, are within acceptable range of a 10 days PHI. The decline curves also suggest that applications more than 30-40 days before the final application will contribute less than 10% to the final residues, so they should not be increased by more than 4 or 5 applications.
Apples. Thiram residues in two Belgian trials (spray concentration 0.16 kg ai/hl, PHI 9 days) were 2.7 and 4.6 mg/kg, and from a French trial (spray concentration 0.16 kg ai/hl, PHI 14 days) 2.4 mg/kg. In two German trials, one at 0.2 kg ai/hl, 15 days PHI (which exceeds 14 days but the spray concentration is slightly higher than 0.16 kg ai/hl), and the other at 0.16 kg ai/hl with a 14-day PHI, thiram residues were 0.49 and 1.9 mg/kg. In three Italian trials according to the registered use pattern of 0.15-0.20 kg ai/hl with a PHI of 10 days thiram residues were 2.5, 4.1 and 1.1 mg/kg.
The pome fruit registration in The Netherlands permits thiram application rates of 1.4-3.0 kg ai/ha or spray concentrations of 0.10-0.20 kg ai/hl with a 7-day PHI. The registration is for a WG formulation, but the trials with WP formulations are considered comparable. The highest thiram residues in apples at rates of 2.4-3.1 kg ai/ha at a 7-day PHI in three trials in The Netherlands were 0.46, 1.8 and 6.3 mg/kg. In four other trials thiram residues were generally not detected (<0.1 mg/kg) at a PHI of 7 days, but the data could not be used because sample storage conditions before analysis were not available.
Four Polish trials in which apples were treated at 2.4 kg ai/ha and harvested 14 days later were evaluated according to the German use pattern on pome fruit (1.5-2.4 kg ai/ha and 10 days PHI). Thiram residues were 3.8, 3.2, 0.87 and 1.7 mg/kg.
In summary thiram residues in apples from trials according to GAP were Belgium 2.7, 4.6 mg/kg, France 2.4 mg/kg, Germany 0.49, 1.9 mg/kg, Italy 1.1, 2.5, 4.1 mg/kg, The Netherlands 0.46, 1.8, 6.3 mg/kg and Poland 1.7, 3.2, 3.8 mg/kg. The 15 residues in rank order (median underlined) were 0.46, 0.49, 0.87, 1.1, 1.7, 1.8, 1.9, 2.4, 2.5, 2.7, 3.2, 3.8, 4.1, 4.6 and 6.3 mg/kg.
Pears. Two trials in Belgium (2.4 kg ai/ha, 13 and 14 days PHI) and two in Germany (2.4 kg ai/ha, 10 days PHI) were evaluated against German GAP for pome fruit (2.4 kg ai/ha and 10 days PHI). The thiram residues were 0.69, 1.6, 1.9 and 0.90 mg/kg. Residue levels resulting from 12 and 14 applications (0.69 and 1.6 mg/kg) were similar to those from 4 applications (0.90 and 1.9 mg/kg).
Three trials in Italy (0.15 kg ai/hl, 10 days PHI) were according to Italian GAP for pome fruit (0.15-0.20 kg ai/hl, 10 days PHI). The thiram residues were 0.54, 4.3 and 5.1 mg/kg. A trial in Spain (0.24 kg ai/hl 14 days PHI) was according to Spanish GAP for pome fruit (0.16-0.24 kg ai/hl, 15 days PHI). The thiram residue was 3.0 mg/kg.
The thiram residues in pears from each of the eight trials in rank order (median underlined) were 0.54, 0.69, 0.90, 1.6, 1.9, 3.0, 4.3 and 5.1 mg/kg.
The Meeting noted that the registered use patterns were for pome fruits, that the use patterns in the apple and pear trials were similar and that the residue levels in the two fruits overlapped. The Meeting therefore agreed to evaluate the combined apple and pear data as applying to pome fruits. The thiram residues in apples and pears taken together in rank order (median underlined) were 0.46, 0.49, 0.54, 0.69, 0.87, 0.90, 1.1, 1.6, 1.7, 1.8, 1.9, 1.9, 2.4, 2.5, 2.7, 3.0, 3.2, 3.8, 4.1, 4.3, 4.6, 5.1 and 6.3 mg/kg. The highest residue, 6.3 mg/kg as thiram, is equivalent to 4.0 mg/kg dithiocarbamates as CS2.
The Meeting estimated a maximum residue level of 5 mg/kg for dithiocarbamates in pome fruits arising from the use of thiram, and noted that this value was the same as the current recommendation for dithiocarbamates in pome fruits. The Meeting estimated an STMR of 1.9 mg/kg for thiram (as thiram) in pome fruit.
Stone fruits. The Italian registered use for thiram on stone fruit permits a spray concentration of 0.15 kg ai/hl and a PHI of 10 days. Thiram residues in peaches in 2 Italian trials matching these conditions were 2.7 and 3.6 mg/kg, and in two Spanish trials under conditions close to Spanish GAP for stone fruit (0.16-0.24 kg ai/hl, 15 days PHI, 14 days in the trials) they were 0.26 and 0.70 mg/kg.
In a French trial on plums which was also according to Spanish GAP the highest residue 14 days after the last of 3 applications was 1.0 mg/kg. In two Italian trials on plums according to Italian GAP for stone fruit the highest residues on day 10 were 0.83 and 0.62 mg/kg.
In two Italian trials on cherries according to Italian GAP the highest residues at the recommended PHI of 10 days were 0.37 and 0.41 mg/kg, but in the second trial a residue of 1.1 mg/kg occurred 15 days after the final treatment. Thiram residues of 0.1 mg/kg were found in cherries from a Spanish trial where the conditions were close to Spanish GAP.
The residue levels in peaches from 2 of the 4 relevant trials appeared to be outside the general population of the stone fruit residues, but 4 trials on peaches were in any case insufficient to support an MRL. The Meeting concluded that the residues in plums (0.62, 0.83 and 1.0 mg/kg) and cherries (0.1,0.37 and 1.1 mg/kg) from the valid trials could be evaluated together. The residues in rank order (medians underlined) were 0.1, 0.37, 0.62, 0.83. 1.0 and 1.1 mg/kg. The highest residue, 1.1 mg/kg as thiram, is equivalent to 0.69 mg/kg dithiocarbamates as CS2.
The Meeting estimated a maximum residue level of 1 mg/kg for dithiocarbamates (as CS2) in plums and cherries arising from the use of thiram, and an STMR of 0.72 mg/kg for thiram (as thiram) in plums and cherries.
Berries and other small fruits. Thiram trials on grapes in France and Germany could not be evaluated because corresponding GAP information was not available.
Thiram trials on strawberries in France could not be evaluated because corresponding information on GAP was not available. Full details of sample storage and handling were not available for the German trials.
The UK use pattern on strawberries allows thiram application of 1.6 kg ai/ha beginning at white bud burst, with repeats at 7-10 day intervals and a PHI of 7 days. In commercial practice there will be no more than 4 or 5 applications in a season. Seven strawberry trials with multiple applications in Belgium were evaluated against the UK use pattern. In four of the trials samples had been taken for analysis just before each application. Generally the number of applications did not seem to influence the level of the residues, although the highest residues were found in two of the trials after 13 and 14 applications. The highest thiram residues (median underlined) in each trial within the range of the UK use pattern resulting from up to 8 applications were 1.4, 1.4, 2.1, 2.1. 2.4, 2.8 and 3.1 mg/kg. The highest residue, 3.1 mg/kg as thiram, is equivalent to 2.0 mg/kg dithiocarbamates as CS2.
The Meeting estimated a maximum residue level of 5 mg/kg for dithiocarbamates arising from the use of thiram, and an STMR of 2.1 mg/kg for thiram (as thiram), in strawberries.
Residue data on beans, Savoy cabbage, green peas, head lettuce and spinach could not be evaluated because there was no matching GAP or because the number of trials was too small.
The UK registration for thiram on tomatoes allows a spray concentration of 0.32 kg ai/hl and a PHI of 7 days. Four tomato trials in France at 0.22 and 0.32 kg ai/hl and 8 and 10 days PHI produced thiram residues of <0.2, 0.2, 0.95 and 1.1 mg/kg. The Meeting agreed that four trials in one year were inadequate to estimate a maximum residue level for tomatoes.
Information on the fate of thiram during the processing of apples and grapes was made available to the Meeting.
The levels of thiram in apple juice, wet pomace, and dry pomace were 0.29, 1.02 and 3.65 times the level in the apples, suggesting that little of the thiram was lost during the drying process.
In four studies in France field-sprayed grapes were processed to juice, wine and raisins. Thiram residues were not detected (<0.1 mg/kg) in wine by an HPLC method, but were found at 0.12-0.98 mg/kg (as thiram) by a CS2 evolution method. The thiram residues measured in the grapes by the CS2 method were also somewhat higher than by the HPLC method. The results obtained by the HPLC method were considered more reliable and were used for estimating processing factors. Since thiram residues were not detected (<0.1 mg/kg) in wine, juice, wet pomace or dry pomace by the HPLC method, the processing factors for grapes to wine for the 4 trials (2 sampling intervals) by the HPLC method were <0.023, 0.033, 0.053, 0.062, <0.071 (median) and 0.083 (3).
In two of the trials thiram residue levels were determined in raisins. In one they were 3.6 and 1.1 times those in the grapes and in the other they were not detected (<0.1 mg/kg). Because of the inconsistency the Meeting could not draw any conclusions about likely residues in raisins.
Monitoring data for dithiocarbamate residues on commodities in trade were provided from The Netherlands, Belgium and Denmark. In most commodities dithiocarbamates were detected in fewer than 15-20% of the samples.
