Methidathion (ISO)

Pesticides

Pesticide -> EPA IRIS

Acaricides, Insecticides

Active substance -> EU Pesticides database: Not approved

Pesticides -> Organophosphate Insecticides

Acaricides, Insecticides

log Kow = 2.20

2.2

2.22

1.495 at 68 °F (EPA, 1998) - Denser than water; will sink

1.51 g/cu cm (20 °C)

1.5 g/cm³

1.495

1.495 @ 20°C

Colorless crystals

Crystals from methanol

Colorless-to-white crystalline solid

Solubility (all in g/L at 20 °C): ethanol (150); acetone (670); toluene (720); hexane (11); n-octanol (14)

Soluble in benzene, methanol and xylene at >60 g/100 mL at 25 °C. Moderately soluble in chloroform and dichloromethane.

In water, 187 mg/L at 20 °C

Solubility in water, g/100ml at 20 °C: 0.0187

102 to 104 °F (EPA, 1998)

39 °C

39 °C

102-104 °F

Danger

H300: Fatal if swallowed [Danger Acute toxicity, oral]

P264, P270, P273, P280, P301+P316, P302+P352, P317, P321, P330, P362+P364, P391, P405, and P501 (click each P-code to see the statement)

Danger

H300+H330 (88.2%): Fatal if swallowed or if inhaled [Danger Acute toxicity, oral; acute toxicity, inhalation]

P260, P261, P262, P264, P264+P265, P270, P271, P272, P273, P280, P284, P301+P316, P302+P352, P304+P340, P305+P354+P338, P316, P317, P320, P321, P330, P333+P317, P361+P364, P362+P364, P391, P403+P233, P405, and P501 (click each P-code to see the statement)

Hazard Traits - Developmental Toxicity; Neurotoxicity

Regulation (EC) No 1107/2009 (amended)

Methidathion is listed in the dataset of EPA Human Health Benchmarks for Pesticides under the Safe Drinking Water Act (40 eCFR 141)

2004/129/EC

The New Jersey Worker and Community Right to Know Act requires public and private employers to provide information about hazardous substances at their workplaces. (N.J.S.A. 34:5A-1 et. seq.)

(Non-Specific -- Organophosphorus Pesticide, n.o.s.) Container may explode in heat of fire. Fire and runoff from fire control water may produce irritating or poisonous gases. Stable in neutral or weak acid solution. Hydrolyzed by alkali. (EPA, 1998)

Combustible. Liquid formulations containing organic solvents may be flammable. Gives off irritating or toxic fumes (or gases) in a fire.

Allergic contact dermatitis reported in workers handling this pesticide; [Kanerva, p. 1812] A cholinesterase inhibitor that may cause convulsions and respiratory failure; May have cumulative effects; Can be absorbed through skin; [ICSC] Severely irritating to eyes; No reports of delayed neuropathy following ingestion poisonings; [HSDB] Human ingestion of 93 mg/kg causes miosis, lachrymation, and coma; [RTECS] A chronic dietary study of dogs produced inhibition of cholinesterase activity, increased hepatic enzymes, gross hepatic lesions, histopathology, and chronic hepatitis; [Reference #1] An irritant that may cause serious eye injury; Highly toxic by ingestion, inhalation, and skin absorption; [Aldrich MSDS] The average of two baseline respective cholinesterase activity determinations three days apart, with no exposures to enzyme inhibiting pesticides for at least 30 days, is recommended for each worker prior to exposure to cholinesterase inhibitors because of large inter-individual differences in published baseline values. To be established at least once a year. Removal from workplace exposures is recommended until the cholinesterase activity returns to within 20% of baseline. [TLVs and BEIs]

Toxicology Review: Residue Reviews 46: 1 (1973)

Toxicology Review: Residue Reviews 63: 1 (1976)

USEPA/Office of Pesticide Programs; Interim Reregistration Eligibility Decision (IRED) for Methidathion. EPA issues an IRED for a pesticide that is undergoing reregistration, requires a reregistration eligibility decision, and also needs a cumulative assessment under FQPA. The IRED, issued after EPA completes the individual pesticide's aggregate risk assessment, may include taking risk reduction measures -- for example, reducing risks to workers or eliminating uses that the registrant no longer wishes to maintain -- to gain the benefits of these changes before the final RED can be issued following the Agency's consideration of cumulative risks.[Available from, as of July 24, 2003: http://www.epa.gov/pesticides/reregistration/status.htm]

Tolerances are established for residues of the insecticide methidathion, including its metabolites and degradates, in or on the commodities in the table in this paragraph. Compliance with the tolerance levels specified in this paragraph is to be determined by measuring only methidathion, S -[(5-methoxy-2-oxo-1,3,4-thiadiazol-3(2 H )-yl)methyl] O,O-dimethyl phosphorodithioate, in or on the commodity.[Table#3285]

Tolerances with regional registration, as defined in part 180.1(l), are established for residues of the insecticide methidathion, including its metabolites and degradates, in or on the commodities in the table in this paragraph. Compliance with the tolerance levels specified in this paragraph is to be determined by measuring only methidathion, S -[(5-methoxy-2-oxo-1,3,4-thiadiazol-3(2H)-yl)methyl] O,O -dimethyl phosphorodithioate, in or on the commodity.[Table#3286]

Classified for restricted use and limited to use by or under the direct supervision of a certified applicator. Formulation: All formulations; Use Pattern: All uses except nursery stock, safflower, and sun flower; Classification: Restricted; Criteria Influencing Restriction: Residue effects on avian species.

Classified for restricted use and limited to use by or under the direct supervision of a certified applicator. Formulation: All formulations; Use Pattern: Nursery stock, safflower, and sunflower; Classification: Unclassified.

For more FIFRA Requirements (Complete) data for METHIDATHION (7 total), please visit the HSDB record page.

Organophosphates, such as METHIDATHION, are susceptible to formation of highly toxic and flammable phosphine gas in the presence of strong reducing agents such as hydrides. Partial oxidation by oxidizing agents may result in the release of toxic phosphorus oxides.

(Non-Specific -- Organophosphorus Pesticide, n.o.s.) Stay upwind; keep out of low areas. Move container from fire area if you can do so without risk. Fight fire from maximum distance. Dike fire control water for later disposal; do not scatter the material. Wear positive pressure breathing apparatus and special protective clothing.

Use water spray. Use dry powder. Use carbon dioxide.

Fresh air, rest. Artificial respiration may be needed. Refer immediately for medical attention.

Remove contaminated clothes. Rinse and then wash skin with water and soap. Refer immediately for medical attention. Wear protective gloves when administering first aid.

First rinse with plenty of water for several minutes (remove contact lenses if easily possible), then refer for medical attention.

Rinse mouth. Give a slurry of activated charcoal in water to drink. Refer immediately for medical attention.

Biological Exposure Indices (BEI) [ACGIH] - Acetylcholinesterase activity in red blood cells = 70% of individual's baseline; Butylcholinesterase activity in serum or plasma = 60% of individual's baseline; Sample at end of shift; [TLVs and BEIs]

Self contained breathing apparatus with a full facepiece operated in pressure demand, or other positive pressure mode /should be used in firefighting/. /Parathion/

If material on fire or involved in fire: Do not extinguish fire unless flow can be stopped. Use water in flooding quantities as fog. Solid streams of water may be ineffective. Cool all affected containers with flooding quantities of water. Apply water from as far a distance as possible. Use "alcohol" foam, dry chemical or carbon dioxide. /Organophosphorus pesticides, liquid, flammable, toxic; Organophosphorus pesticides, liquid, toxic/

If material on fire or involved in fire: Use water in flooding quantities as fog. Extinguish fire using agent suitable for type of surrounding fire. (Material itself does not burn or burns with difficulty.) Use foam, dry chemical, or carbon dioxide. /Organophosphorus pesticides, solid, toxic/

VENTILATE AREA OF SPILL OR LEAK. COLLECT FOR RECLAMATION, OR ABSORB IN VERMICULITE, DRY SAND, EARTH, OR A SIMILAR MATERIAL. /PARATHION/

Spills of parathion on floors ... absorbed with an absorbing clay. Sweeping compound ... facilitate the removal of all visible traces of parathion contaminated clay. /Parathion/

Environmental considerations: Air spill: Apply water spray or mist to knock down vapors. /Organophosphorus pesticides, liquid, flammable, toxic; Organophosphorus pesticides, liquid, toxic; Organophosphorus pesticides, solid, toxic/

Environmental considerations: Water spill: Use natural barriers or oil spill control booms to limit spill travel. Remove trapped material with suction hoses. /Organophosphorus pesticides, liquid, flammable, toxic; Organophosphorus pesticides, liquid, toxic; Organophosphorus pesticides, solid, toxic/

Environmental considerations: Land spill: Dig a pit, pond, lagoon, holding area to contain liquid or solid material. /SRP: If time permits, pits, ponds, lagoons, soak holes, or holding areas should be sealed with an impermeable flexible membrane liner./ Dike surface flow using soil, sand bags, foamed polyurethane, or foamed concrete. Absorb bulk liquid with fly ash, cement powder, or commercial sorbents. /Organophosphorus pesticides, liquid, flammable, toxic; Organophosphorus pesticides, liquid, toxic; Organophosphorus pesticides, solid, toxic/

SRP: The most favorable course of action is to use an alternative chemical product with less inherent propensity for occupational harm/injury/toxicity or environmental contamination. Recycle any unused portion of the material for its approved use or return it to the manufacturer or supplier. Ultimate disposal of the chemical must consider: the material's impact on air quality; potential migration in soil or water; effects on animal and plant life; and conformance with environmental and public health regulations.

Potential candidate for rotary kiln incineration, with a temperature range of 820 to 1,600 °C and a residence time of seconds. Also, a potential candidate for fluidized bed incineration, with a temperature range of 450 to 980 °C, and a residence time of seconds. Also, a potential candidate for liquid injection incineration with a temperature range of 650 to 1,600 °C, and a residence time of 0.1 to 2 seconds. /Parathion/

Personal protection: complete protective clothing including self-contained breathing apparatus. Do NOT let this chemical enter the environment. Sweep spilled substance into covered sealable containers. Carefully collect remainder. Then store and dispose of according to local regulations.

SRP: The scientific literature for the use of contact lenses by industrial workers is inconsistent. The benefits or detrimental effects of wearing contact lenses depend not only upon the substance, but also on factors including the form of the substance, characteristics and duration of the exposure, the uses of other eye protection equipment, and the hygiene of the lenses. However, there may be individual substances whose irritating or corrosive properties are such that the wearing of contact lenses would be harmful to the eye. In those specific cases, contact lenses should not be worn. In any event, the usual eye protection equipment should be worn even when contact lenses are in place.

The protective clothing should be kept in separate places where it cannot be contaminated with toxic chemicals. It should be forbidden to keep this clothing in living quarters. Protective clothing must be washed at least once a week and each time it is contaminated with pesticides. Before washing the clothing should be soaked for several hours in a calcium carbonate solution. /Pesticides/

Smoking, eating, and drinking before washing should be absolutely prohibited when any pesticide ... is being handled or used. /Pesticides/

SRP: Contaminated protective clothing should be segregated in such a manner so that there is no direct personal contact by personnel who handle, dispose, or clean the clothing. Quality assurance to ascertain the completeness of the cleaning procedures should be implemented before the decontaminated protective clothing is returned for reuse by the workers. Contaminated clothing should not be taken home at end of shift, but should remain at employee's place of work for cleaning.

For more Preventive Measures (Complete) data for METHIDATHION (10 total), please visit the HSDB record page.

Based on the analysis of 6,846-6,936 urine samples collected from the general population via the Health and Nutrition Examination Survey II, National Center of Health Statistics, methidathion was identified in 5.3% of the samples with a maximum concentration of 1,810 ppb(1).

Dimethyl phosphorothionate (DMTPK) and phosphorodithioate (DMDTPK) as the potassium salts, metabolites of methidathion, were identified in urine samples as an indication of worker exposure to methidathion(1).

If the compound has been ingested, rapid gastric lavage should be performed using 5% sodium bicarbonate. For skin contact, the skin should be washed with soap and water. If the compound has entered the eyes, they should be washed with large quantities of isotonic saline or water. In serious cases, atropine and/or pralidoxime should be administered. Anti-cholinergic drugs work to counteract the effects of excess acetylcholine and reactivate AChE. Atropine can be used as an antidote in conjunction with pralidoxime or other pyridinium oximes (such as trimedoxime or obidoxime), though the use of '-oximes' has been found to be of no benefit, or possibly harmful, in at least two meta-analyses. Atropine is a muscarinic antagonist, and thus blocks the action of acetylcholine peripherally.

The potentiating effects of methidathion (purity not specified) with profenofos and methacrifos were investigated in male and female Tif:RAIf rats by comparing the theoretical LD50 values, based on an assumption of strictly additive toxicity, with that of the experimentally derived LD50 values for the equitoxic mixtures. There was no potentiating effect of methidathion with profenofos, whereas a slight enhancement of acute toxicity was observed with an equitoxic mixture of methidathion and methacrifos.

/The authors/ investigated /in rats/ the effect of subchronic administration of methidathion (MD) on ovary evaluated ameliorating effects of vitamins E and C against MD toxicity. Experimental groups were as follows: control group; a group treated with 5 mg/kg body weight MD (MD group); and a group treated with 5 mg/kg body weight MD plus vitamin E and vitamin C (MD + Vit group). MD and MD + Vit groups were given MD by gavage five days a week for four weeks at a dose level of 5 mg/kg/day by using corn oil as the vehicle. Serum malondialdehyde (MDA: an indicator of lipid peroxidation) concentration, serum activity of cholinesterase (ChE), and ovary histopathology were studied. The level of MDA increased significantly in the MD group compared with the control (P < 0.005). Serum MDA decreased significantly in the MD + Vit group compared with the MD group (P < 0.05). The activities of ChE decreased significantly both in the MD and MD + Vit groups compared with the controls ( P < 0.05). However, the decrease in the MD + Vit groups was less than in the MD group; the ChE activity in the MD + Vit group was significantly higher compared with MD group (P < 0.05). Number of ovarian follicles were significantly lower in the MD group compared to the controls (P < 0.05). Number of atretic follicles were significantly higher in the MD group than in the controls (P < 0.05). Follicle counts in MD + Vit group showed that all types of ovarian follicles were significantly higher, and a significant decrease in the number of atretic follicles compared with the MD group (P < 0.05). In conclusion, subchronic MD administration caused an ovarian damage, in addition, LPO may be one of the molecular mechanisms involved in MD-induced toxicity. Treatment with vitamins E and C after the administration of MD reduced LPO and ovarian damage.

Because different classes of enzymes may be inhibited, the effects of organophosphorus pesticide poisoning may be complex and potentially at least could involve interactions with drugs as well as with other pesticides or chemicals. Potentiation may also involve solvents or other components of formulated pesticides. Certain drugs such a phenothiazines, antihistamines, CNS depressants, barbiturates, xanthines (theophylline), aminoglycosides and parasympathomimetic agents are to be avoided because of increased toxicity. /Organophosphorus pesticides/

...The LD50 value of... methidathion... /was/...10.5... mg/kg of female mice, orally... . Diphenhydramine (20 mg/kg) injected sc 5 min after dosing mice ...markedly increased the LD50 value by...114%... .

For more Interactions (Complete) data for METHIDATHION (10 total), please visit the HSDB record page.

Acute exposure to cholinesterase inhibitors can cause a cholinergic crisis characterized by severe nausea/vomiting, salivation, sweating, bradycardia, hypotension, collapse, and convulsions. Increasing muscle weakness is a possibility and may result in death if respiratory muscles are involved. Accumulation of ACh at motor nerves causes overstimulation of nicotinic expression at the neuromuscular junction. When this occurs symptoms such as muscle weakness, fatigue, muscle cramps, fasciculation, and paralysis can be seen. When there is an accumulation of ACh at autonomic ganglia this causes overstimulation of nicotinic expression in the sympathetic system. Symptoms associated with this are hypertension, and hypoglycemia. Overstimulation of nicotinic acetylcholine receptors in the central nervous system, due to accumulation of ACh, results in anxiety, headache, convulsions, ataxia, depression of respiration and circulation, tremor, general weakness, and potentially coma. When there is expression of muscarinic overstimulation due to excess acetylcholine at muscarinic acetylcholine receptors symptoms of visual disturbances, tightness in chest, wheezing due to bronchoconstriction, increased bronchial secretions, increased salivation, lacrimation, sweating, peristalsis, and urination can occur. Certain reproductive effects in fertility, growth, and development for males and females have been linked specifically to organophosphate pesticide exposure. Most of the research on reproductive effects has been conducted on farmers working with pesticides and insecticdes in rural areas. In females menstrual cycle disturbances, longer pregnancies, spontaneous abortions, stillbirths, and some developmental effects in offspring have been linked to organophosphate pesticide exposure. Prenatal exposure has been linked to impaired fetal growth and development. Neurotoxic effects have also been linked to poisoning with OP pesticides causing four neurotoxic effects in humans: cholinergic syndrome, intermediate syndrome, organophosphate-induced delayed polyneuropathy (OPIDP), and chronic organophosphate-induced neuropsychiatric disorder (COPIND). These syndromes result after acute and chronic exposure to OP pesticides.

LD50; Species: Branta canadensis (Canada goose) adult males and females; oral 8.41 mg/kg (95% confidence limit 4.20-16.8 mg/kg)

LC50; Species: Anas platyrhynchos (Mallard Duck) juvenile; diet 543 ppm for 8 days (95% confidence interval: 410-720 ppm)

LC50; Species: Anas platyrhynchos (Mallard Duck) juvenile; diet 543 ppm for 8 days (95% confidence interval: 410-720 ppm)

LD50; Species: Anas platyrhynchos (Mallard Duck) 3-4 month old female; oral 23.6 mg/kg (95% confidence limit 16.5-33.8 mg/kg)

For more Ecotoxicity Values (Complete) data for METHIDATHION (41 total), please visit the HSDB record page.

TERRESTRIAL FATE: Based on a classification scheme(1), Koc values of 34(2) and 761(3), indicate that methidathion is expected to have very high to low mobility in soil(SRC). Volatilization of methidathion from moist soil surfaces is not expected to be an important fate process(SRC) given an estimated Henry's Law constant of 7.2X10-9 atm-cu m/mole(SRC), based upon its vapor pressure, 3.37X10-6 mm Hg(4), and water solubility, 187 mg/L(5). Methidathion is not expected to volatilize from dry soil surfaces(SRC) based upon its vapor pressure(4). Biodegradation in soil may be an important environmental fate process based on half-lives of 11 and 10 days in aerobic and anaerobic soil systems, respectively(6).

AQUATIC FATE: Koc values in pond sediment of 7 and 56(1) and high water solubility of 187 mg/L(2), indicate that methidathion is not expected to adsorb to suspended solids and sediment(SRC). Volatilization from water surfaces is not expected(3) based upon an estimated Henry's Law constant of 7.2X10-9 atm-cu m/mole(SRC), derived from its vapor pressure, 3.37X10-6 mm Hg(4), and water solubility(2). According to a classification scheme(5), BCFs of 1.5(6) to 6.4(7), suggest bioconcentration in aquatic organisms is low(SRC). Biodegradation in aquatic systems may be important based on an avg half-life of 12.42 days measured using filtered Limon River water, between Mara and Paez, State of Zula, Argentina(8). Methidathion is relatively stable to hydrolysis in neutral, or slightly acidic media(9); however, it is rapidly hydrolyzed under basic and strongly acidic conditions(9).

ATMOSPHERIC FATE: According to a model of gas/particle partitioning of semivolatile organic compounds in the atmosphere(1), methidathion, which has a vapor pressure of 3.37X10-6 mm Hg at 25 °C(2), will exist in both the vapor and particulate phases in the ambient atmosphere. Vapor-phase methidathion is degraded in the atmosphere by reaction with photochemically-produced hydroxyl radicals(SRC); the half-life for this reaction in air is estimated to be 2.5 hrs(SRC), calculated from its rate constant of 1.5X10-10 cu cm/molecule-sec at 25 °C(SRC) that was derived using a structure estimation method(3). Particulate-phase methidathion may be removed from the air by wet and dry deposition(SRC). Photolysis of thin films of methidathion via sunlight yielded O,O,S-trimethyl phosphorothioate, O,S,S-trimethyl phosphorodithioate, and O,O,S-trimethyl phosphorodithioate as the major photodegradation products(4).

Methidathion was qualitatively identified in raw agricultural commodities during FY78-86 according to FDA's program for monitoring pesticides in foods(1-2). Methidathion was qualitatively identified in 193 fruit composites from five Canadian cities during 1976-78(3). Methidathion was qualitatively identified in 1 of 106 grapefruit, 7 of 139 lemon, and 13 of 220 orange samples in a pesticide residue survey of produce taken during 1989-1991(4).

From Oct 1, 1981 to Sept 30, 1986, methidathion was found in 1, 1, 6, 1 and 1 US agricultural commodity at a concentration ranges of >0-0.05 ppm, 0.05-0.1 ppm, 0.1-0.5 ppm, 0.5-1.0 ppm and 1.0-2.0 ppm, respectively, no agricultural commodities contained methidathion at >2.0 ppm(1). This study does not distinguish between domestic and imported commodities or between surveillance and compliance samples(1). From Oct 1, 1981 to Sept 30, 1986, methidathion was found in 2 US domestic agricultural commodities conducted by surveillance sampling at 0.1-0.5 ppm and 1 commodity at 0.5-1.0 ppm(2). From Oct 1, 1981 to Sept 30, 1986, methidathion was found in 1 US imported agricultural commodities conducted by surveillance sampling at >0-0.05 ppm, 1 commodity at 0.05-0.1 ppm, 2 at 0.1-0.5 ppm, and 1 at 1.0-2.0 ppm(2). Methidathion was detected in 10.9% of oranges tested as part of a Belgian food commodity survey conducted from 1991-1993 at a mean concentration of 0.029 ppm; lemons, 38.2% positive, 0.147 ppm mean concentration(3). Of 21,982 samples of Canadian fruit and vegetable commodities 140 tested positive for methidathion, sampled from Jan 1, 1992 through March 31, 1994, with 17 samples <0.05 ppm, 21 samples at 0.10 ppm, 78 samples at 0.50 ppm, 20 samples at 1.0 ppm, 3 samples at 20.0 ppm, and 1 sample >20.0 average ppm(4). Methidathion was detected in 143 orange samples at a maximum residue of 1.4 ppm, conducted as part of a 1985-1991 survey of domestic foods likely to be consumed by children and infants(5).

Following orchard spraying, methidathion exhibited a half-life of 10.5 days on olives. Residual methidathion concentrations at differwnt acidities during storage were observed(1).

Methidathion was detected in wine, lees, and cake at 0.03-0.20, 0.03-4.90, and 0.03-1.00 mg/kg, respectively; however, it was not detected in the distilled spirits of wine nor its byproducts(1). Agricultural products monitored in Hyogo Prefecture, Japan from April 1995 to March 2000, found methidathion in 3 of 287 imported samples (orange, grapefruit) at concentrations of <0.1 ug/g, and in 3 of 478 domestic agricultural products (tomato) at concentrations <0.05 ug/g(2). Methidathion was not detected (detection limit 0.1-0.2 ug/kg) in 21 juice samples (apple, peach, orange, pineapple, grape) purchased in Spanish supermarkets(3). Methidathion was not detected (detection limit 0.003 mg/kg) in 130 commercial egg samples taken from Sao Paulo City, Brazil from Oct 2003 to July 2005(4). Methidathion was not detected in canned peaches from the 1999 season or from fresh peaches sampled Aug 2000, 210 samples were collected in Imathia and Pella, Greece(5). Methidathion was detected in four of 50 honey samples taken from Spain and Portugal at concentrations of 0.025 to 0.068 ug/g(6). Methidathion was not detected (detection limit 2.55 ng/g) in 275 honey samples collected in 2004 from 33 locations throughout Turkey(6). Of 173 agricultural products analyzed in 2006 in Japan, six were reported to contain methidathion at 14-732 ng/g(7). Methidathion was not detected (detection limit 1 ug/kg) in 298 raw milk samples obtained directly from tank trucks during delivery at nine Italian dairy plants, sampling took place Oct 2004, Jan and May 2005(8).

Chicken samples collected from super markets and groceries in Coimbra, Portugal were tested for methidathion, resulting in 2 of 15 samples testing positive, 3.25 and 4.41 ug/kg in muscle and 199.04 and 1205.0 ug/kg in skin(1).

Methidathion with a (14)C carbonyl group was rapidly metabolized and eliminated by lactating cows. Not only did (14)C appear in the urine, but a substantial portion was exhaled as CO2. Milk from the cows /receiving methidathion with a (14)C carbonyl group/ contained 1-2% of the radioactivity; of this, a small proportion was sulfoxide and sulfone, but no parent compound was detected. ... Failure of the compound to be excreted in milk has been confirmed.

ENVIRONMENTAL: Methidathion was not detected (detection limit 1 ug/kg) in 298 raw milk samples obtained directly from tank trucks during delivery at nine Italian dairy plants, sampling took place Oct 2004, Jan and May 2005(1).

In a metabolism study conducted in CD rats, 14(C) labeled methidathion was administered at single and preconditioned doses of 0.3 or 3.0 mg/kg. The compound was metabolized and excreted within 24 hours in both sexes... . The major metabolites that were detected in this study were the organic soluble sulfides, sulfoxide, and sulfone derivatives. Water soluble urinary metabolites included a cysteine conjugate and desmonomethyl methidathion.

Milk from the cows /receiving methidathion with a (14)C carbonyl group/ contained 1-2% of the radioactivity; of this, a small proportion was sulfoxide and sulfone, but no parent compound was detected. ...Failure of the compound to be excreted in milk has been confirmed.

Methidathion is a cholinesterase or acetylcholinesterase (AChE) inhibitor. A cholinesterase inhibitor (or 'anticholinesterase') suppresses the action of acetylcholinesterase. Because of its essential function, chemicals that interfere with the action of acetylcholinesterase are potent neurotoxins, causing excessive salivation and eye-watering in low doses, followed by muscle spasms and ultimately death. Nerve gases and many substances used in insecticides have been shown to act by binding a serine in the active site of acetylcholine esterase, inhibiting the enzyme completely. Acetylcholine esterase breaks down the neurotransmitter acetylcholine, which is released at nerve and muscle junctions, in order to allow the muscle or organ to relax. The result of acetylcholine esterase inhibition is that acetylcholine builds up and continues to act so that any nerve impulses are continually transmitted and muscle contractions do not stop. Among the most common acetylcholinesterase inhibitors are phosphorus-based compounds, which are designed to bind to the active site of the enzyme. The structural requirements are a phosphorus atom bearing two lipophilic groups, a leaving group (such as a halide or thiocyanate), and a terminal oxygen.

Tolerances are established for residues of the insecticide methidathion, including its metabolites and degradates, in or on the commodities in the table in this paragraph. Compliance with the tolerance levels specified in this paragraph is to be determined by measuring only methidathion, S -[(5-methoxy-2-oxo-1,3,4-thiadiazol-3(2 H )-yl)methyl] O,O-dimethyl phosphorodithioate, in or on the commodity.[Table#3285]

Tolerances with regional registration, as defined in part 180.1(l), are established for residues of the insecticide methidathion, including its metabolites and degradates, in or on the commodities in the table in this paragraph. Compliance with the tolerance levels specified in this paragraph is to be determined by measuring only methidathion, S -[(5-methoxy-2-oxo-1,3,4-thiadiazol-3(2H)-yl)methyl] O,O -dimethyl phosphorodithioate, in or on the commodity.[Table#3286]

Classified for restricted use and limited to use by or under the direct supervision of a certified applicator. Formulation: All formulations; Use Pattern: All uses except nursery stock, safflower, and sun flower; Classification: Restricted; Criteria Influencing Restriction: Residue effects on avian species.

Classified for restricted use and limited to use by or under the direct supervision of a certified applicator. Formulation: All formulations; Use Pattern: Nursery stock, safflower, and sunflower; Classification: Unclassified.

For more FIFRA Requirements (Complete) data for METHIDATHION (7 total), please visit the HSDB record page.

Releases of CERCLA hazardous substances are subject to the release reporting requirement of CERCLA section 103, codified at 40 CFR part 302, in addition to the requirements of 40 CFR part 355. Methidathion is an extremely hazardous substance (EHS) subject to reporting requirements when stored in amounts in excess of its threshold planning quantity (TPQ) of 500 or 10,000 lbs. Extremely hazardous substances that are solids are subject to either of two threshold planning quantities ... The lower quantity applies only if the solid exists in powdered form and has a particle size less than 100 microns; or is handled in solution or in molten form; or meets the criteria for a National Fire Protection Association (NFPA) rating of 2, 3 or 4 for reactivity. If the solid does not meet any of these criteria, it is subject to the upper ... threshold planning quantity ... .

Organophosphates poison insects and mammals primarily by phosphorylation of the acetylcholinesterase enzyme at nerve endings. ... At sufficient dosage, loss of enzyme function allows accumulation of acetylcholine (the impulse- transmitter substance) at cholinergic neuroeffector junctions (muscarinic effects, and at skeletal myoneural junctions and in autonomic ganglia (nicotinic effects). Organophosphates also impair nerve impulse transmission in the brain ... . /Organophosphate pesticides/

Male and female CFI mice were treated dermally with (14)C- radiolabelled methidathion ... in acetone solution or formulated in petroleum hydrocarbon with emulsifier (the ratio of active ingredient to petroleum hydrocarbon to emulsifier was 6:10:1). The actual dermal dose was stated to be 12 mg/kg bw. ... The half-lives for the testing solutions on the skin were calculated for the acetone group to be 9.1 and 10.5 hr for males and females, and for the formulated group to be 10.4 and 10.9 hr for males and females, respectively. ...

In a metabolism study conducted in CD rats, C-14 labeled methidathion was administered at single and preconditioned doses of 0.3 or 3.0 mg/kg. ...The half-life for elimination was approximately 8 hr.

/In rats the/ product of final oxidation and main metabolite (up to 36% of applied dose /2-methoxy and thiadazole ring labeled (14)C/) was ...CO2. 2 ...Metabolites in urine were 2-methoxy-4-methylsulfinylmethyl-delta2-1,3,4-thiadiazolin-5-one (25%) and corresponding sulfone (7%). The methylthiomethyl derivative did not appear in significant amount. ...When labeled supracide was admin to lactating cow, analysis of milk, urine, and feces indicated that extensive degradation ...had occurred. No supracide or its oxygen analog was found in milk. ...In rumen ...degradation /to water sol metabolites/ ...was ...due to microbial activity. ...The locus l migratoria degraded the thiadiazole ring. Some CO2 and unidentified water-soluble metabolites were formed.

The most important metabolites observed /in rats following oral dose/ were 4-methylsulfinylmethyl- and 4-methylsulfonylmethyl-2-methoxy-1,3,4-thiadiazole-5-one. These metabolites originate by methylation and oxidation of the mercaptomethyl deriv liberated after hydrolysis of the p-s bond. Dimethyl phosphate, dimethyl phosphorothioate, methyl phosphate, inorganic phosphate and desmethyl gs-13005 were ...found in urine and/or feces. ...Cotton plants of delta-pine smoothleaf variety were grown and treated with (32)P labelled gs-13005 ...revealed ...desmethyl gs-13005, the oxon ...analog of gs-13005, dimethyl phosphorothioate, mono- and di-methyl phosphate, inorganic phosphate, unidentified metabolite and some unextracted radioactivity. ...In 5th instar tobacco budworms, gs-13005 was completely metabolized during 1st 4 hr. Small amount of thiolate analog and desmethyl analog formed ...but was depleted after 4 hr. Dimethyl phosphorothioate and dimethyl phosphate were major hydrolytic products. ...Other polar metabolites were ...observed.

5-Methoxy-1,3,4-thiadiazol-2-one is a metabolite of supracide ... . The first oxidative degradation reaction takes place with dithioate going to thiolate.

Details of the metabolism of the acidic group of methidathion have been explored using insecticide in which every carbon atom in this group was marked with (14)C. Up to 36% of the dose applied to rats was recovered as carbon dioxide. ... In another study in rats, it was shown that averages of 22.4, 25.8, and 17.7% of the applied dose were recovered as carbon dioxide when the 2-methoxy, carbonyl, and methylene carbons, respectively, of the acidic groups were labeled with (14)C. Dimethyl phosphate (33.6%) and dimethyl phosphorothioate (24.2%) were the main urinary metabolites of methidathion, but 11.1% was recovered during the first 48 hours as desmethyl methidathion. Almost 80% of intraperitoneal doses of (32)P methidathion was excreted in 48 hr, including 7.1% in the feces.

For more Metabolism/Metabolites (Complete) data for METHIDATHION (11 total), please visit the HSDB record page.

Metabolism of organophosphates occurs principally by oxidation, by hydrolysis via esterases and by reaction with glutathione. Demethylation and glucuronidation may also occur. Oxidation of organophosphorus pesticides may result in moderately toxic products. In general, phosphorothioates are not directly toxic but require oxidative metabolism to the proximal toxin. The glutathione transferase reactions produce products that are, in most cases, of low toxicity. Paraoxonase (PON1) is a key enzyme in the metabolism of organophosphates. PON1 can inactivate some organophosphates through hydrolysis. PON1 hydrolyzes the active metabolites in several organophosphates insecticides as well as, nerve agents such as soman, sarin, and VX. The presence of PON1 polymorphisms causes there to be different enzyme levels and catalytic efficiency of this esterase, which in turn suggests that different individuals may be more susceptible to the toxic effect of organophosphate exposure.

Male and female CFI mice were treated dermally with (14)C-radiolabelled methidathion in the carbonyl carbon of the thiadiazole ring in acetone solution or formulated in petroleum hydrocarbon with emulsifier (the ratio of active ingredient to petroleum hydrocarbon to emulsifier was 6:10:1). The actual dermal dose was stated to be 12 mg/kg bw. The test solutions for both sexes were well absorbed through the skin as measured over a 72-hr period, with residual radioactivity on the skin in the acetone group of 0.47-0.67% and in the formulated product of 0.35-1.27% of the dose. The highest concentrations of radioactivity were recovered in the expired CO2 (50.85-64.1%) and urine (14.48-23.47%). Radioactivity found in tissues (0.3-0.7%) and blood (0.03-0.21%) was minimal. Total recovery represented 83-94% of the administered dose. The half-lives for the testing solutions on the skin were calculated for the acetone group to be 9.1 and 10.5 hr for males and females, and for the formulated group to be 10.4 and 10.9 hr for males and females, respectively. Blood and tissue levels appeared to plateau approximately 4 hr post-dosing, with tissue levels rarely exceeding 4 ppm.

Oral doses /in rats/ of three differently labeled forms ...were rapidly excreted in urine (up to 45%) and expired air (up to 36%). 24 hr after daily dosing ...for 10 days, <2% remained in tissues examined, and none after 48 hr. The lactating goat excreted around 1% of dose ...into milk in 72 hr.

Rats were dosed orally .../studies/ indicated complete absorption .../labeled/gs-13005 was distributed rapidly but radioactive content of all organs was below limit of detection 48 hr after dosage except for traces in muscles. Most ...was excreted in urine as polar metabolites... .

...Oral dose of (14)C-carbonyl supracide ...was ...degraded by cow. 16-51% of (14)C was thought to have been excreted in expired air, 43% in urine, 4% in feces and 1% in milk. Serum levels of (14)C peaked within 5 hr of dosing.

For more Absorption, Distribution and Excretion (Complete) data for METHIDATHION (12 total), please visit the HSDB record page.

Used as insecticide/acaricide; [Merck Index] Used as a broad spectrum insecticide/acaricide on various crops and ornamentals (primarily citrus and other fruits, artichokes, nuts, and cotton); [Reference #1]

A restricted use pesticide in the US (high acute oral toxicity); All US registrations voluntarily cancelled in December 2012, with distribution of existing stocks ending in December 2014; [Reference #1]

Farming (Pesticides) [Category: Industry]

For methidathion (USEPA/OPP Pesticide Code: 100301) ACTIVE products with label matches. /SRP: Registered for use in the U.S. but approved pesticide uses may change periodically and so federal, state and local authorities must be consulted for currently approved uses./

Insecticide, acaricide

Alfalfa weevil in alfalfa; scales in citrus; spider mites, bollworm, budworm, lygus bug, and whitefly in cotton.

1997. U.S. Methidathion 314,091 lbs active ingredient applied per year.

1992. U.S. Methidathion 372,953 lbs active ingredient applied per year.

THE TREATMENT OF 2-METHOXY-1,3,4-THIADIAZOL-5(4H)-ONE WITH FORMALDEHYDE, CONVERSION TO THE 4-HALOGENMETHYL DERIVATIVE WITH THIONYL CHLORIDE OR PHOSPHORUS TRIBROMIDE, & REACTION WITH DIMETHYL HYDROGEN PHOSPHORODITHIOATE.

Methidathion is produced by reaction of 2-methoxy-1,3,4-thiadiazol-5(4H)-one with dimethyldithiophosphoric acid via the N-chloromethyl derivative or in a one-step reaction in the presence of formaldehyde.

Emulsifiable concentrates, wettable powder.

Premix Partners: Buprofezin.

Supracide 2E Insecticide (Gowan Company), Methidathion 24.4%.

Supracide 25-W (Gowan Company), Methidathion 25%.

Methidathion Technical (Gowan Company), Methidathion 95%.

Acaricides, Insecticides

INSECTICIDES

The WHO Recommended Classification of Pesticides by Hazard identifies methidathion (technical grade) as Class IB: highly hazardous; Main Use: insecticide.

IT IS COMPATIBLE WITH CAPTAN, THIRAM, ZINEB & ACARICIDES.

RECOMMENDED FOR USE AGAINST PESTS OF FRUIT TREES, VINEYARDS, CITRUS, HOPS, LUCERNE, POTATOES, SUGAR BEET, COFFEE, TEA, DATES, PLUMS, COTTON & MORE. NON-PHYTOTOXIC.