WORLD HEALTH ORGANIZATION FOOD AND AGRICULTURE ORGANIZATION ORGANISATION MONDIALE DE LA SANTE ORGANISATION POUR L'ALIMENTATION ET L'AGRICULTURE VBC/DS/80.42 ORIGINAL: ENGLISH DATA SHEETS ON PESTICIDES No. 42 March 1980 DIMETHOATE It must be noted that the issue of a Data Sheet for a particular pesticide does not imply endorsement of the pesticide by WHO or FAO for any particular use, or exclude its use for other purposes not stated. While the information provided is believed to be accurate according to data available at the time when the sheet was compiled, neither WHO nor FAO are responsible for any errors or omissions, or any consequences therefrom. The issue of this document does Ce document ne constitue pas une not constitute formal publication. Il ne doit faire publication. It should not be l'objet d'aucun compte rendu ou reviewed, abstracted or quoted résumé ni d'aucune citation sans without the agreement of the l'autorisation de l'Organisation Food and Agriculture des Nations Unies pour Organization of the United l'Alimentation et l'Agriculture Nations or of the World Health ou de l'Organisation Mondiale de Organization. la Santé. CLASSIFICATION: Primary use: Insecticide Secondary use: Acaricide Chemical group: Organophosphorus compound Date issued: March 1980 1. GENERAL INFORMATION 1.1 COMMON NAME: Dimethoate (ISO) 1.1.1 Identity: O,O-dimethyl S-[2-(methylamino)-2-oxoethyl] phosphorodithioate 1.1.2 Synonyms: OMS-94 Rogor(R) OMS-111 Cygon(R) Perfekthion(R) Ridmite(R) De-Fend(R) Roxion(R) Local synonyms: 1.2 SYNOPSIS: An insecticide and acaricide of moderate mammalian toxicity which is used in housefly control and against a broad range of agricultural insect and mite pests. It is active after metabolism, both as a contact and as a systemic insecticide. 1.3 SELECTED PROPERTIES 1.3.1 Physical characteristics The pure compound forms colourless crystals with a camphor-like odour, m.p. 51-52°C. Technical dimethoate, about 93% pure, varies from off-white crystals to a grey, semi-crystalline material. 1.3.2 Solubility Water: 25 g/l at 21°C. Soluble in most organic solvents except saturated hydrocarbons such as hexane. 1.3.3 Stability Stable in aqueous solution but is readily hydrolysed by aqueous alkali; heating converts it to the SCH3 isomer. 1.3.4 Vapour pressure 8.5 x 10-6 mmHg at 25°C. 1.4 AGRICULTURE, HORTICULTURE AND FORESTRY 1.4.1 Common formulations 200, 400 and 500 g/l emulsion concentrates, 200 g/kg water-dispersible powder, 50 mg/kg granules: 200 g/technical compound/l for ULV. 1.4.2 Susceptible pests Aphids, leafhoppers, lygus bugs, pear psylla. Various mites of crops and ornamental plants. 1.4.3 Use pattern As a systemic pesticide on various fruits and vegetables, on alfalfa and fodder crops well before harvesting or grazing, on cotton against sucking insects: as a residual insecticide at 1 g/m2 for houseflies in barns, stables, piggeries, etc. Also used for houseflies in sugar baits. 1.4.4 Unintended effects Cytotoxic for some fruit trees, including citrus. 1.5 PUBLIC HEALTH PROGRAMMES No recommended use. 1.6 HOUSEHOLD USE No recommended use. 2. TOXICOLOGY AND RISKS 2.1 TOXICOLOGY - MAMMALS 2.1.1 Absorption route Readily absorbed by the gastrointestinal tract and to a lesser extent through the intact skin and by inhalation. 2.1.2 Mode of action Cholinesterase inhibition after metabolism. Oral dimethoate is biotransformed in the liver microsomes by conversion into its oxygen analogue, which is the active form, by hydrolysis of the methyl ester group, and by removal of the methyl-amido group. 2.1.3 Excretion products Studies on rat and man with 32p-labelled dimethoate show rapid absorption and excretion, 79-90% of the radioactivity being found in the urine after 24 hours. Direct degradation of a C-N bond is considered largely responsible for its selective toxicity and relative safety for mammals. The chief metabolite in animals is the thiocarboxy derivative of dimethoate. 2.1.4 Toxicity, single dose Oral: LD50 rat 500-600 mg/kg: pure dimethoate about 150 mg/kg: technical product The difference in toxicity between pure dimethoate and the technical product is due to a trace impurity produced by partial hydrolysis which potentiates the toxicity of pure dimethoate. Therefore the LD50 of a given technical product may vary within these limits. The oral LD50 value of 150 mg/kg is recommended for classification purposes. Dermal: LD50 rat 353 mg/kg 2.1.5 Toxicity repeated doses Oral: Given 30 (mg/kg)/day for three weeks, guinea-pigs showed some weight loss and weakness but no cholinergic effects. In similar tests on rats, the highest non-lethal dose was 20 (mg/kg)/day. Inhalation: Twelve male rats were kept for 28 days in a miniature greenhouse, with plants and sides of the chamber sprayed daily with 0.570 aqueous dimethoate formulation. No blood cholinesterases inhibition or toxic effects occurred. Cumulation of compound: Dimethoate is not cumulative in body tissues. Cumulation of effect: Repeated daily exposure may cumulatively inhibit cholinesterase activity. 2.1.6 Dietary studies Short-term: Groups of 10 male rats were fed levels of 1, 5, 25 and 125 mg dimethoate/kg diet for 15 weeks. At the highest concentration, a slight fall in the rate of gain of weight was observed as well as mild symptoms of poisoning. In the group fed 25 mg/kg and at higher concentrations, a significant reduction in plasma and erythrocyte cholinesterase activity was observed, while in the animals fed 5 mg/kg, a reduction of 20% in plasma cholinesterase activity only was found. In another study, groups of 20 male rats were fed for 6-12 months with various levels of laboratory grade dimethoate. At 800 mg/kg intoxication developed within a few days. No toxic effects were seen at 50 mg/kg. Marked inhibition of erythrocyte cholinesterase occurred at 50 mg/kg but at 10 mg/kg and below neither erythrocyte nor plasma cholinesterase showed significant inhibition. The maximum no-effect level corresponded to 0.5-0.8 mg/kg body weight. Dogs were fed levels of 2, 10 and 50 mg/kg diet for 13 weeks. The erythrocyte cholinesterase activity was only slightly reduced at 50 mg/kg. Long-term: See below. 2.1.7 Supplementary studies of toxicity Carcinogenicity: Dimethoate was administered in feed to groups of 50 male and 50 female rats for 80 weeks, followed by 35 weeks of observation. Initial doses were not well tolerated; therefore, they were reduced during the study. The "time-weighted average doses" for rats were 155 and 310 mg/kg diet for males and 192 and 384 mg/kg for females. All surviving rats were killed between 113 and 115 weeks. Similarly, dimethoate was administered in feed to groups of 50 male and 50 female mice at concentrations of 250 and 500 mg/kg diet for 80 weeks. High-dose males were returned to the control diet at 60 weeks, and low-dose males at 69 weeks. All surviving mice were killed between 93 and 94 weeks. In both studies, tremor and hyperexcitability, indications of dimethoate toxicity, were observed in the treated animals. Pathologic evaluation revealed no statistically significant increase in tumours associated with dimethoate treatment in either species. Reproduction and teratogenicity: A three-generation reproduction study was conducted on mice at levels of 5, 15 and 50 mg/kg diet, with two litters produced by generation. No effect was seen on fertility, lactation or survival of the pups to weaning, gross appearance, weight of major organs, or gross and microscopic appearance of tissues. In another study, a dietary level of 60 mg/kg produced reduced mating success and longer reproduction time. At birth, litter size and weight were not reduced, but pup mortality increased significantly with treatment. Growth rate of the pup was generally lower also. No teratogenic or other adverse effects on organs were observed. 2.1.8 Modification of toxicity Acute oral toxicity of dimethoate was not potentiated by any of 17 other insecticides. There is a theoretical possibility that the toxicity of some batches might vary due to the presence of impurities. 2.2 TOXICOLOGY - MAN 2.2.1 Absorption See 2.1.1. 2.2.2 Dangerous doses Single: Not known. Studies on the degradation of dimethoate by 11 specimens of human liver have been conducted. On the basis of comparisons with other species for which data both on toxicity and on degradation in liver are known, it is predicted that the acute oral LD50 of dimethoate to humans is about 30 mg/kg. Repeated: Not known. 2.2.3 Observations of occupationally exposed workers No reports. 2.2.4 Observations on exposure of the general population No reports. 2.2.5 Observations of volunteers Twenty subjects ingested 2.5 mg of dimethoate in aqueous solution, corresponding to about 0.04 mg/kg body weight daily for four weeks. No toxic effect, or significant change in the blood cholinesterase activity were observed. Similar results were found in two subjects who ingested daily during 21 days, 0.13 mg/kg and 0.26 dimethoate mg/kg body weight respectively. Thirty-six male and female volunteers were given daily oral doses of dimethoate of 5, 15, 30, 46 and 60 mg for periods of 14 to 57 days. There was no effect on the blood cholinesterase levels with intakes of 5 and 15 mg daily, but effects were noted at 30 mg and above. 2.2.6 Reported mishaps Human cases of poisoning have occurred due to ingestion. 2.3 TOXICITY TO NON-MAMMALIAN SPECIES 2.3.1 Fish Toxic to fish but disappears from water rapidly under field conditions. 2.3.2 Birds Toxic to birds. 2.3.3 Other species Toxic to bees. 3. FOR REGULATORY AUTHORITIES RECOMMENDATIONS ON REGULATION OF COMPOUND 3.1 RECOMMENDED RESTRICTIONS ON AVAILABILITY (For definition of categories, see introduction). Liquid formulations 10% and above category 3, below 10% category 4. Solid formulations above 25% category 3, all other formulations category 4. 3.2 TRANSPORTATION AND STORAGE All formulations - Should be transported in clearly labelled rigid or leakproof containers and stored in these containers, under lock and key, secure from access by unauthorized persons and children. No food or drink should be transported or stored in the same compartment. 3.3 HANDLING All formulations - Protective clothing should be provided for those handling the compound. Adequate washing facilities should be available at all times during handling and should be close to the site of handling. Eating, drinking and smoking should be prohibited during handling and before washing after handling. 3.4 DISPOSAL AND/OR DECONTAMINATION OF CONTAINER All formulations - Containers may be decontaminated (for method, see part 4). Decontaminated containers should not be used for food and drink. If not decontaminated, containers should be burned or crushed and buried below topsoil. Care must be taken to avoid subsequent contamination of water sources. 3.5 SELECTION, TRAINING AND MEDICAL SUPERVISION OF WORKERS All formulations - Pre-employment medical examination of workers desirable. Workers suffering from active hepatic or renal disease should be excluded from contact. Pre-employment and periodic cholinesterase tests for workers desirable. Special account should be taken of the workers' mental ability to comprehend and follow instructions. Training of workers in techniques to avoid contact essential. 3.6 ADDITIONAL REGULATIONS RECOMMENDED IF DISTRIBUTED BY AIRCRAFT All formulations - Pilots and loaders should have special training in application methods and recognition of early symptoms of poisoning, and must wear a suitable respirator. Flagmen, if used, should wear overalls and be located well away from the dropping zone. 3.7 LABELLING All formulations - Minimum cautionary statement: Dimethoate is an organophosphorus compound which inhibits cholinesterase. It is poisonous if swallowed. It may be absorbed through the skin or inhaled as dusts or mists. Avoid skin contact; wear protective gloves, clean protective clothing and a dust mask when handling the material. Wash thoroughly with soap and water after using. Keep the material out of reach of children, and well away from foodstuffs, animal feed and their containers. If poisoning occurs, call a physician. Atropine is a specific antidote and artificial respiration may be needed. 3.8 RESIDUES IN FOOD: Maximum residue limits for dimethoate have been recommended by the Joint FAO/WHO Meeting on Pesticide Residues. These are subject to change at annual reviews. 4. PREVENTION OF POISONING IN MAN AND EMERGENCY AID 4.1 PRECAUTIONS IN USE 4.1.1 General Dimethoate is an organophosphorus pesticide of moderate toxicity. It penetrates the intact skin and is also absorbed by inhalation of dust and spray mists, and from the intestinal tract. Most concentrated formulations should be handled only by trained personnel wearing protective clothing. Manufacture and formulation - T.L.V. No information. Closed systems of forced ventilation may be required to reduce as much as possible the exposure of workers to the chemical. Protective equipment for the skin and respiratory protection may be desirable. 4.1.3 Mixers and applicators When opening the container and when mixing, protective impermeable boots, clean overalls, gloves and a visor should be worn. Mixing, if not mechanical, should always be carried out with a paddle of appropriate length. When spraying tall crops or during aerial application, a face mask should be worn, as well as an impermeable hat, clothing, boots and gloves. The applicator should avoid contact with the mouth. Particular care is needed when equipment is being washed after use. All protective clothing should be washed immediately after use, including the insides of gloves. Splashes must be washed immediately from the skin or eyes with large quantities of water. After washing, eyes that have been splashed should be examined by a doctor. Before eating, drinking or smoking, hands and other exposed skin should be washed. 4.1.4 Other associated workers (including flagmen in aerial operations) Persons exposed to dimethoate and associated with its application should wear protective clothing and observe the precautions described in 4.1.3 under "Mixers and applicators". 4.1.5 Other populations likely to be affected With good agricultural and manufacturing practice subject to 4.2 below, other populations should not be exposed to hazardous amounts of dimethoate. 4.2 ENTRY OF PERSONS INTO UNTREATED AREAS: Unprotected persons should be kept out of tall crops for four days; and out of other crops for 24 hours. 4.3 DECONTAMINATION OF SPILLAGE AND CONTAINERS: Residues in containers should be emptied in a diluted form into a deep pit taking care to avoid contamination of ground waters. The empty container should be filled with 5% sodium hydroxide solution which should remain in the container overnight. Then containers may be further decontaminated by rinsing two or three times with water and scrubbing the sides. Impermeable gauntlets should be worn during the work and a soakage pit should be provided for the rinsing. Decontaminated containers should not be used for food and drink. Spillage of dimethoate and its formulations should be removed by washing with 5% sodium hydroxide solution and then rinsing with large quantities of water. 4.4 EMERGENCY AID 4.4.1 Early symptoms of poisoning These may include excessive sweating, headache, weakness, giddiness, nausea, vomiting, stomach pains, blurred vision, slurred speech and muscle twitching. Later there may be convlusions, coma, loss of reflexes and loss of sphincter control. 4.4.2 Treatment before person is seen by a physician, if these symptoms appear following exposure The person should stop work immediately, remove contaminated clothing and wash the affected skin with soap and water, if available, and flush the area with large quantities of water. If swallowed, vomiting should be induced, if the person is conscious. In the event of collapse, artificial respiration should be given, bearing in mind that if mouth-to-mouth resuscitation is used, vomit may contain toxic amounts of dimethoate. 5. FOR MEDICAL AND LABORATORY PERSONNEL 5.1 MEDICAL DIAGNOSIS AND TREATMENT IN CASES OF POISONING 5.1.1 General information Dimethoate is an organophosphorus pesticide of moderate toxicity which is readily absorbed from the gastrointestinal tract. Absorption may occur through the dermal route and by inhalation. It is converted in vivo to the oxygen analogue which then inhibits acetylcholinesterase. Continued exposure to low amounts may inhibit acetylcholinesterase to dangerous levels. 5.1.2 Symptoms and signs Initial symptoms of poisoning may include excessive sweating, headache, weakness, giddiness, nausea, vomiting, stomach pains, blurred vision, slurred speech and muscle twitching. More advanced symptoms of poisoning may be convulsions, coma, loss of reflexes and loss of sphincter control. 5.1.3 Laboratory The most important laboratory finding is reduction in activity of blood cholinesterases. Urinary levels of organic phosphorus containing metabolites may also be used as a measure of exposure. Neither method is specific for dimethoate. 5.1.4 Treatment If the pesticide has been ingested, unless the patient is vomiting, rapid gastric lavage should be performed using 5% sodium bicarbonate, if available. For skin contact, the skin should be washed with soap and water. If the compound has entered the eyes, they should be washed with isotonic saline or water. Persons without signs of respiratory inefficiency but with manifest peripheral symptoms should be treated with 2-4 mg of atropine sulfate by intravenous injection. More atropine may be given as needed. Persons with severe intoxication, with respiratory difficulties convulsions and unconsciousness, should immediately be given atropine and a reactivator. In such severe cases 4-6 mg of atropine sulfate should be given initially followed by repeated doses of 2 mg at 5-10 minute intervals. The patient's condition including respiration, blood pressure, pulse frequency, salivation and convulsions should be carefully observed to further administration of atropine. If the patient is cyanotic, artificial respiration should be given at the same time as atropine sulfate. The airways should be kept free and artificial respiration should be applied, if required, preferably by mechanical means. If necessary, intubation should be performed. Contraindications are morphine, barbiturates, phenothiazine, tranquillizers and central stimulants of all kinds. Pralidoxime and toxogonin are not regarded as effective antidotes in dimethoate poisoning. 5.1.5 Prognosis If the acute toxic effect is survived and adequate artificial respiration has been given, if needed, the chances of complete recovery are good. However, in very severe cases, particularly if artificial respiration has been inadequate, prolonged anoxia may give rise to permanent brain damage. 5.1.6 References of previous reported cases None. 5.2 SURVEILLANCE TESTS Test Normal level* Action level* Symptomatic level Plasma cholinesterase 100% 50% Variable Erythrocyte cholinesterase 100% 70% Usually < 40% Urinary levels of ether extractable organic phosphorus may also be used to determine the degree of exposure. 5.3 LABORATORY METHODS References only are given. 5.3.1 Detection and assay of compound Residues of dimethoate may be determined by GLC (Abbott et al., 1970) or colorimetrically, see: Chillwell & Beecham (1960), de Pietri-Tonelli et al. (1965) and Van Middlem & Waites (1964). *Expressed as percentage of pre-exposure activity. 5.3.2 Other tests in cases of poisoning Levels of cholinesterase in blood provide the most useful diagnosis of poisoning. See: Michel, N. O. (1949) J. Lab. Clin. Med., 34, 1564-1568. Ellman, G. L. et al. (1961) Biochem. Pharmacol., 7, 88-95. Urinary levels of dimethyl phosphate and phosphorothionate (Shafik & Emos, 1969) can also be used to determine exposure.
See Also: Dimethoate (EHC 90, 1989) Dimethoate (PIM 388)