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Extension Toxicology Network

A Pesticide Information Project of Cooperative Extension Offices of Cornell University, Michigan State University, Oregon State University, and University of California at Davis. Major support and funding was provided by the USDA/Extension Service/National Agricultural Pesticide Impact Assessment Program.


Publication Date: 9/95


Trade names for products containing demeton-s-methyl include Meta-Systox I, Meta-isosystox, Azotox, Bay-18436, Bay-25/154, DSM, Duratox, Metasystox 55, Mifatox, and Persyst (1, 2).


Demeton-s-methyl is no longer registered for use in the United States (11).


Demeton-s-methyl is a systemic and contact insecticide and acaricide that is a member of the organophosphate family of chemicals. It is absorbed and translocated within the plant in sufficient concentrations to kill insects that feed on the plant by sucking its juices. It is used to control aphids, sawflies, and spider mites in fruits, vegetables, potatoes, cereals, ornamentals, and in forestry. Demeton-s-methyl replaces methyl demeton, a mixture of demeton-s-methyl and demeton-o-methyl sold as Meta-Systox. Demeton- s-methyl is more toxic to insects than demeton-o-methyl (1, 7). Classified as Category I - highly toxic, products containing demeton-s-methyl bear the SIGNAL WORD: DANGER (3). It is available as an emulsifiable concentrate (1).



Demeton-s-methyl inhibits acetyl-cholinesterase, an enzyme critical to the normal function of the nervous system. Symptoms of toxicity are similar to those caused by other organophosphate insecticides including headaches, nausea, vomiting, diarrhea, sweating, and dizziness. In addition, symptoms in some cases of poisoning include tremors, lack of coordination, and hiccough. With time, anorexia and loss of the ability to concentrate may occur (1, 5). Demeton-s-methyl is non-irritating to the skin and eyes (10).

The LD50 for demeton-s-methyl, the dose at which half of the test animals who ate it died, is approximately 60 mg/kg for rats and 110 mg/kg for guinea pigs. The LD50 for rats whose skin is exposed is approximately 85 mg/kg. The concentration in air that kills half of the rats exposed for four hours, the LC50, is 500 mg/m3 of air (9).


Reproductive Effects

Demeton-s-methyl does not appear to cause reproductive effects. Daily oral doses of oxydemeton (Metasystox R), a derivative of demeton, caused decreased cholinesterase in pregnant rats, as well as decreased body weight. The females also developed tremors, but there was no evident effect on the fetuses (7).

Teratogenic Effects

Demeton-s-methyl's capacity to cause teratogenic effects is not clear. Studies show that 10 mg/kg of demeton, the parent compound of demeton-s- methyl, is mildly teratogenic when given to mouse embryos between 7 and 10 days old. It causes digestive, nervous, and skeletal problems, malformed digits and vertebrae, and cleft palate. However, when daily doses of up to 4.5 mg/kg of oxydemetonmethyl, a metabolic product of demeton-s-methyl, are given to pregnant rats from days 6 to 15 of gestation, there is no evidence of teratogenic effects (7).

Mutagenic Effects

Test results indicate that demeton-s-methyl is mutagenic. Mutagenicity studies show effects on sex chromosomes in fruit flies treated with 80 mg/kg of demeton-s-methyl. Microbes also mutated when exposed to 5 mg/plate of demeton-s-methyl (6).

Carcinogenic Effects

Demeton-s-methyl is an organo-phosphate compound. Studies of other organo-phosphosphate compounds show they do not share the chemical characteristics of most tumor causing compounds and most organophosphate compounds tested do not cause tumors in rats and mice (1).

Organ Toxicity

Chronic exposure to demeton-s-methyl can cause inner ear irritation and excess blood in the tissue of respiratory organs. Rats receiving 15 daily applications of 5 mg of demeton-methyl, a mixture of demeton-s-methyl and demeton-o-methyl, on their skin suffered liver damage and cholinesterase inhibition in the brain (6).

Fate in Humans and Animals

Organophosphorus pesticides like demeton-s-methyl are rapidly absorbed into the body through mucous membranes of the digestive tract, the respiratory system, and the skin. Demeton-s-methyl is broken down in the body and rapidly eliminated in urine (2).


Effects on Birds

The LC50 for Japanese quail is approximately 50 ppm, indicating that demeton-s-methyl is highly toxic to this species (3).

Effects on Aquatic Organisms

Demeton-s-methyl is moderately to slightly toxic to aquatic organisms. The LC50 for rainbow trout exposed for 48 hours is 4.5 ppm (3). For carp and Japanese killifish, the LC50 is 10 ppm (2).

Effects on Plants

Demeton-s-methyl is toxic to some ornamental plants, particularly chrysanthemum varieties. It is not toxic to most other plants when used according to directions (2).

Effects on Other Animals (Nontarget species)

Demeton-s-methyl is toxic to bees (2).


Breakdown of Chemical in Soil and Groundwater

Organophosphorus insecticides are relatively non-persistent in the environment. Applied to crops and sometimes soil, they persist for only a few hours to a few months. Compounds in this class react with the soil and bind well to soils with a high organic content. They do not move freely in wet soils and leaching does not appear to be a major factor (8).

Breakdown of Chemical in Surface Water

No information is currently available.

Breakdown of Chemical in Vegetation

No information is currently available.


Demeton-s-methyl is a pale yellow oil that has a sulfur-like odor. It is incompatible with alkaline materials (10). It is hydrolyzed rapidly in alkaline media and more slowly in acidic and neutral aqueous media (2). It is non-corrosive (10). When heated to decomposition, demeton-s-methyl emits very toxic fumes (9).

Physical Properties:

CAS No.: 919-86-8
Chemical Name: S-[2-(ethylthio)ethyl]O,O-dimethyl phosphorothioate (2)
Solubility in water: 3.3 g/l at 20 degrees C (2)
Solubility in other solvents: Rapidly soluble in common polar organic solvents including dichloromethane, 2-propanol, toluene and n-hexane (2, 4)
Boiling Point: 74 degrees C at 0.15 mm Hg (10)
Volatility: 3.6 x 10 to the minus 4 mm Hg at 20 degrees C (10)

Exposure Guidelines:

ADI: 0.0003 mg/kg (for sum of demeton-s-methyl, demeton-s-methylsulphon and oxydemeton-methyl) (2)
NOEL: 1 mg/kg (rats) (2)
TLV-TWA: 0.5 mg/m3 (10)
TLV-ELR: Short term exposures should not exceed 1.5 mg/m3 for more than a total of 30 minutes during the weekday. Under no circumstances should exposure exceed 2.5 mg/m3 (10)


Bayer AG/ Miles
P.O. Box 4913
Kansas City, MO 64120
Telephone: 816-242-2000
Emergency: 816-242-2582

Review by Basic Manufacturer:

Comments solicited: October, 1994
Comments received: January, 1995


  1. Hayes Jr., W. R. and E.R. Laws, Jr. (eds.) 1991. Handbook of Pesticide Toxicology. Academic Press, Inc. New York, NY.
  2. The Agrochemicals Handbook, Third Edition. 1994. Royal Society of Chemistry Information Systems, Unwin Brothers Limited. Surrey, England.
  3. Meister, R.T. 1994. Farm Chemicals Handbook '94. Meister Publishing Company, Willoughby, OH.
  4. Smith, G.J. 1993. Toxicology & Pesticide Use in Relation to Wildlife: Organophosphorus & Carbamate Compounds. C.K. Smoley. Boca Raton, FL.
  5. Budavari, Susan (ed.). 1989. The Merck Index, Eleventh Edition. Merck and Company Inc. Rahway, NJ.
  6. National Institute for Occupational Safety and Health (NIOSH). 1993. Registry of Toxic Effects of Chemical Substances (RTECS). NIOSH. Cincinnati, OH.
  7. Chambers, J. E. and Patricia Levi. 1992. Organophosphates Chemistry, Fate, and Effects. Academic Press, Inc. New York, NY.
  8. Amdur, M. O., John Doull & Curtis Klaassen. 1991. Casarett and Doull's Toxicology. Pergamon Press. New York, NY.
  9. Sax, N. I. 1984. Dangerous Properties of Industrial Materials, Sixth Edition. Van Nostrand Reinhold Co. New York, NY.
  10. U.S. Department of Health and Human Services. 1993. Hazardous Substance Data Base. HHS. Washington, DC.
  11. U.S. EPA Region 5. Phone call with David Star. August 31, 1994.