E X T O X N E T
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
TRADE OR OTHER NAMES
Product names include Monitor, Nitofol, Tamaron, Swipe, Nuratron,
Vetaron, Filitox, Patrole, Tamanox, SRA 5172, and Tam (1, 2). Methamidophos is
also a breakdown product of the organophosphate insecticide acephate (Orthene)
Methamidophos is classified by EPA as a Class I compound, and must bear
the signal word "Danger-Poison" on commercial products (1). Pesticides in
this toxicity class are restricted use pesticides (RUP). Tolerances for
residues of methamidophos on raw agricultural products range from 0.5 ppm in
or on melons to 1.0 ppm in or on broccoli and tomatoes. Check with specific
state regulations for local restrictions which may apply.
Methamidophos is a highly active, systemic, residual organophosphate
insecticide/acaricide/avicide with contact and stomach action. Its mode of
action in insects and mammals is by decreasing the activity of an enzyme
important for nervous system function called acetylcholinesterase. This
enzyme is essential in the normal transmission of nerve impulses.
Methamidophos is a potent acetylcholinesterase inhibitor (5).
It is effective against chewing and sucking insects and is used to
control aphids, flea beetles, worms, whiteflies, thrips, cabbage loopers,
Colorado potato beetles, potato tubeworms, armyworms, mites, leafhoppers, and
many others. Crop uses include broccoli, Brussel sprouts, cauliflower,
grapes, celery, sugar beets, cotton, tobacco, and potatoes. It is used abroad
for many vegetables, hops, corn, peaches, and other crops (6). Commercially
available formulations include soluble concentrate, emulsifiable concentrate,
wettable powder, granules, ultra-low volume spray and water miscible spray
Generally, methamidophos is not considered phytotoxic if used as
directed, but defoliation has occurred when applied as foliar spray to
deciduous fruit. It is compatible with many other pesticides, but do not use
with alkaline materials (4). Methamidophos is slightly corrosive to mild
steel and copper alloys. This compound is highly toxic to mammals, birds, and
bees. Do not graze treated areas, and be sure to wear protective clothing
including respirator, chemical goggles, rubber gloves, and impervious
protective clothing (1, 2).
Methamidophos is highly toxic via oral, dermal and inhalation routes of
exposure. The oral doses of methamidophos that resulted in the mortality of
half of the test organisms (LD50 values) are 21 and 16 mg/kg body weight for
male and female rats respectively, 30-50 mg/kg body weight in guinea pigs and
10-30 mg/kg body weight in rabbits. Dermal LD50 values include 50 mg/kg body
weight in rats and 118 mg/kg body weight in rabbits (7). Inhalation LD50
values include 9 mg/kg in rats, and 19 mg/kg in mice (8).
Signs and Symptoms of Acute Poisoning
Early symptoms of acute organo-phosphate poisoning are dependent on route
of exposure, and usually develop during or shortly after exposure (within 12
hours) (9). If inhaled, tightness in the chest, wheezing, headache, blurred
vision, pinpoint pupils, tearing and runny nose are common early symptoms. If
ingested, nausea, vomiting, diarrhea, and cramps are the most common early
signs of poisoning. Sweating and twitching in the area of absorption are seen
with skin exposure. Weakness, shakiness, blurred vision, tightness in the
chest, sweating, confusion, changes in heart rate, convulsions, coma, and
cessation of breathing may occur with significant inhalation, ingestion or
dermal exposure (9). An intermediate syndrome has been described in cases of
poisonings in Sri Lanka, where patients experienced paralysis of limb, neck,
and respiratory muscles 24-96 hours after exposure. Delayed neurological
problems (delayed peripheral neuropathy) have been described 2-4 weeks after
large exposures to organophosphates, and include a loss of feeling and pins
and needles type of pains in the feet, legs, and hands (10, 11). Atropine is
an antidote for organophosphate poisoning (9).
People with high blood pressure, gastrointestinal disorders, heart,
liver, lung, or nervous system problems may be more sensitive to
A 56-day rat feeding study resulted in a No Observable Effects Level
(NOEL) of 0.03 mg/kg/day. The reference dose (RfD) is based on this study.
In another study, dogs were fed up to 32 parts per million (ppm) (or 32
mg/1000 g of food per day) methamidophos for 1 year without observed adverse
effects on body weights, organ weights, food consumption, blood chemistry, and
urine chemistry. Measurable cholinesterase inhibition was found at all
treatment levels (4).
A two generation feeding study in rats showed a decrease in the
percentage of females delivering offspring at all dose levels (0.15, 0.5, and
1.65 mg/kg/day). A systemic NOEL was 0.5 mg/kg/day based on reduced body
weights during premating period (13). In humans, reduced sperm count and
sperm viability were seen in men who were exposed to the product Tamaron in
Some fetal liver pathologic changes were observed when pregnant rabbits
were exposed to methamidophos (14). In two teratology studies, no birth
defects were observed at the highest levels tested (3 mg/kg/day in rats, and
2.5 mg/kg/day in rabbits). Decreased body weights were observed in offspring
and mothers in the rat study at 3 mg/kg/day. In rabbits, a maternal low effect
level (LEL) of 0.1 mg/kg/day (lowest dose tested) was observed based on low
body weights (12).
Methamidophos has tested positive for genotoxicity, or ability to induce
changes in chromosomes, in some tests and negative in others. It may be
weakly mutagenic (12).
There is no evidence of carcinogenicity in tests with rats and mice.
The primary target of organophosphate compounds is the nervous system.
Some liver damage has been observed in rabbits. Reduced sperm count and
reduced sperm viability have been observed in humans.
Fate in Humans and Animals
Methamidophos is rapidly absorbed through the stomach, lungs and skin.
It is eliminated primarily in the urine.
Effects on Birds
Methamidophos is very toxic to birds. Oral LD50 values were 8-11 mg/kg
in tests with bobwhite quail (1).
Effects on Aquatic Organisms
Methamidophos is toxic to aquatic organisms. The concentration in water
that is lethal to half of the test organisms (LC50) ranges from 25-51 mg/l in
96-hour tests with rainbow trout, 46 mg/l in guppies, 100 mg/l in carp and 100
mg/l in goldfish (1, 2). Freshwater, estuarine and marine crustaceans are
extremely sensitive to methamidophos. Concentrations as low as 0.22 ng/l
(.00000022 mg/l) were lethal to larval crustaceans in 96-hour toxicity tests
Effects on Other Animals (Nontarget species)
Methamidophos is toxic to bees. A field study of the effects of
methamidophos on honey bees during alfalfa pollination demonstrated that the
chemical can severely reduce the foraging activity of bees for a prolonged
period of time after application (15).
Breakdown of Chemical in Soil and Groundwater
In aerobic soils, the half-life of methamidophos is as follows: 1.9 days
in silt, 4.8 days in loam, 6.1 days in sand, and 10-12 days in sandy loam
Breakdown of Chemical in Surface Water
The half-life of the chemical in water is 309 days at pH 5.0, 27 days at
pH 7.0, and 3 days at pH 9.0. The chemical will break down in the presence of
sunlight, and has a half-life of 90 days in water at pH 5 when there is
Breakdown of Chemical in Vegetation
Methamidophos is taken up through the roots and leaves. In studies of
methamidophos in tomato plants, the half-lives in fruit and leaves were
measured as 4.8-5.1 days and 5.5-5.9 days, respectively (3).
PHYSICAL PROPERTIES AND GUIDELINES
|RfD: ||OPP: 0.00100 mg/kg/day; EPA: 0.00005 mg/kg/day; WHO: 0.00400 mg/kg/day
|NOEL: ||0.03 mg/kg/day (rat feeding study)
|LEL: ||0.05 mg/kg/day
|Form: ||Crystalline solid, with off-white color and pungent odor
|Chemical Name: ||O,S-Dimethylphosphora-midothiolate
|Molecular Formula: ||C2H8NO2PS
|CAS #: ||10265-92-6
|Molecular Weight: ||141.12
|Solubility in Water: ||90g/l @ 20 degrees C
|Log Kow: ||-1.74
|Specific Gravity: ||1.31 @ 44.5 degrees C
|Melting Point: ||112 degrees F, 44.5 degrees C
|Vapor Pressure: ||3 X 10 to the minus 4 mmHg @ 30 degrees C
Bayer Agricultural Products
P. O. Box 4913
Kansas City, MO 64120
Review by Basic Manufacturer:
Comments solicited: May and October, 1995
Comments received: not received
Meister, R.T. 1995. Farm Chemicals Handbook '95. Meister Publishing
Company. Willoughby, OH.
Kidd, H. and D. James (eds.). 1994. Agrochemicals Handbook. Third
Edition. Royal Society of Chemistry. Cambridge, England.
Antonious, G.F. and J.C. Snyder. 1994. Residues and Half-Lives of
Acephate, Methamidophos, and Pirimiphos-Methyl in Leaves and Fruit of
Greenhouse-Grown Tomatoes. Bull. Environ. Contam. Toxicol. 52:141-148.
HSDB: Hazardous Substances Data Bank. 1990. National Library of
Medicine, Bethesda, MD (CD-ROM Version). Micromedix, Inc., Denver, CO.
Hussain, M.A. 1987. Anticholines-terase Properties of Methamidophos
and Acephate in Insects and Mammals. Bull. Environ. Contam. Toxicol. 38:131-
Thomson, W.T. 1992. Agricultural Chemicals Book 1. Thomson
Publications. Fresno, CA.
RTECS: Registry of Toxic Effects of Chemical Substances. 1990.
National Institute for Occupational Safety and Health, Cincinnati, OH (CD-ROM
Version). Canadian Centre for Occupational Health and Safety, Hamilton,
TOMES Hazard Management
Morgan, D.P. 1989. U.S. Environmental Protection Agency. Recognition
and Management of Pesticide Poisonings. Fourth Edition. Health Effects
Division, Office of Pesticide Programs, U.S. EPA, Washington, DC.
Senanayake, N. and L. Karalliedde. 1987. Neurotoxic Effects of
Organophosphorus Insecticides. N. Engl. J. Med. 316:761-763.
Senanayake, N. and M.K. Johnson. 1982. Acute Polyneuropathy After
Poisoning by a New Organophosphate Insecticide. N. Engl. J. Med. 306: 155-
Integrated Risk Information System (IRIS). (10-01-93).
Juarez, L.M. and J. Sanchez. 1989. Toxicity of the Organophosphorus
Insecticide Metamidophos (O,S-Dimethyl Phosphoramidothioate) to Larvae of the
Freshwater Prawn Macrobrachium rosenbergii (De Man) and the Blue Shrimp
Penaeus stylirostris Stimpson. Bull. Environ. Contam. Toxicol. 43: 302-309.
Gary, N.E. and K. Lorenzen. 1989. Effect of Methamidophos on Honey
Bees (Hymenoptera: Apidae) During Alfalfa Pollination. J. Econ. Entomol.
U.S. Environmental Protection Agency. 1989. Pesticide Environmental
Fate One Line Summary: Methamidophos. U.S. EPA Environmental Fate and Effects
Division. Washington, DC.