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E X T O X N E T
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Pesticide Information Profiles
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TRADE OR OTHER NAMES: Product names include Monitor, Nitofol, Tamaron, Swipe, Nuratron, Vetaron, Filitox,
Patrole, Tamanox, SRA 5172, and Tam (156, 13). Methamidophos is also a breakdown product of the organophosphate
insecticide acephate (Orthene) (157, 158).
REGULATORY STATUS: Methamidophos is classified by EPA as a Class I compound, and must bear the signal word
"Danger-Poison" on commercial products (156). 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.
INTRODUCTION: 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 (159). 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 (138). Commercially available formulations include soluble concentrate, emulsifiable concentrate,
wettable powder, granules, ultra-low volume spray and water miscible spray concentrate (158). 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 (158). 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 (156, 13).
- Acute Toxicity: 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 (160).
Inhalation LD50 values include 9 mg/kg in rats, and 19 mg/kg in mice (161).
- 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) (162). 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 (162). 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 (163, 164).
Atropine is an antidote for organophosphate poisoning (162). People with high blood pressure, gastrointestinal disorders,
heart, liver, lung, or nervous system problems may be more sensitive to methamidophos.
- Chronic Toxicity: 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
- Reproductive Effects: 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 (166). In humans, reduced sperm count and sperm viability were seen in men who
were exposed to the product Tamaron in China.
- Teratogenic Effects: Some fetal liver pathologic changes were observed when pregnant rabbits were exposed to
methamidophos (167). 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 (165).
- Mutagenic Effects: 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 (165).
- Carcinogenic Effects: There is no evidence of carcinogenicity in tests with rats and mice.
- Organ Toxicity: 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
- 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 (156, 13). 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 (167).
- 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 (168).
- 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 (169).
- 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 sunlight (169).
- 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,
PHYSICAL PROPERTIES AND GUIDELINES
- Appearance: Crystalline solid, with off-white color and pungent odor
- Chemical Name: O,S-Dimethylphosphora-midothiolate
- CAS Number: 10265-92-6
- Molecular Weight: 141.12
- Water Solubility: 90g/l @ 20 degrees C
- Solubility in Other Solvents: Not Available
- Melting Point: 112 degrees F, 44.5 degrees C
- Vapor Pressure: 3 X 10 to the minus 4 mmHg @ 30 degrees C
- Partition Coefficient: -1.74
- Adsorption Coefficient: Not Available
- ADI: Not Available
- MCL:Not Available
- RfD: OPP: 0.00100 mg/kg/day; EPA: 0.00005 mg/kg/day; WHO: 0.00400 mg/kg/day
- PEL: Not Available
- HA: Not Available
- TLV: Not Available
Bayer Agricultural Products
P. O. Box 4913
Kansas City, MO 64120
References for the information in this PIP can be found in Reference List Number 5
DISCLAIMER: The information in this profile does not in any way replace or supersede the information on the pesticide
product label/ing or other regulatory requirements. Please refer to the pesticide product label/ing.