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The information in this profile may be out-of-date. It was last revised in 1996. EXTOXNET no longer updates this information, but it may be useful as a reference or resource.

Please visit the National Pesticide Information Center (NPIC) to find updated pesticide fact sheets. If you don't find a fact sheet related to your question, feel free to call 1-800-858-7378. NPIC is open five days a week from 8:00am to 12:00pm Pacific Time.

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E X T O X N E T

Extension Toxicology Network

Pesticide Information Profiles

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

EXTOXNET primary files maintained and archived at Oregon State University

METHAMIDOPHOS

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).

TOXICOLOGICAL EFFECTS

• 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 levels (158).
• 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.

ECOLOGICAL EFFECTS

• Effects on Birds: Methamidophos is very toxic to birds. Oral LD50 values were 8-11 mg/kg in tests with bobwhite quail (156).
• 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).

ENVIRONMENTAL FATE

• 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, respectively (157).

PHYSICAL PROPERTIES AND GUIDELINES

Physical Properties:

• 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

Exposure Guidelines:

• 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

BASIC MANUFACTURER

Bayer Agricultural Products
P. O. Box 4913
Kansas City, MO 64120

REFERENCES

References for the information in this PIP can be found in Reference List Number 5