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The information in this profile may be out-of-date.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

Revised June 1996

Methomyl

Trade and Other Names: Common names include metomil and mesomile. Trade names include Acinate, Agrinate, DuPont 1179, Flytek, Kipsin, Lannate, Lanox, Memilene, Methavin, Methomex, Nudrin, NuBait, Pillarmate and SD 14999.

Regulatory Status: Methomyl is a highly toxic compound in EPA toxicity class I. It is classified as Restricted Use Pesticide (RUP) by EPA because of its high acute toxicity to humans. The Signal Words for products containing methomyl depend upon the formulation of the product. Restricted Use Pesticides may be purchased and used only by certified applicators. Reentry periods for farm workers of 1 to 7 days are required, depending on the crop.

Chemical Class: carbamate

Introduction: Methomyl was introduced in 1966 as a broad spectrum insecticide. It is also used as an acaricide to control ticks and spiders. It is used for foliar treatment of vegetable, fruit and field crops, cotton, commercial ornamentals, and in and around poultry houses and dairies. It is also used as a fly bait. Methomyl is effective in two ways: (a) as a "contact insecticide," because it kills target insects upon direct contact, and (b) as a "systemic insecticide" because of its capability to cause overall "systemic" poisoning in target insects, after it is absorbed and transported throughout the pests that feed on treated plants. It is capable of being absorbed by plants without being "phytotoxic" or harmful, to the plant.

Formulation: Not Available

Toxicological Effects:

• Acute toxicity: Methomyl is highly toxic via the oral route, with reported oral LD50 values of 17 to 24 mg/kg in rats [10], 10 mg/kg in mice, and 15 mg/kg in guinea pigs [5]. Symptoms of methomyl exposure are similar to those caused by other carbamates and cholinesterase inhibitors [5]. These may include weakness, blurred vision, headache, nausea, abdominal cramps, chest discomfort, constriction of pupils, sweating, muscle tremors, and decreased pulse. If there is severe poisoning, symptoms of twitching, giddiness, confusion, muscle incoordination, slurred speech, low blood pressure, heart irregularities, and loss of reflexes may also be experienced. Death can result from discontinued breathing, paralysis of muscles of the respiratory system, intense constriction of the openings of the lung, or all three [5]. It is moderately toxic via inhalation with a reported 4-hour inhalation LC50 in male rats of 0.3 mg/L [4]. Inhalation of dust or aerosol may cause irritation, lung and eye problems, with symptoms of chest tightness, blurred vision, tearing, wheezing, and headaches appearing upon exposure. Other systemic symptoms of cholinesterase inhibition may appear within a few minutes to several hours of exposure [39]. It is slightly toxic via the dermal route, with a reported dermal LD50 of 5880 mg/kg in rabbits [10], and is absorbed only slowly through the skin [5]. However, if sufficient amounts are absorbed through the skin, symptoms similar to those induced by ingestion or inhalation will develop [5]. Within fifteen minutes to four hours of exposure, the immediate area of contact may show localized sweating and uncoordinated muscular contractions [5]. In rabbits, application of methomyl resulted in mild eye irritation [5]. Pain, short-sightedness, blurring of distant vision, tearing, and other eye disturbances may occur within a few minutes of eye contact with methomyl [11].
• Chronic toxicity: Prolonged or repeated exposure to methomyl may cause symptoms similar to the pesticide's acute effects [5]. Repeated exposure to small amounts of methomyl may cause an unsuspected inhibition of cholinesterase, resulting in flu-like symptoms, such as weakness, lack of appetite, and muscle aches. Cholinesterase-inhibition may persist for two to six weeks. This condition is reversible if exposure is discontinued. Since cholinesterase is increasingly inhibited with each exposure, severe cholinesterase-inhibition symptoms may be produced in a person who has had previous methomyl exposure, while a person without previous exposure may not experience any symptoms at all [5]. In a 24-month study with rats fed doses of 2.5, 5 or 20 mg/kg/day, effects were only observed at the highest dose tested, 20 mg/kg/day. At this very high dose, red blood cell counts and hemoglobin levels were significantly reduced in female rats [39]. In a 2-year feeding study with dogs, 5 mg/kg/day caused no observed adverse effects [39]. It is not likely that chronic effects would be seen in humans unless exposures were unexpectedly high, as with chronic misuse.
• Reproductive effects: Methomyl fed to rats at dietary doses of 2.5 or 5 mg/kg for three generations caused no adverse effect on reproduction, nor was there any evidence of congenital abnormalities [8]. No fetotoxicity was observed in offspring of pregnant rats given 33.9 mg/kg/day on day 6 to 21 of gestation [5]. Based on these data it appears unlikely that methomyl will have reproductive effects.
• Teratogenic effects: No teratogenic effects were found in the fetuses of female rabbits that were fed approximately 15 to 30 mg/kg/day during the 8th to 16th day of gestation [8]. In rats, no embryonic or teratogenic effects were observed at the highest dietary dose administered, approximately 34 mg/kg/day [5]. Thus, methomyl does not appar to be teratogenic.
• Mutagenic effects: In a number of assays (including Ames test, a reverse mutation assay, a recessive lethal assay, three DNA damage studies, an unscheduled DNA synthesis assay, and in vivo and in vitro cytogenetic assays), methomyl was not mutagenic [8,39]. There is no evidence that methomyl is a mutagenic or genotoxic.
• Carcinogenic effects: There was no evidence of carcinogenicity in either rats or dogs that ingested high doses of methomyl in 2-year feeding studies [39]. Methomyl was not carcinogenic in 22- and 24-month studies with rats fed doses of up to 20 mg/kg, nor in a two-year study with mice fed dietary doses of up to 93.4 mg/kg/day [5]. The evidence suggests that methomyl is not carcinogenic.
• Organ toxicity: Lungs, skin, eyes, gastrointestinal tract, kidneys, spleen, and blood-forming organs have been affected in various experiments, depending on route of entry, duration of exposure, and dosage.

• Fate in humans and animals: Methomyl is quickly absorbed through the skin, lungs, and gastrointestinal tract and are broken down in the liver. Breakdown products are readily excreted via respiration and urine [10]. Although they do not appear to accumulate in any particular body tissue, they may alter many other enzymes besides the cholinesterases [8].

Ecological Effects:

• Effects on birds: Methomyl is highly toxic to birds. The acute oral LD50 in bobwhite quail is 24.2 mg/kg [39]. The oral LD50 of methomyl is 28 mg/kg in hens. All deaths occurred within ten minutes of dosing. The clinical signs of toxicity included tearing of the eyes, salivation, occasional convulsions, and respiratory disorders. In Japanese quail, the LD50 is 34 mg/kg [39]. The LD50 of a 90% pure formulation is 15.9 mg/kg in eight-month-old mallards, and 15.4 mg/kg in three- to four-month-old male pheasants [22]. The LD50 for starlings is 42 mg/kg and for redwinged blackbirds is 10 mg/kg [39].
• Effects on aquatic organisms: Methomyl is moderately to highly toxic to fish and highly toxic to aquatic invertebrates. The 96-hour LC50 in rainbow trout for a liquid formulation of methomyl is 3.4 mg/L and for bluegill sunfish is 0.8 mg/L [10]. The 48-hour LC50 for Daphnia magna (a small, freshwater crustacean) is 0.0287 mg/L [14]. A 28-day fish residue study indicated that methomyl did not accumulate in fish tissue [39]. Methomyl is unlikely to bioconcentrate in aquatic systems [14].
• Effects on other organisms: Methomyl is highly toxic to bees both by direct contact and through ingestion [39]. The LD50 for a 90% pure formulation of methomyl is 11.0 to 22.0 mg/kg in mule deer [22]. Symptoms of acute poisoning in these animals included drowsiness, drooling, diarrhea, and tremors [22].

Environmental Fate:

• Breakdown in soil and groundwater: Methomyl has low persistence in the soil environment, with a reported half-life of approximately 14 days [14]. Because of its high solubility in water, and low affinity for soil binding methomyl may have potential for groundwater contamination [14]. It is very mobile in sandy loam and silty clay loam soils, but only slight leaching was observed in a silt loam and in a sandy soil. Methomyl is rapidly degraded by soil microbes [14]. Methomyl residues are not expected to be found in treated soil after the growing season in which it is applied [14].
• Breakdown in water: Aqueous solutions of methomyl have been reported to decompose more rapidly on aeration, in sunlight, or in alkaline media [14]. The estimated aqueous half-life for the insecticide is 6 days in surface water and over 25 weeks in groundwater [14]. In one experiment, the hydrolysis half-lives of methomyl in solutions at pHs of 6.0, 7.0 and 8.0 were 54, 38, and 20 weeks respectively. In pure water, the hydrolysis half-life has been estimated to be 262 days [14].
• Breakdown in vegetation: Following soil treatment, plants take up methomyl through their roots and move it throughout the plant by a process called "translocation." When methomyl is applied to plants, its residues are short-lived [40]. After it is applied to leaves, it has a 3 to 5 day half-life [10]. Less than 3% methomyl remained in cabbage plants 1 week after they were given foliar treatment with the insecticide [8].

Physical Properties:

• Appearance: Methomyl is a white, crystalline solid with a slight sulfurous odor.
• Chemical Name: S-methyl N-(methylcarbamoyloxy) thioacetimidate [10]
• CAS Number: 16752-77-5
• Molecular Weight: 162.21
• Water Solubility: 57.9 g/l @ 25 C [10]
• Solubility in Other Solvents: v.s. in methanol, acetone, ethanol, and isopropanol [10]
• Melting Point: 79 C [10]
• Vapor Pressure: 6.65 mPa @ 25 C[10]
• Partition Coefficient: Not Available
• Adsorption Coefficient: 72 [13]

Exposure Guidelines:

• ADI: 0.03 mg/kg/day [10]
• MCL: Not Available
• RfD: 0.025 mg/kg/day [20]
• PEL: 2.5 mg/m3 (8-hour) [31]
• HA: 0.2 mg/L [39]
• TLV: Not Available

Basic Manufacturer:

DuPont Agricultural Products
Walker's Mill, Barley Mill Plaza
P.O. Box 80038
Wilmington, DE 19880-0038

• Phone: 800-441-7515
• Emergency: 800-441-3637
  

References:

References for the information in this PIP can be found in Reference List Number 3

DISCLAIMER: The information in this profile does not in any way replace or supersede the information on the pesticide product labeling or other regulatory requirements. Please refer to the pesticide product labeling.