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The information in this profile may be out-of-date.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

Methoxychlor

Trade and Other Names: Trade names for methoxychlor include Chemform, Dimethoxy-DT, DMDT, ENT 1716, Higalmetox, Methoxychlore, Marlate, Methoxy-DDT, OMS 466 and Prentox.

Regulatory Status: Methoxychlor is a practically nontoxic compound in EPA toxicity class IV. It is a General Use Pesticide (GUP), and labels for products containing it must bear the Signal Word CAUTION.

Chemical Class: organochlorine

Introduction: Methoxychlor is an organochlorine insecticide effective against a wide range of pests encountered in agriculture, households, and ornamental plantings. It is registered for use on fruits, vegetables, forage crops, and in forestry. Methoxychlor is also registered for veterinary use to kill parasites on dairy and beef cattle.

Methoxychlor is one of a few organochlorine pesticides that have seen an increase in use since the ban on DDT in 1972. It is quite similar in structure to DDT, but has relatively low toxicity and relatively short persistence in biological systems. It is available in wettable and dustable powders, emulsifiable conentrates, granules, and an aerosol. It may be found in formulations with malathion, parathion, piperonyl butoxide, and pyrethrins.

Formulation: It is available in wettable and dustable powders, emusifiable concetnrates, granules, and as an aerosol. It may be found in formulations with malathion, parathion, piperonyl butoxide, and pyrethrins.

Toxicological Effects:

• Acute toxicity: Methoxychlor is practically nontoxic via the oral route, with reported oral LD50 values of 5000 to 6000 mg/kg in rats [2,9], 1850 mg/kg in mice and 2000 mg/kg in hamsters [2]. The lowest oral dose that can cause lethal effects in humans is estimated to be 6400 mg/kg, and the lowest dose through the skin that produces toxic effects in humans is 2400 mg/kg based on behavioral symptoms [17]. It is reportedly slightly to practically nontoxic dermally, with a reported dermal LD50 in rabbits of greater than 2000 mg/kg [9]. Symptoms of high acute exposure include central nervous system depression, progressive weakness, and diarrhea [2]. Extremely high doses can cause death within 36 to 48 hours.
• Chronic toxicity: Rats fed methoxychlor at doses of 500 mg/kg/day for 2 years showed practically no weight gain, but this was attributed to refusal of food rather than any toxic effects of the compound [2,17]. At doses of 1500 mg/kg/day in rats, severe reductions in weight appeared, and most animals died within 45 days [2]. Rabbits were more susceptible than rats; doses of 200 mg/kg/day were fatal in most cases within 15 days [2]. Data from experiments in dogs are contradictory; dogs experienced weight loss at approximately 25 mg/kg/day over 6 months, and doses of 50 mg/kg/day caused convulsions and subsequent death in some animals within 9 weeks [2]. In other studies, dogs fed up to 300 mg/kg/day in the diet for 1 year, and about 63 mg/kg/day by stomach tube for 5 months showed no signs of injury or observable effects [2]. Massive doses in swine, rats, and monkeys produced pathological changes in liver, kidney, mammary glands, and uteri. Other data suggest that the liver effects in rats may be temporary, and one study showed no effects on the liver in rats [2]. Human volunteers taking oral doses up to 2.0 mg/kg/day for 8 weeks showed no detectable effects on overall health, blood and enzyme biochemistry and no observable changes in bone marrow, liver, small intestine, or testes [2].
• Reproductive effects: Available evidence suggests that high doses of technical methoxychlor (88 to 90% pure) or its metabolites may have estrogenic or reproductive effects [2]. In rats, dietary doses of about 125 mg/kg/day reduced mating, and many did not produce litters [2]. Rats fed doses of about 50 mg/kg/day had normal fertility and fecundity, but their offspring had abnormal reproductive functioning [2]. Male and female weanling rats fed methoxychlor through puberty and mating had normal fertility overall, but female rats had reduced fertility when paired with untreated males [2]. In mice, 200 mg/kg/day administered on days 6 to 15 of pregnancy decreased fertility and birthweight [2]. Testicular atrophy was observed in rats at levels of approximately 500 mg/kg/day over an unspecified period, but not in dogs at doses of 100 mg/kg/day [2]. Wistar rats given 100 mg/kg/day for 14 (females) to 70 (males) days showed pathological changes in reproductive tissues [2]. It is unlikely that methoxychlor will cause reproductive effects in humans at expected exposure levels.
• Teratogenic effects: In mice, 200 mg/kg/day administered on days 6 to 15 of pregnancy resulted in delayed ossification and wavy ribs in the offspring [2]. When a methoxychlor formulation containing 50% active ingredient and 50% unknown compounds was administered to pregnant female rats, adverse effects on the fetus occurred only at doses large enough to be toxic to the dams [65]. At 400 mg/kg/day, the pesticide killed rat embryos [65]. These suggest that teratogenic effects in humans are unlikely under normal conditions.
• Mutagenic effects: Most mutation assays have proven to be negative [65]. There is no convincing evidence that methoxychlor is toxic to genetic material.
• Carcinogenic effects: Tumor incidence was statistically similar in unexposed rats and those given as much as 80 mg/kg/day over 2 years [2]. Dogs given about 250 mg/kg/day over an unspecified period did not show evidence of tumors [17]. Two strains of mice were fed diets containing up to approximately 90 mg/kg/day methoxychlor for 2 years showed no significant incidence of liver tumors, but one strain did have increased testicular tumors [17]. In rats, about 25 mg/kg/day produced slight increases in liver cancers. The data suggest that methoxychlor is unlikely to show carcinogenic activity in humans .
• Organ toxicity: Central nervous system depression occurs with acute exposure; data from animals studies indicate that target organs for methoxychlor include the kidneys, liver, mammary glands, and uterus.

• Fate in humans and animals: Available evidence suggests that methoxychlor does not accumulate to any significant degree in fat or other tissues of mammals. At high dietary doses in rats, storage was minimal over the 18-week course of the study, and nondetectable within 2 weeks after the study [2]. Mice excreted 98.3% of a single oral 50 mg/kg dose in urine and feces within 24 hours [2]. The major metabolites in mouse feces and urine were the monophenol and bisphenol [2,9]. Other metabolites (e.g., dihydroxybenzophenone) were also present, but are not the primary metabolites [2,9]. These compounds are typically eliminated in a conjugated form (i.e., bound to an innocuous molecule, such as glutathione, sulfate, etc.) [2]. It is thought that these metabolites may form reactive intermediates if not successfully conjugated and eliminated [2]. Lactating cows treated twice in 14 days with sprays of 0.25 to 0.5% methoxychlor (2 quarts per animal) had residues of 2 to 3 ppm in milk. After 14 days, levels were at the limit of detection (0.005 ppm) [17].

Ecological Effects:

• Effects on birds: Methoxychlor is slightly toxic to bird species, with reported acute oral LD50 values of greater than 2000 mg/kg in the mallard duck, sharp-tailed grouse, and California quail [53]. The reported 5-day dietary LC50 in Japanese quail is greater than 5000 ppm. Reported 8-day LC50 values are greater than 5000 ppm in bobwhite quail and ring-necked pheasants [9]. Dietary levels of as high as about 145 mg/kg/day had no effects on reproductive function of male and female chickens over 8 to 16 weeks [2].
• Effects on aquatic organisms: Methoxychlor is very highly toxic to fish and aquatic invertebrates. Reported 96-hour LC50 values (for the technical grade material, ca. 90% pure) are less than 20 ug/L for cutthroat trout, atlantic salmon, brook trout, lake trout, northern pike, and large mouth bass [55]. Reported LC50 values are between 20 and 65 ug/L in rainbow trout, goldfish, fathead minnow, channel catfish, bluegill, and yellow perch [55]. Aquatic invertebrates with 96- or 48-hour LC50 values of less than 0.1 mg/L include Daphnia, scuds, sideswimmers, and stoneflies [55]. Predicted bioconcentration factors were the highest in the mussel (12,000) and in the snail (8570) [66]. This indicates that methoxychlor would accumulate in aquatic organisms that do not rapidly metabolize the compound. Practically no metabolism was seen in Daphnia or mayflies [55]. Fish reportedly break down methoxychlor fairly rapidly and thus tend not to accumulate it appreciably [12], but this may vary according to species and/or life-stage. No magnification of residues was observed in largemouth bass fingerlings fed contaminated Daphnia, and no evidence of metabolism was seen in rainbow trout [55].
• Effects on other organisms: The compound is nontoxic to bees [9].

Environmental Fate:

• Breakdown in soil and groundwater: Methoxychlor is very persistent in soil, with a reported representative half-life of approximately 120 days [14]. However, rates may be as fast as 1 week in some instances. Methoxychlor degrades much more rapidly in soil that has a supply of oxygen (aerobic) than in soil without oxygen (anaerobic). Methoxychlor is tightly bound to soil and is insoluble in water, so it is not expected to very mobile in moist soils [9,14]. Actual mobility will depend on site-specific factors (e.g., soil organic matter and rainfall). The risk to groundwater should be slight, but may be greater if application rates are very high, or the water table is very shallow [17]. Movement of the pesticide is more likely via adsorption to suspended soil particles in runoff. In the EPA pilot groundwater survey, methoxychlor was found in a number of wells in New Jersey (not quantified) and at extremely low concentrations (from 0.1 to 1.0 ng/L, or ppt) in water from the Niagara River, the James River, and a Lake Michigan tributary [65].
• Breakdown in water: Methoxychlor is practically insoluble in water, and so will most likely reach surface waters via runoff as described above. In hydrosoils (sediments in an aquatic environment), degradation of methoxychlor to methoxychlor olefin (MDE) occured only under aerobic conditions [55]. In open water the major products of breakdown in a neutral solution are anisoin, anisil, and p,p-dimethoxydichloroethene (DMDE). The half-life in distilled water is 37 to 46 days but in some river waters the half-life may be as rapid as 2 to 5 hours [9,17]. Methoxychlor evaporates very slowly, but the evaporation may contribute to the cycling of the product in the environment [12].
• Breakdown in vegetation: On mature soybean foliage, the washoff rate was 8% per cm of rainfall, with a total of 33.5% washoff for a season [17]. Dislodgeable residues account for less than 1% of the amount applied.

Physical Properties:

• Appearance: Pure methoxychlor is a colorless crystalline solid; technical methoxychlor (88 to 90% pure) is a grey powder [9].
• Chemical Name: 1,1,1-trichloro-2,2-bis(4-methoxyphenyl)ethane [9]
• CAS Number: 72-43-5
• Molecular Weight: 345.65
• Water Solubility: 0.1 mg/L @ 25 C [9]
• Solubility in Other Solvents: v.s. in most organic solvents [9]
• Melting Point: 77 C (technical product) [9]
• Vapor Pressure: Very low [9]
• Log P: Not Available
• Partition Coefficient: 80,000 [14]

Exposure Guidelines:

• ADI: 0.1 mg/kg/day [27]
• MCL: 0.04 mg/L [8]
• RFD: 0.005 mg/kg/day [8]
• PEL: Not Available
• HA: Not Available
• TLV: 10.0 mg/m3 (8-hour) [56]

Basic Manufacturer:

Drexel Chemical Company

1700 Channel Avenue

Memphis, TN 38113

• Phone: 901-774-4370
• Emergency: Not Available
  

References:

References for the information in this PIP can be found in Reference List Number 6

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.