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The information in this profile may be out-of-date.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

Isofenphos

Trade and Other Names: Trade names for products containing isofenphos include Amaze, Oftanol and Pryfon.

Regulatory Status: Isofenphos is a highly toxic insecticide in EPA toxicity class I. Products containing it must bear the Signal Words DANGER - POISON on their labels. Isofenphos is a Restricted Use Pesticide (RUP), which may be purchased and applied only by certified applicators.

Chemical Class: organophospate

Introduction: Isofenphos is an organophosphate insecticide used to control soil-dwelling insects such as white grubs, cabbage root flies, corn roundworms, and wireworms. The product is used in vegetables including maize and carrots, on soil insects in fruit crops like bananas, and in soils with turfgrass. It is a selective contact and stomach poison in insects. It is applied as a preplant or preemergence soil treatment. It is also used to control termites in and around structures. The insecticide may also be found in formulations with the fungicide thiram. It is available in emulsifiable concentrate, dry seed treatment, granular, and wettable powder formulations.

Formulation: It is available in emulsifiable concentrate, dry seed treatment, granular, and wettable powder formulations.

Toxicological Effects:

• Acute toxicity: Isofenphos is highly toxic via the oral route, with a reported acute oral LD50 of 28 to 38 mg/kg in rats and 91.3 to 127 mg/kg in mice [8,13]. It is moderately toxic via the dermal route, with a reported dermal LD50 of 188 mg/kg in rats [13,41]. The acute dermal LD50 in rabbits ranges from 162 to 315 mg/kg [8,13]. The compound causes no damage to the skin or mucous membranes in rats. It is highly toxic via the inhalation route, with the reported airborne LC50s of 1.3 mg/L and 0.144 mg/L in rats [8,13]. The hamster 4-hour inhalation LC50 is 0.23 mg/L [8]. Typical of other organophosphate insecticides, this compound is a cholinesterase inhibitor. Effects that may occur from exposure include: increased secretions, difficulty in breathing, diarrhea, urination, pupil contraction, and slowness of the heart [74]. At very high doses, convulsions and coma may ensue. Isofenphos may be more toxic when it is combined with the insecticide malathion [74].
• Chronic toxicity: The primary chronic effect of isofenphos in animals and in humans is suppression of cholinesterase activity. Doses in the diets of rats and mice below 0.05 mg/kg/day had no effect on blood stream (plasma) cholinesterase activity. Rats and dogs fed 1 mg/kg/day of isofenphos for 3 months exhibited no compound-related effects. Rats fed low doses of isofenphos for 2 years showed no effects on cholinesterase activity related to the compound below the 1 mg/kg/day dose [8]. Repeated doses at 2 mg/kg/day for 90 days did not affect the nerves of the hens. Other tests on rats, however, showed evidence of nerve damage after repeated low-level exposures to some organophosphates [75].
• Reproductive effects: Female rats fed isofenphos at doses of 0.05 to 5.0 mg/kg/day through three successive litters exhibited no adverse effects on reproduction at the lowest dose, but maternal body weight reductions occurred doses above 0.05 mg/kg/day. At 0.5 mg/kg/day, isofenphos produced decreases in reproductive success, in some studies but not others [8]. However, some evidence of abnormalities associated with toxicity to the developing embryos were reported at 0.15 mg/kg/day [8]. The available data on reproductive effects due to isofenphos are inconclusive, but suggest that such effects are unlikely in humans at expected exposure levels.
• Teratogenic effects: Pregnant rats fed low to moderate amounts (0.3 to 3 mg/kg/day) of isofenphos during gestation had no compound induced malformations in their offspring [8]. Pregnant rabbits fed isofenphos over a range of doses 1 to 5 mg/kg/day during gestation had offspring with no teratogenic effects [8]. These data indicate that isofenphos is not teratogenic.
• Mutagenic effects: A number of tests on the mutagenic potential of isofenphos were all negative [8,74]. Additional tests using concentrations as high as 3.15 ml/plate confirmed the nonmutagenicity of isofenphos [8,74].
• Carcinogenic effects: Mice fed isofenphos at doses of up to 5 mg/kg/day for 2 years in their food had no dose-related increases in tumors. There was however, high mortality in all of the groups in the study, including the controls [74]. Rats fed low to high doses of isofenphos (0.05 mg/kg/day to 5 mg/kg/day) for 2 years had no treatment-related alterations in thirty different tissues examined [8,74]. The evidence suggests that isofenphos is not carcinogenic.
• Organ toxicity: Target organs affected by isofenphos exposure include the central and peripheral nervous systems and the blood.

• Fate in humans and animals: Rats, pigs, and cows all eliminate isofenphos rapidly. Rats excreted nearly all of the compound in urine and the remaining small amount in feces within 3 days of the initial dosing. Pigs eliminated 80% in urine and most of the remaining amount in feces within a day. Cows had a very similar pattern of excretion over a 2-day interval. Less than 1% of the initial dose was detected in the milk [8]. When domestic hens were fed moderate doses of isofenphos (4 mg/kg/day) for 3 days, a significant amount of the compound was eliminated within 2 days. Residues of the compound in the tissues and eggs were less than 3% of the administered dose. The major residues found in excreta, tissues, and eggs were isofenphos and isopropyl salicylate [8]. Though the compound can be temporarily stored in tissues, the concentrations drop after exposure to the compound ceases. Rats fed isofenphos at high doses (15 mg/kg/day) for 6 days had small amounts of the compound in muscle, liver tissue, fat, and in the kidneys. Within 5 days after dosing, the concentration in these tissues had fallen to very low levels [15]. Cows had a peak blood plasma level 2 hours after the dose. Levels fell sharply within a day.

Ecological Effects:

• Effects on birds: Isofenphos is highly toxic to birds. The reported 5-day dietary LC50 in Japanese quail is 299 ppm [6,14]. In northern bobwhite quail, the reported acute LD50 is 13 to 19 mg/kg [6]. For hens, the LD50 ranges between 6 and 20 mg/kg. Red-winged blackbirds are also susceptible to isofenphos [66]. Laboratory studies on blackbirds and the death of over 100 birds linked to the application of the product Oftanol, support its toxicity to this species.
• Effects on aquatic organisms: Isofenphos is moderately toxic to fish. The LC50 values of the compound for various species of fish are relatively consistent. The reported 96-hour LD50 values are 2 mg/L in goldfish, 2 to 4 mg/L in carp, 1 to 2 mg/L in orfe, and 1 mg/L in rudd [13]. Studies indicate possible adverse effects on aquatic protozoa at concentrations exceeding 20 mg/L [76]. The potential of the compound to significantly bioaccumulate is relatively low. When channel catfish were exposed to 0.01 mg/L for a month residues accumulated to 75 times the water concentration within the first 7 days and only slowly declined during treatment period. After the end of treatment, fish tissue residues decreased by 87% within the first day and by 96% within 10 days. All of the extractable tissue residue was isofenphos [76].
• Effects on other organisms: No data are currently available.

Environmental Fate:

• Breakdown in soil and groundwater: Isofenphos is moderately to highly persistent in the soil environment. At a normal field-use rate, a single application of isofenphos had a half-life of 30 to 300 days, with a typical time of 150 days [19]. Isofenphos is poorly bound to soils, but has a low water solubility [19]. Isofenphos slowly leaches in all soils and the breakdown products leach more rapidly. The first step in the breakdown of the compound requires the presence of oxygen (oxidation). The products of this process are isopropyl salicylate and cyclic isofenphos. Degradation was not greatly influenced by the addition of organic matter to the soil [77]. The evaporation of breakdown products accounts for a substantial portion of the loss of these residues from the soil.
• Breakdown in water: The maximum concentrations expected as the result of runoff following a field application of the compound would be very low in water (0.07 mg/L) and in sediment (0.04 mg/L) [76]. The rate of its chemical breakdown in the presence of water (hydrolysis) is markedly increased by acidic or basic conditions and by increases in temperature. Its breakdown in the presence of sunlight is very slow.
• Breakdown in vegetation: Plants, such as corn and onions, absorb isofenphos from the soil and, in the case of corn, translocate it to plant leaves and stems. Plants generally transform the compound to its oxygen analog. The oxygen analog is then moved within the plant to the upper plant parts where further changes occur. Cereal grains, leafy and root vegetables, and edible oil crops grown in soil treated 9 months earlier can take up the residues of the insecticide even when it is present at very low concentrations (0.005 mg/kg). The principal residues found in plants are isofenphos and an isofenphos-oxygen analog [8].

Physical Properties:

• Appearance: Isofenphos is a colorless oil at room temperature [13].
• Chemical Name: O-ethyl O-2-isopropoxycarbonylphenyl isopropylphosphoramidothioate [13]
• CAS Number: 25311-71-1
• Molecular Weight: 345.40
• Water Solubility: 24 mg/L @ 20 C [13]
• Solubility in Other Solvents: s. in cyclohexone, toluene, acetone, and diethyl ether [13]
• Melting Point: Less than -12 C [13]
• Vapor Pressure: 0.53 mPa @ 20 C [13]
• Partition Coefficient: Not Available
• Adsorption Coefficient: 600 [19]

Exposure Guidelines:

• ADI: 0.0001 mg/kg/day [38]
• MCL: Not Available
• RfD: Not Available
• PEL: Not Available
• HA: Not Available
• TLV: Not Available

Basic Manufacturer:

Miles, Inc.
8400 Hawthorn Road
P.O. Box 4913
Kansas City, MO 64120

• Phone: 816-242-2429
• Emergency: 800-242-2582
  

References:

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 labeling or other regulatory requirements. Please refer to the pesticide product labeling.