E X T O X N E T
Extension Toxicology Network
A Pesticide Information Project of Cooperative Extension Offices of
Cornell University, Michigan State University, Oregon State University, and
University of California at Davis. Major support and funding was provided
by the USDA/Extension Service/National Agricultural Pesticide Impact
Publication Date: 5/94
TRADE OR OTHER NAMES
Trade names for products containing isofenphos include Amaze, Oftanol
and Pryfon. The insecticide may also be found in formulations with the
Isofenphos is a Restricted Use Pesticide (RUP). Restricted Use
Pesticides may be purchased and applied only by a certified applicator.
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 on vegetables including maize and
carrots, on soil insects in fruit crops like bananas, and on 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 building structures.
Isofenphos is transported to a limited extent from the roots to plant
leaves and stems. In the United States, it is mostly used in the corn-
growing areas of the Ohio Valley.
Isofenphos is a highly toxic insecticide that requires the signal
words DANGER-POISON on its label. Typical of other organophosphate
insecticides, this compound is a cholinesterase inhibitor (8).
Cholinesterase inhibitors are a general group of enzymes that inhibit the
activity of specific cells within the organisms. One typical enzyme,
acetyl cholinesterase, regulates inter-cellular nerve activity.
Although no mention of acute toxic effects were found in the reference
material, acute effects would likely be similar to those from other
organophosphates. These include increased secretions, breathing
difficulty, diarrhea, urination, pupil contraction and slowness of the
heart. At very high doses, convulsions and coma may ensue. Other parts of
the body where cholinesterase is found include the blood stream
(butrylcholinesterase) and the brain. During its use in a Japanese beetle
eradication program in California, no illnesses were found attributable to
the insecticide (8).
In the studies described below, isofenphos was administered in
polyethylene glycol which is also a toxic compound. In rats, isofenphos
has an oral LD50 between 28 to 38 mg/kg. In other species the LD50 for
isofenphos is 91.3 to 127 mg/kg (mice), 150 mg/kg (female rabbits), and
greater than 25 mg/kg (female dogs). All of these values indicate high
acute toxicity (3).
In one study, the dermal LD50 for isofenphos was 188 mg/kg for rats
and for rabbits it was 162 mg/kg when applied over a seven day period (4).
However, in another study, the rat dermal LD50 for a four hour period was
greater than 1,000 ug/kg (2). The compound caused no damage to the skin or
membranes. In rabbits, the acute dermal LD50 ranges from 162 to 315 mg/kg
The rat LC50 is between 0.144 mg/l and 1.3 mg/l (2). The four hour
inhalation LC50 for hamsters was 0.23 mg/l (2). These values indicate that
the compound is highly toxic to organisms which inhale even small amounts
Isofenphos becomes more toxic when it is combined with the insecticide
malathion. This increased effect did not occur when the compound was
combined with fensulfothion, fenamiphos or phoxim.
The primary chronic effect of isofenphos in animals and in humans is
in the suppression of cholinesterase activity in the bloodstream (plasma).
Dietary doses in rats below 0.05 mg/kg and in mice of 0.16 mg/kg/day had no
effect on blood stream (plasma) cholinesterase activity. In dogs, the No
Observable Effect Level (NOEL), based on plasma cholinesterase activity was
also very low. Rats and dogs fed relatively small amounts (1 mg/kg) of
isofenphos for 3 months exhibited no compound-induced dose related effects.
Rats fed low doses of isofenphos for two years showed no compound-related
effects on cholinesterase activity below the 1 mg/kg dose (1).
Isofenphos has the potential to adversely affect the nervous system.
However, the large single dose which produced this effect in hens is
substantially higher than its LD50. Repeated doses of 2 mg/kg for 90 days
did not affect the nerves of the hens (5). Tests on rats, however, showed
evidence of nerve damage due to repeated low-level exposures to some
Female rats fed low to moderate amounts of isofenphos (0.05 to 5.0
mg/kg) through three successive litters exhibited no adverse reproductive
effects at the lowest dose. The female rats experienced a reduction in
their body weight gains at doses above 0.05 mg/kg. At slightly higher
doses (0.5 mg/kg) isofenphos produced a decrease in the pregnancy rate.
The FAO has reported that the no-effect-level in a multi-generation rat
study was 0.5 mg/kg. There was, however, some evidence of abnormalities
associated with toxicity to the developing embryos at the low dose of 0.15
There were no isofenphos related malformations in the offspring of
pregnant rats fed low to moderate amounts (0.3 to 3.0 mg/kg) of this
pesticide during gestation (10). Pregnant rabbits fed isofenphos over a
similar range of doses (1 to 5 mg/kg) during gestation had offspring with
no teratogenic effects (11). For these two species, the compound does not
appear to cause any birth defects at low to moderate levels of exposure.
A number of tests on the mutagenic potential of isofenphos were all
negative (12, 13). It is unlikely that the
compound is mutagenic to humans.
Mice fed moderate to large amounts of isofenphos (up to 5 mg/kg) for
two 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 (2). Rats fed low to high doses of isofenphos (0.05 mg/kg to 5
mg/kg) for two years had decreased body and organ weights though no
presence of tumors (14). Isofenphos has shown no potential for
Three male pesticide applicators working 5 to 11 days (not more than
three consecutive days) had no significant changes in blood cholinesterase
activity when compared to control subjects (8).
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 three days of the initial dosing. Pigs, within one day, eliminated
eighty percent in urine and most of the remaining amount in feces. Cows
had a very similar pattern of excretion over a two day interval. Less than
one percent of the initial dose was detected in the milk (8).
When domestic hens were fed moderate doses of isofenphos (4 mg/kg) for
three days, a significant amount of the compound was eliminated within two
days. Residues of the compound in the tissues and eggs were less than
three percent of the administered dose. The major residues found in
excreta, tissues, and eggs were isofenphos and isopropyl salicylate (8).
Though the compound can be stored in tissues, the concentrations drop
after exposure to the compound ceases. Rats fed isofenphos at high doses
(15 mg/kg/day) for six days had small amounts of the compound in muscle,
liver tissue, fat, and in the kidneys. Within five days after dosing, the
concentration in these tissues had fallen to very low levels (8). Cows had
a peak blood plasma level two hours after the dose and these levels fell
sharply within a day.
Effects on Birds
The wide range of toxicity for isofenphos in birds makes any general
characterization of its toxicity to this group of animals difficult.
Bobwhite quail were fed isofenphos in their diet for five days and
this was followed by three days of no isofenphos. Their LC50 in this study
was 145 ppm. Studies indicated a dietary LC50 of 299 ppm for Japanese
quail and an eight-day LD50 for Japanese quail from 5.0 to 12.5 mg/kg (1).
For white Leghorn hens, the LD50 ranges from 3 and 6 mg/kg (5). These
values indicate that the compound is highly toxic to quail and to hens.
Red-winged blackbirds are also susceptible to isofenphos (6). Male
blackbirds were fed granules containing 15% active ingredient of
isofenphos. Forty percent of the blackbirds died after eating 20 granules
and none after eating 5 or 10 granules (6).
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 96-
hour LC50 in goldfish is 2 mg/l. In carp the LC50 ranged between 2 mg/l
and 4 mg/l and for rainbow trout the LC50 is 1.8 mg/l (1). There are some
possible effects on aquatic protozoa at concentrations exceeding 20 mg/l
The potential of the compound to significantly bioaccumulate, as
predicted from its solubility in water, is relatively low.
Effects on Other Organisms (Nontarget species)
No other information is available.
Breakdown of Chemical in Soil and Groundwater
Isofenphos is moderately persistent in soil. 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 (7).
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. This first step is relatively slow.
Degradation was not greatly influenced by the addition of organic matter to
the soil (7). The evaporation of breakdown products account for a
substantial portion of the loss of these residues from the soil.
Isofenphos exposed to sunlight has a half-life of 72 days in sandy
loam soil (9).
Breakdown of Chemical in Surface Water
The maximum concentrations expected as the result of runoff following
a field application of the compound would be very low in water (0.00007
mg/l) and in sediment (0.00004 mg/l) (9).
Isofenphos is relatively stable in acidic and neutral solutions. Half
of the compound dissolved in 263 days at pH 2 and in 525 days at pH 7. The
half-life is much shorter in alkaline solutions, 52 hours at pH 11.5 (1).
Breakdown in Vegetation
Plants, such as corn and onions, absorb isofenphos from the soil and
convert 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 nine 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
PHYSICAL PROPERTIES AND GUIDELINES
Isofenphos is a colorless oil, and the technical grade active
ingredient is a yellow-brown liquid.
|ADI: ||0.001 mg/kg (WHO)
|CAS #: ||25311-71-1
|Chemical name: ||0-ethyl 0-2-isopropoxycar-bonylphenylisopropylphosphoramid-othioate
|Molecular weight: ||345.40 g/mol
|Solubility in water: ||24 mg/l
|Solubility in solvents: ||cyclohexanone, toluene > 600g/kg
|Melting point: ||< -12 degrees C
|Vapor pressure: ||0.53 mPa at 20 degrees C
|Partition coefficient (octanol/water) (log): ||4.04
|Adsorption coefficient: ||600
|Chemical class/use: ||organophosphate insecticide
Agricultural Chemicals Division
8400 Hawthorn Road
P.O. Box 4913
Kansas City, MO 64120
Telephone: (816) 242-2000
Emergency: (816) 242-2582
FAX: (816) 242-2592
Review by Basic Manufacturer:
Comments solicited: April, 1993
Comments received: June, 1993
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Food and Agriculture Organization of the United Nations. (1981).
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Smith, Gregory J. (1993). Pesticide Use and Toxicology in Relation
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Wilson, B.W., M. Hooper, E. Chow, R.J. Higgins and J.B. Knaak.
(1984). Antidotes and Neuropathic Potential of Isofenphos. Bulletin of
Environmental Contamination and Toxicology, 33: 386-394.
Balcomb, R., R. Stevens and C. Bowen. (1984). Toxicity of 16
Granular Insecticides to Wild-caught Songbirds. Bulletin of Environmental
Contamination and Toxicology, 33: 302-307.
Somasundaram, L., K.D. Racke and J.R. Coats. (1987). Effects of
Manuring on the Persistence and Degradation of Soil Insecticides. Bulletin
Environ. Toxicol., 39: 579-586.
Broadberg, R.K. (1990). Estimation of Exposure of Persons in
California to Pesticide Products Containing Isofenphos. California
Department of Food and Agriculture. Division of Pest Management,
Environmental Protection and Worker Safety. Sacramento, CA.
Environmental Fate and Effects Division. U.S. Environmental
Protection Agency. (1990). Pesticide Environmental Fate One Line Summary
Bayer AG Institute fur Toxicologie. (1977). Rat Teratology study.
Unpublished study #35482. 10/25/77. EPA Accession No. 096657.
Bayer AG Institute fur Toxicologie. (1977). Rabbit Teratology study.
Unpublished study #46931. 11/10/75. EPA Accession No. 096657.
Bayer AG Institute fur Toxicologie. (1977). Mice Mutagenicity study.
Unpublished study #39567. 11/07/77. EPA Accession No. 096657.
Oesch, F. (1977). Ames Mutagenicity test. Unpublished study #53954.
9/29/77. EPA Accession No. 096657.
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