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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 Assessment Program.


Publication Date: 9/93


Trade names include Difonate, Dy-fonate, Dyphonate, and Stauffer N 2790.


Fonofos is a soil organophosphate insecticide primarily used on corn. It is also used on sugar cane, peanuts, tobacco, turf, and some vegetable crops. It controls aphids, corn borer, corn rootworm, corn wireworm, cutworms, white grubs, and some maggots.

Some or all formulations of fonofos are classified as Restricted Use Pesticides (RUP) by the EPA. Restricted Use Pesticides may be purchased and used only by certified applicators.



This highly toxic chemical interferes with the nervous system by inhibiting an enzyme, cholinesterase, and is labeled with a DANGER signal word. Symptoms of fonofos exposure may be delayed from a few minutes to up to twelve hours after exposure. Early symptoms include blurred vision, headache, and dizziness. Skin contact often brings about sweating and muscle twitching. Eye contact causes tearing, pain, and blurring. Ingestion may cause nausea, abdominal cramps, and diarrhea (6). Deaths resulting from high exposures are often due to respiratory arrest.

The oral LD50 in male rats ranges from 6.8 to 18.5 (3) while it ranges from 3.2 to 7.9 mg/kg in female rats (10). The dermal LD50 of fonofos is 25 mg/kg in female rabbits, 147 mg/kg in rats, and 278 mg/kg in guinea pigs (6).


Fonofos can be absorbed through the skin, gastrointestinal tract, and respiratory tract. Most occupational poisonings have resulted from absorption through the skin and by inhalation.

Cholinesterase inhibition may persist for as long as three months after chronic exposure. Symptoms are similar to those resulting from acute exposures. Over 21 human poisonings by fonofos have been recorded. One woman exposed orally to a large amount of fonofos, developed nausea, sweating, and cardiorespiratory stoppage. She also had muscle twitching, low blood pressure and pulse rate, and pinpoint pupils. She recovered after two months of hospitalization.

Dietary feeding of fonofos at low levels to dogs for 14 weeks produced no effects at or below the very low level of 0.20 mg/kg (13).

Reproductive Effects

A long-term reproduction test in rats showed no effects on female reproductive ability at high doses about 2.0 mg/kg) of fonofos (9, 10).

Teratogenic Effects

Pregnant mice were fed very high doses of fonofos during the sensitive period of gestation. At these levels, some abnormal bone development and brain changes were observed in the fetuses (10).

Mutagenic Effects

Fonofos was not mutagenic in five microbial assays nor in a human DNA synthesis test (10). Thus, it is unlikely to cause to cause mutations in humans.

Carcinogenic Effects

Male and female rats which ingested very high daily doses (relative to the LD50) of fonofos for two years showed no cancerous effects (10). However no carcinogen status has been officially established for fonofos.

Organ Toxicity

Fonofos exposure may affect the eyes, respiratory system, and central nervous system. A fonofos product, Dyfonate, is broken down in rat livers into several metabolites, one of which is a potent cholinesterase inhibitor known as an oxon (5).

Fonofos and its metabolites are mostly absorbed from the stomach, detoxified in the liver, and then excreted (4). Other than the fonofos oxon (an anticholinesterase metabolite), fonofos' breakdown products are less toxic than the parent compound (3).

Fate in Animals and Humans

Fonofos is quickly excreted in animals. Ninety-six hours after rats received a single high oral dose of fonofos, three quarters of the dose was found in the urine, a third in the feces, a small amount in expired air, and only a trace in the tissues (4). The small amount of residues found in storage organs and tissues was eliminated within two to 16 days after exposure (10). In another study, rats given nearly pure fonofos excreted almost all of it within four days (10).


Fonofos is highly toxic to birds, freshwater fish, and salt water organisms. The metabolites of fonofos are less toxic to these organisms than is the parent material. The LD50 for fonofos in mallard ducks is 128 mg/kg. Its LC50 in bluegill sunfish is 0.028 mg/l, and 0.05 mg/l in trout.

Organophosphates, such as fonofos, do not bioaccumulate in the environment or in animals (11). Although it is practically insoluble in water, it is quickly broken down by interaction with water.

Fonofos is toxic to bees (13).


Fonofos is rapidly metabolized in plant tissues to non-toxic compounds which, when ingested by an animal, are readily excreted (8).

Moderately persistent, fonofos has a soil half-life of a few months in general (7). On a silty clay loam, the half-life was 82 days after a high level of application, while at lower application rates, the half- life was 46 days (2). It can be transported in runoff primarily in sediment. It is immobile in sandy loam and silt loam soils, but is mobile in quartz sand. Soil microbes, such as fungi, rapidly degrade fonofos (10).

Fonofos resists soil leaching, and has been detected only rarely in ground water and then at very low levels. Fonofos has been detected in California groundwater at 0.01 to 0.03 ppb, and in Iowa groundwater at 0.1 ppb (10).

The compound is not readily absorbed by plant foliage and it is not translocated throughout plants (13).


Fonofos is a relatively non-reactive insecticide. This organophosphate is a yellow liquid with a disagreeable, sulfurous odor. Its molecular weight is 246.32. Its chemical name is O-ethyl-S-phenyl ethylphosphonodithioate.

Exposure Guidelines:

NOEL: 0.2 mg/kg/day (10)
ADI: 0.002 mg/kg/day (10)
TLV: 0.1 mg/m3 (air); 100 ug/m3(skin) (1)
Drinking water
health advisory:
Drinking Water Equivalent Level (DWEL): 0.07 ug/L (10)

Physical Properties:

CAS #: 944-22-9
Solubility in water: almost insoluble (12)
Solubility in solvents: miscible in organic solvents like kerosene, xylene, and isobutyl methyl ketone.
Melting point: 32 degrees C at 0.3 mm Hg (9).
Boiling point: 130 degrees C at .1 mm Hg (3).
Vapor pressure: 2.1 x 10 to the minus 4 power mm Hg at 25 degrees C (3)


Zeneca Ag Products
Wilmington, Delaware 19897
Telephone: 800-323-8633

Review by Basic Manufacturer:

Comments solicited: November, 1992
Comments received:


  1. American Conference of Governmental Industrial Hygienists. Threshold Limit Values for Chemical Substances. 1984-85.
  2. Ahmad, N., Walgenbach, D.D. and Sutter, G.R. 1979. Bull. Environm. Contam. Toxicol. 23, 423-429.
  3. Hayes, W.J. 1982. Pesticides Studied in Man. pp. 425-427. Williams & Wilkins.
  4. Hoffman, L.J., Ford, I.M., and Menn, J.J. 1971. Dyfonate metabolism studies. I. Absorption, distribution, and excretion of Dyfonate in rats. Pesticide Biochem. and Physiol. 1: 349-355.
  5. McBain, J.B., Yamamoto, I., and Casida, J.E. 1971. Mechanism of Activation and deactivation of dyfonate by rat liver microsomes. Life Sci. 10: 947-954. Pergamon Press.
  6. Occupational Health Services Inc. Material Safety Data Sheet: Fonofos. 4/10/87. OHS. NY.
  7. Rao, P.S.C. and Davidson, J.M. 1980. Estimation of pesticide retention and transformation parameters required in nonpoint source pollution models. In Environmental Impact of Nonpoint Source Pollution. Overcash, M.R. and Davidson, J.M., eds. Ann Arbor Science.
  8. Stauffer Chemical's Product Safety Information Booklet. 1986. Fonofos. pp. 22-27.
  9. U.S. Environmental Protection Agency. "Chemical Fact Sheet for Fonofos." Fact Sheet No. 36. 30 September 1984.
  10. U.S. Environmental Protection Agency. Office of Drinking Water. "Fonofos Health Advisory." Draft Report. August 1987.
  11. Wagner, S.L. 1983. Clinical Toxicology of Agricultural Chemicals. Noyes Data Corp. Park Ridge, NJ.
  12. Windholz, M. et al., eds. 1983. Fonofos. p. 604. The Merck Index. 10th edition. Merck & Co. Inc.
  13. ACGIH. 1992. Documentation of the Threshold Limit Values and Biological Exposure Indices, 6th edition. American Conference of Governmental and Industrial Hygienists, Inc. Cincinnati, OH.