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Benomyl

Publication Date: 5/94

TRADE OR OTHER NAMES

Commercial names for products containing benomyl are Agrodit, Benex, Benlate, Benosan, Fundazol, Fungidice 1991, and Tersan. Benomyl is compatible with many other pesticides (7).

REGULATORY STATUS

Benomyl is a General Use Pesticide (GUP).

INTRODUCTION

Benomyl is a systemic, benzimidazole fungicide that is selectively toxic to microorganisms and to invertebrates, especially earthworms. It is used against a wide range of fungal diseases of field crops, fruits, nuts, ornamentals, mushrooms, and turf. The World Health Organization gives benomyl a Class 0 rating which means it is unlikely to present an acute hazard during normal use (1).

TOXICOLOGICAL EFFECTS

ACUTE TOXICITY

Benomyl containing products carry the signal word CAUTION, indicating that it has a low acute toxicity. Benomyl is of such a low acute toxicity to mammals, it has been impossible or impractical to administer doses large enough to firmly establish an LD50. Thus the LD50 is greater than 10,000 mg/kg/day for rats and greater than 3400 mg/kg for rabbits (using a 50% wettable powder formulation), however, skin irritation may occur for workers exposed to benomyl. Because of its high LD50 there is a low risk for acute poisoning from this compound (12).

Some individuals are more sensitive to this compound than others. Skin reactions are seen in rats, guinea pigs and humans. Most organisms can become sensitized to the compound as well. Benomyl is readily absorbed into the body by inhaling the dust, but there are no reports of toxic effects to humans by this route of exposure. The inhalation LC50 for rats ranges from 0.82 mg/l (using 50% Benlate fungicide) to 4.01 mg/l for Fungicide 1991 (50% benomyl) (12).

CHRONIC TOXICITY

Rats fed benomyl from very low doses to very high doses in their diet for three weeks showed no immunological changes. When rats were fed diets containing about 150 mg/kg/day for two years, no toxic effects were observed (11).

Dogs fed benomyl in their diets for three months had no major toxic effects but did show evidence of altered liver function at the highest dose (150 mg/kg). The damage progressed to more severely impaired liver function and liver cirrhosis after two years (3).

Reproductive Effects

A three-generation study on rats showed no reproductive or lactational differences at low doses (150 mg/kg/day) administered in the diet (11). In another rat study where mothers were fed 1000 mg/kg/day for four months, the offspring showed a decrease in viability and fertility (12).

In rats the testes were the most affected sites at relatively low doses (about 15 mg/kg). Male rats had decreased sperm counts, decreased testicular weight, and lower fertility rates. The animals recovered from these effects 70 days after feeding with the pesticide had stopped (1).

Teratogenic Effects

Benomyl can cause birth defects in test animals (10). However, the route of exposure makes a difference in the teratogenicity of benomyl. Rats fed low amounts (150 mg/kg/day) in the diet for three generations showed no birth defects. No teratogenicity was observed in another study of rats given 300 mg/kg/day on days 6 through 15 (10). At higher doses, some birth defects were noted, but they were accompanied by toxicity to the fetus. Benomyl administered to rats by stomach tube (up to 62.5 mg/kg/day) on days 7 through 16 of pregnancy produced birth defects in the young (10).

Mutagenic Effects

There are conflicting negative and positive results in numerous mutagenicity assays on several different types of animals. As a result, no conclusions can be drawn (11).

Carcinogenic Effects

Tumors in the livers of both male and female mice were observed in lifetime studies at doses of about 40 to 400 mg/kg/day. The metabolite carbendazim (MBC) was responsible for irregular cell growths (carcinomas) in glands (5).

In a two-year dietary study of benomyl, albino rats were fed between 0 to 2,500 mg/kg/day of benomyl. There were no significant adverse effects at any dose level attributable to benomyl (12).

As a result of conflicting studies, the U.S. Environmental Protection Agency classified benomyl as a possible human carcinogen (5).

Fate in Humans and Animals

Benomyl's metabolism has been studied in the mouse, rat, rabbit, dog, sheep, and cow. Benomyl is rapidly broken down to carbendazim (MBC), further to other compounds, such as 5-HBC, and then eliminated. In a rat study, benomyl, carbendazim (MBC) and 5-HBC were found in rat blood in the first 6 hours. After 18 hours, only 5-HBC was present. The urine contained about 40-70% of the dose, the feces 20-45%. No residues were found in muscle or fat. Benomyl and its metabolites do not accumulate in tissues over long term exposure periods (7, 11). Carbendazim (MBC) and the parent compound benomyl have similar toxicological properties, but the former is not a skin sensitizer (4).

ECOLOGICAL EFFECTS

Effects on Birds

As with mammals, it is difficult to reach the toxic dose in some birds. Bobwhite quail and mallard ducks had five-day dietary LC50s for benomyl greater than 10,000 ppm. In redwing blackbirds, the LD50 value is 100 mg/kg which indicates that benomyl is moderately toxic to these species (12).

Effects on Aquatic Organisms

The order of susceptibility to benomyl for various fish species from least susceptible to most susceptible is catfish, bluegill, rainbow trout, goldfish, and loach with catfish fry. The LC50 values for the compound in fish range from 0.05 mg/l to 14 mg/l for adults and down to 0.006 mg/l for catfish fry (12).

The main breakdown product carbendazim (MBC) had the same order of toxicity as benomyl. Crayfish are much more resistant with a benomyl induced LC50 greater than 100 mg/l. The estimated bioconcentration factor (BCF) ranges from 159 in rainbow trout up to 460 in bluegill sunfish indicating that benomyl does not tend to significantly concentrate in living tissue (2, 8). Effects on Other Organisms (Nontarget species)

A single application of benomyl to turf grass can substantially reduce some soil dwelling organisms. The compound is very lethal to earthworms at low concentrations over a long time period. The seven-day LC50 was 1.7 mg/l and the 14-day LC50 is 0.4 mg/l (3). Benomyl also decreases the mixing of soil and thatch. The effects lasted for up to twenty weeks (9).

The compound is relatively non-toxic to bees (7). The breakdown product carbendazim is also relatively non-toxic to bees. The compound has little effect on soil microbes at recommended application rates.

ENVIRONMENTAL FATE

Breakdown of Chemical in Soil and Groundwater

Benomyl is strongly bound to soil and does not dissolve in water in flooded rice fields to any significant extent (4). When applied to turf, it has a half-life of three to six months and when applied to bare soil the half-life is six to twelve months. Where four successive annual applications were applied, residues did not accumulate from one year to the next (3).

Breakdown of Chemical in Surface Water

Benomyl completely degrades to carbendazim (MBC) within several hours when in acidic or neutral water. The half-life of carbendazim is two months. In strongly alkaline water, benomyl breaks down to another compound, STB (12).

Breakdown of Chemical in Vegetation

Since benomyl is a systemic fungicide, it is absorbed by plants through the roots or the above-ground tissues. Once it is in the plant, it accumulates in veins and at the leaf margins (3). The metabolite carbendazim (MBC) seems to be the fungicidally active agent. However, benomyl residues are quite stable, with 48 to 97% remaining as the parent compound 21 to 23 days after application (3). The residues are easily extracted from the plant in hot water (4).

PHYSICAL PROPERTIES AND GUIDELINES

Benomyl is a tan crystalline solid compound with a molecular mass of 290.3 g/mol. There is little or no odor associated with the technical material. Formulations include wettable powder, dry flowable powder and dispersible granules.

Exposure Guidelines:

RfD: 0.05 mg/kg/day
HA: N/A
ADI: 0.02 mg/kg
PEL: 10 mg/m3 OSHA TWA (inhalable dust)

Physical Properties:

CAS #: 17804-35-2
Chemical name: methyl 1-[(butylamino) carbonyl]-H-benzimidazol-2-yl carbamate
Solubility in water: 2 mg/l (pH 9.0); 3.6 mg/l (pH 5.0)
Solubility in other solvents at 25 degrees C: chloroform 9.4 g/100 g; heptane 0.4 g/100g; ethanol 1 g/100g; acetone 5.3 g/100 g
Melting Point: decomposes without melting above 300 degrees C
Vapor Pressure: negligible (< 5 x 10 to the minus 6 Pa) @ 25 degrees C
Partition Coefficient (octanol/water) (log): 1.36
Adsorption Coefficient: 1860 mg/g
Chemical class/use: benzimidazole fungicide

BASIC MANUFACTURER

Du Pont Agricultural Products
Walker's Mill, Barley Mill Plaza
P.O. Box 80038
Wilmington, DE 19880-0038
Telephone: (800) 441-7515
Emergency: (800) 441-3637

Review by Basic Manufacturer:

Comments solicited: October, 1992
Comments received: January, 1994

REFERENCES

  1. World Health Organization. 1986. Organophosphorus Insecticides: A General Introduction. IPCS International Programme on Chemical Safety, Environmental Health Criteria 63.
  2. National Library of Medicine. 1992. Hazardous Substances Databank. TOXNET, Medlars Management Section, Bethesda, MD.
  3. Forest Service. 1986. Pesticide Background Statements, Volume II: Fungicides and Fumigants. U. S. Department of Agriculture, Agriculture Handbook No. 661.
  4. Food and Agriculture Organization of the United Nations. 1983. Pesticide Residues in Food - 1983: Evaluations, FAO Plant Production and Protection Paper 61.
  5. National Research Council. 1987. Regulating Pesticides in Food: The Delaney Paradox, National Academy Press, Washington, DC.
  6. Occupational Health Services, Inc. 1988. Hazard line, New York, NY.
  7. The Agrochemicals Handbook: Third Edition. 1991. Royal Society of Chemistry, Unwin Brothers Ltd., Surrey, England.
  8. Howard, P. H., ed. 1991. Handbook of Environmental Fate and Exposure Data for Organic Chemicals, Volume III: Pesticides. Lewis Publishers, Chelsea, MI.
  9. Potter, D.A., M.C. Buxton, C.T. Redmond, C.T. Patterson and A.J. Powell. 1990. Toxicity of pesticides to earthworms (Oligochaeta: Lumbricidae) and effects on thatch degradation in Kentucky bluegrass turf. Journal of Economic Entomology, 83(6): 2362-2369.
  10. Cummings, A. M., M. T. Ebron-McCoy, J. M. Rogers, B. D. Barbee and S. T. Harris. 1992. Developmental effects of Methyl Benzimidazole carbamate following Exposure during Early Pregnancy. Fundamental and Applied Toxicology, 18: 288-293.
  11. Edwards, I. R., D. G. Ferry and W. A. Temple. 1991. Fungicides and Related Compounds. in Handbook of Pesticide Toxicology, Volume 3: Classes of Pesticides. Wayland J. Hayes, Jr. and Edward R. Laws, Jr., eds. Academic Press, Inc., New York, NY.
  12. World Health Organization. 1993. Environmental Health Criteria 148. Benomyl. Geneva, Switzerland.