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
Commercial names for products containing benomyl are Agrodit, Benex,
Benlate, Benosan, Fundazol, Fungidice 1991, and Tersan. Benomyl is
compatible with many other pesticides (7).
Benomyl is a General Use Pesticide (GUP).
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).
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).
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
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).
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).
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).
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).
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).
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
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.
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
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
|RfD: ||0.05 mg/kg/day
|ADI: ||0.02 mg/kg
|PEL: ||10 mg/m3 OSHA TWA (inhalable dust)
|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
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
World Health Organization. 1986. Organophosphorus Insecticides: A
General Introduction. IPCS International Programme on Chemical Safety,
Environmental Health Criteria 63.
National Library of Medicine. 1992. Hazardous Substances Databank.
TOXNET, Medlars Management Section, Bethesda, MD.
Forest Service. 1986. Pesticide Background Statements, Volume II:
Fungicides and Fumigants. U. S. Department of Agriculture, Agriculture
Handbook No. 661.
Food and Agriculture Organization of the United Nations. 1983.
Pesticide Residues in Food - 1983: Evaluations, FAO Plant Production and
Protection Paper 61.
National Research Council. 1987. Regulating Pesticides in Food: The
Delaney Paradox, National Academy Press, Washington, DC.
Occupational Health Services, Inc. 1988. Hazard line, New York, NY.
The Agrochemicals Handbook: Third Edition. 1991. Royal Society of
Chemistry, Unwin Brothers Ltd., Surrey, England.
Howard, P. H., ed. 1991. Handbook of Environmental Fate and
Exposure Data for Organic Chemicals, Volume III: Pesticides. Lewis
Publishers, Chelsea, MI.
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.
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.
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.
World Health Organization. 1993. Environmental Health Criteria
148. Benomyl. Geneva, Switzerland.