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Bensulide

Publication Date: 9/93

TRADE OR OTHER NAMES

Trade names for bensulide include Betamec, Betasan, Benzulfide, Disan, Exporsan, Prefar, Pre-San, and R-4461. It may be used in combination with other pesticides such as thiobencarb and molinate.

INTRODUCTION

Bensulide is a selective organophosphate herbicide. It is used on vegetable crops such as carrots, cucumbers, peppers, and melons and in cotton and turfgrass to control annual grasses (such as bluegrass and crabgrass) and broadleaf weeds. It is often formulated as an emulsifiable liquid and applied before the weed seeds germinate in order to prevent them from germinating (10).

TOXICOLOGICAL EFFECTS

ACUTE TOXICITY

This slightly toxic herbicide is labeled with a CAUTION signal word. Although its toxicity is not high, it can cause convulsions in humans when large amounts are ingested. Other symptoms of acute poisoning range from nausea and vomiting at mild exposure levels to abdominal cramps, loss of muscle coordination and slurring of speech, coma and death at higher levels of acute exposure (4). Specific concentrations or doses were not noted with the listed symptoms.

The oral LD50 for rats ranges from 271 mg/kg (5) to 770 mg/kg (2). The dermal LD50 is 3,950 mg/kg for rats and 2,000 mg/kg for rabbits. In tests with rodents, Betasan, a bensulide containing product, did not cause eye irritation (4).

CHRONIC TOXICITY

Bensulide inhibits cholinesterase, a chemical which is critical to the proper functioning of the nervous system. Symptoms of human chronic exposure are fairly typical of other organophosphate pesticides and may include chest tightness, nausea, abdominal cramps, diarrhea, headache, dizziness, weakness, blurring, tearing, loss of muscle coordination, and face muscle twitches. No information was found about its potential to cause birth defects.

Reproductive Effects

Bentasan fed to adult Japanese quail for three weeks at moderate doses (50 mg/kg) adversely affected egg hatchability. Fertility however was unaffected (4). This evidence is not sufficient to draw any conclusions about the risk of bensulide to human reproduction.

Mutagenic Effects

This chemical was not mutagenic in the one bacterial assay that was performed (4). More information is needed before bensulide's mutagenic potential can be appropriately assessed.

Carcinogenic Effects

Appropriate studies for establishing carcinogenic potential have not been performed. However, in a 90-day feeding trial, rats and dogs tolerated daily doses close to the lethal dose without any noticeable tumor growth (7).

Organ Toxicity

Bensulide can inhibit the enzyme cholinesterase and affect brain, nerve, and some blood cells (6). Rabbits exposed to bensulide have suffered mild eye irritation.

ECOLOGICAL EFFECTS

Bensulide is moderately to highly toxic to aquatic organisms, including rainbow trout and bluegill (4). The LC50 for bensulide in rainbow trout is 0.7 mg/l, in the bluegill it is 0.8 mg/l and is 1-2 mg/l in the goldfish.

It is only slightly toxic to wild birds. The bensulide herbicide, Betasan, was fed to adult Japanese quail for three weeks and egg hatchability was significantly reduced at the highest dose (about 50 mg/kg), but fertility was not affected. Blood cholinesterase was inhibited at lower doses, but recovered within two weeks after the treatments stopped (4).

The compound is very highly toxic to bees (9). The LC50 of bensulide is about 12.5 ug/bee (4).

ENVIRONMENTAL FATE

Bensulide is persistent in both plants and soil. Because it strongly binds to soils, bensulide does not evaporate easily but can be carried off site with sediment or in dust. The rate of application, temperature, soil organic matter, and soil acidity can all affect the breakdown of the product.

Root growth of target plants can be inhibited or delayed by bensulide treatment. In one study, root growth of bermudagrass was greatly reduced by low dose bensulide applications (1). The compound is rapidly absorbed by roots and foliage and is translocated to the active growing portions of the plant (root or stem tips) where it works to stop cell division and plant growth (9). When applied to roots, bensulide is not translocated to leaves except as metabolites (4). Bensulide is effective for four months to one year, depending on the rate of application.

Bensulide leaches very little in sand, clay, or organic soils. It is tightly bound in the top 0 to 2 inches of soil. Thus, it persists long enough to control summer weeds like crabgrass, barnyard grass, lambsquarters, and deadnettle.

The rate of degradation increases with increasing soil temperature and organic matter, but decreases with increasing basicity (4). Bensulide is slowly broken down by soil micro-organisms. At 70-80 degrees F, the half of the compound is still present after four months in a moist loam soil, and after six months in a moist, loamy sand (4). Since bensulide binds to soil, it can be transported to surface water through runoff. As of 1988 it had not been found in groundwater or in wellwater (8). In rice fields the half-life of the compound averages only four to six days (9).

Some decomposition by sunlight occurs over several days (4).

PHYSICAL PROPERTIES AND GUIDELINES

Bensulide is a viscous, colorless liquid or a white crystalline solid, with a molecular weight of 397.52. It is a member of the organophosphates. Products containing bensulide may be amber colored liquids. While it is corrosive to copper, it is non-corrosive to other metals. No incompatibilities are known. Bensulide shows little reactivity, and is of low fire hazard.

Its chemical name is S-(O,O-diisopropyl phosphorodithioate) ester of N-(2-mercaptoethyl) benzenesulfonamide. Bensulide is stable and has an indefinite storage life under normal conditions.

Exposure Guidelines:

NOEL: 25 mg/kg/day for 90 days (rat); 12.5 mg/kg/day for 90 days (dog).

Physical Properties:

CAS #: 741-58-2
Solubility in water: 25 mg/L at 20 degrees C (low solubility)
Solubility in solvents: soluble in 300 mg/l kerosene at 20 degrees C. It is also miscible with acetone, ethanol, xylene, and methyl-isobutyl ketone.
Melting point: 34.4 degrees C
Boiling point: decomposes at 200 degrees C in 18 to 48 hours.
Vapor pressure: <133 uPa at 20 degrees C.

BASIC MANUFACTURER

Zeneca Ag Products
Wilmington, DE 19897
Telephone: (302) 886-1000

Review by Basic Manufacturer:

Comments solicited: November, 1992
Comments received:

REFERENCES

  1. Ashton, F.M. and Crafts, A.S. Bensulide, Mode of Action of Herbicides. pp. 396-398. New York: John Wiley & Sons Inc., 1973.
  2. Buchel, K.H., ed. and Holmwood, G.M., translator. Chemistry of Pesticides. p. 391. New York: John Wiley & Sons Inc, 1983.
  3. Gosselin, R.E.; Smith, R.P.; Hodge, H.C.; and Braddock, J.E. Clinical Toxicology of Commercial Products, 5th edition. Williams & Wilkins, 1984.
  4. National Library of Medicine. Hazardous Substances Databank. Bensulide. June, 1992
  5. Occupational Health Services Inc. Material Safety Data Sheet: Bensulide. 4/10/87 New York.
  6. Lewis, R.J. and Tatken, R.L., eds. Registry of Toxic Effects of Chemical Substances. Tracor-Jitco Inc.: Rockville, MD.
  7. Worthing, C.R., ed. 1983 "Bensulide." The Pesticide Manual. 7th ed. British Crop Protection Council.
  8. US Environmental Protection Agency. 1988. Pesticides in Ground Water Data Base. 1988 Interim Report. Office of Pesticide Programs, Environmental Fate and Effects Division, Environmental Fate and Ground Water Branch.
  9. The Agrochemicals Handbook. 1991. The Royal Society of Chemistry. Cambridge, England.
  10. Farm Chemicals Handbook. 1992. Meister Publishing Company, Willoughby, Ohio. 0901.