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Bacillus Thuringiensis

Publication Date: 5/94

TRADE OR OTHER NAMES

Berliner (B.t. variety kurstaki): Dipel, Thuricide, Bactospeine, Leptox, Novabac, Victory. Certan (B.t. variety aizawa). Teknar (B.t. variety israelensis).

REGULATORY STATUS

This microbial insecticide was originally registered in 1961 as a general use insecticide. A registration standard, issued in 1986 by the U.S. Environmental Protection Agency (EPA), required manufacturers to make minor changes in label precautions and to provide additional data on the effects of B.t. on nontarget organisms. While EPA considers the toxicological data base for B.t. complete, the Agency is still requiring more ecological effects data. Check with specific state regulations for local restrictions which may apply.

INTRODUCTION

Bacillus thuringiensis (B.t.) is a naturally-occurring soil bacterium that produces poisons which cause disease in insects. A number of insecticides are based on these toxins (8). B.t. is considered ideal for pest management because of its specificity to pests and because of its lack of toxicity to humans or the natural enemies of many crop pests (14). There are different strains of B.t., each with specific toxicity to particular types of insects: B.t. aizawai (B.t.a.) is used against wax moth larvae in honeycombs; B.t. israelensis (B.t.i.) is effective against mosquitoes, blackflies and some midges; B.t. kurstaki (B.t.k.) controls various types of lepidopterous insects, including the gypsy moth and cabbage looper. A new strain, B.t. san diego, has been found to be effective against certain beetle species and the boll weevil. In order to be effective, B.t. must be eaten by insects in the immature, feeding stage of development referred to as larvae. It is ineffective against adult insects. Monitoring the target insect population before application insures that insects are in the vulnerable larval stage (9). More than 150 insects, mostly lepidopterous larvae, are known to be susceptible in some way to B.t. (5).

Bacteria are primitive one-celled organisms, which belong to a group of organisms called prokaryotes. Prokaryotes are neither plants nor animals. Like certain members of the plant kingdom, such as ferns and mushrooms, B. t. forms asexual reproductive cells, called spores, which enable it to survive in adverse conditions. During the process of spore formation, B.t. also produces unique crystalline bodies as a companion product. The spores and crystals of B.t. must be eaten before they can act as poisons in the target insects. B.t. is therefore referred to as a stomach poison (7). B.t. crystals dissolve in response to intestinal conditions of susceptible insect larvae. This paralyzes the cells in the gut, interfering with normal digestion and triggering the insect to stop feeding on host plants. B.t. spores can then invade other insect tissue, multiplying in the insect's blood, until the insect dies. Death can occur within a few hours to a few weeks of B.t. application, depending on the insect species and the amount of B.t. ingested (7, 13).

TOXICOLOGICAL EFFECTS

ACUTE TOXICITY

No complaints were made after eighteen humans ate one gram (g) of commercial B.t. preparation daily for five days, on alternate days. Some inhaled 100 milligrams (mg) of the powder daily, in addition to the dietary dosage (6). Humans who ate one g/day of B.t.k. for three consecutive days were not poisoned or infected (12).

Since it was one of the first biological control agents registered for use against insects in the U.S., B.t. had to undergo a testing program which was more thorough than that which the EPA currently requires for biological pesticides. As a result, there are no data gaps in the toxicity information required by the EPA for registration purposes. A wide range of studies have been conducted on test animals, using several routes of exposure. (The highest dose tested was 6.7 x 10 to the 11th spores per animal.) The results of these tests suggest that the use of B.t. products can cause few, if any, negative effects. B.t. did not have acute toxicity in other tests conducted on birds, dogs, guinea pigs, mice, rats, humans, or other animals. When rats were injected with B.t.k., no toxic or virus- like effects were seen. No oral toxicity was found in rats, mice or Japanese quail fed protein crystals from B.t. var. israelensis (19).

Very slight irritation was observed in test animals from inhalation and dermal exposure. This may have been caused by the physical rather than the biological properties of the B.t. formulation tested (14). Mice survived one or more 1-hour periods of breathing mist that contained as many as 6.0 x 10 to the 10th spores of B.t. per cubic meter (m3) (6). No toxic effects were observed in rats that had a B.t. formulation put directly into their lungs, at rates of 5 mg/kg of body weight (1).

The amount of formulated insecticide that killed 50% of the rats experimentally fed the material ranges from 2.65 to greater than five grams per kilogram (g/kg) (1, 12). This amount is referred to as the lethal dose fifty (LD50) for B.t. in rats. Single oral dosages of up to 10,000 milligram per kilogram (mg/kg) of body weight did not produce toxicity in mice, rats or dogs (1).

The dermal LD50 for a formulated B.t. product in rabbits was 6,280 mg/kg. Some reversible abnormal redness of the skin was observed when 1 mg/kg/day of formulated B.t. product was put on scratched skin for 21 days. No general, systemic poisoning was observed. A single dermal application of 7.2 g/kg of B.t. was not toxic to rabbits (1).

B.t. crystals have caused deaths in test animals when they were injected directly into the abdominal cavity. This suggests that B.t. can be toxic to mammals, but that when exposure is through normal routes of exposure (oral, dermal or inhalation), metabolism or elimination of the toxin prevents poisoning in mammals (19).

CHRONIC TOXICITY

No complaints were made by eight men after they were exposed for seven months to fermentation broth, moist bacterial cakes, waste materials, and final powder created during the commercial production of B.t. (6).

There is no evidence of chronic B.t. toxicity in dogs, guinea pigs, rats, humans or other test animals. Thirteen-week dietary administration of B.t. to rats at dosages of 8,400 mg/kg did not produce toxic effects (14).

Reproductive Effects

This literature review did not produce any information on the effects of B.t. exposure to reproductive systems.

Teratogenic Effects

There is no evidence indicating that formulated B.t. can cause birth defects in mammals (1).

Mutagenic Effects

B. thuringiensis appears to have mutagenic potential in plant tissue. Extensive use of B.t. on food plants might be hazardous, given its mutagenic potential (6).

Carcinogenic Effects

Tumor-producing effects were not seen in two-year chronic studies during which rats were given dietary doses of 8,400 mg/kg of B.t. formulation (1).

Organ Toxicity

No additional information was found on the harmful effects of B.t. to organs.

Fate in Humans and Animals

While B.t. interferes with insect digestion, it does not persist in the digestive systems of mammals that ingest it. When placed in the eyes of rabbits, Bt var. israelensis was still present after 1 week, but there was no infection or other harmful effect to the eye. When injected into the gut of mice, Bt var. israelensis was detected in the spleen and heart blood for as long as 80 days, but there were no infections (16).

ECOLOGICAL EFFECTS

Effects on Birds

B.t. is not toxic to birds (2, 15). It biodegrades and does not persist in the digestive systems of birds (9). The LD50 for bobwhite quail was greater than 10,000 mg of B.t. per kg body weight. When autopsies were performed on these birds, no pathology was attributed to B.t. Field observations of 74 bird species did not reveal any population changes after aerial spraying of the B.t. formulation (1).

Effects on Aquatic Organisms

B.t. has not been reported as having harmful effects in fish (2). Rainbow trout and bluegills exposed for 96 hours to B.t. technical material, at concentrations of 560 and 1,000 parts per million (ppm), did not show adverse effects. A small marine fish (Anguilla anguilla) was not negatively affected by exposure to 1,000-2,000 times the level of B.t. expected during spray programs. Field observations of populations of brook trout, common white suckers and smallmouth bass did not reveal adverse effects one month after aerial application of the B.t. formulation (1).

Effects on Other Animals (Nontarget species)

Applications of labeled rates of formulated B.t. have not been toxic to beneficial or predator insects (1). Treatment of honeycombs with B.t. var. aizawai will not have a detrimental effect upon bees, nor on the honey produced (4). Normal exposure rates do not cause harm to honey bees. Very high concentrations (108 spores/ ml sucrose syrup) of B.t. var. tenebrionis, which is used against beetles such as the Colorado potato beetle, reduced longevity of honey bee adults but did not cause disease (17).

As of 1986, EPA had not completed its assessment of the potential impact of certain uses of B.t. on endangered and/or threatened species of moths and butterflies. Concern was expressed regarding its potential to kill endangered species of butterflies, along with target pests (9).

Users of B.t. are encouraged to consult local officials or the nearest EPA regional office responsible for protecting endangered species before using B.t. products in counties where susceptible endangered species of Lepidoptera are known to be present. (In California: Los Angeles, Contra Costa, Mendocino, San Francisco, San Mateo, Monterey, and Kern Counties; in Florida: Date and Monroe Counties; in Washington: Pacific and Tillamook Counties; and Lane County in Oregon) (12). Death occurs in some nontarget insect species when B.t. is applied at rates used for mosquito control. Results of other experimental testing do not suggest that B.t. adversely affects nontarget insects or aquatic invertebrates. It did not have negative effects on frogs and salamanders (2).

ENVIRONMENTAL FATE

B.t. is a naturally-occurring pathogen that readily breaks down in the environment. As a biological entity, it is subject to death and inactivation in the same fashion as all living things (1, 5). B.t. is degraded very rapidly when exposed to UV light. Its half-life under normal sunlit conditions is 3.8 hours. Formulations of B.t. spores and crystals encapsulated in starch lost all spore viability and insecticidal activity within 4 days (18). Due to its short biological half-life and its specificity, B.t. is less likely than other chemical pesticides to cause field resistance in target insects. In enclosed situations, however, B.t. resistance has been reported in a stored grain pest, the Indian meal moth (9). Because this material readily biodegrades in the environment, it poses little or no disposal problem (11).

Breakdown of Chemical in Soil and Groundwater

Under suitable conditions, B.t. can persist for several months in soil. Its spores are released into the soil from decomposing dead insects after they have been killed by the bacterium. B.t. is rapidly inactivated in soils that have a pH below 5.1 (1, 5).

Microbial pesticides such as B.t. are classified as immobile because they do not move, or leach, with groundwater. Because of their rapid biological breakdown and low toxicity, they pose no threat to groundwater.

Breakdown of Chemical in Water

The EPA has not issued restrictions for the use of B.t. around bodies of water. It can be effective for up to 48 hours in water. Afterwards, it gradually settles out or adheres to suspended organic matter (2).

Breakdown of Chemical in Vegetation

Since it does not spread, B.t. must be applied to the parts of the plants that are normally attacked by lepidopterous larvae, or to the particular zones of water in which dipterous larvae feed. It is relatively short-lived on foliage because the ultraviolet (UV) light of the sun destroys it very rapidly, and rain washes it onto the soil. The bacterium is nonphytotoxic, or not poisonous to plants, and has not shown any adverse effect upon seed generation or plant vigor (2).

PHYSICAL PROPERTIES AND GUIDELINES

The insecticidal action of B.t. is attributed to protein crystals produced by the bacterium. The vegetative cells of B.t. are approximately one micrometer (mcm) in width and 5 mcm in length, and are motile (12). The commercial product contains about 2.5 x 10 to the 11th viable spores per gram. Typical agricultural formulations that contain spores and protein crystals include wettable powders, spray concentrates, liquid concentrates, dusts, baits, and time release rings (4, 6, 14).

B.t. products should be stored in a cool, dry place. Some loss of effectiveness can be expected in products stored for more than six months (2). Formulated products are compatible with most insecticides, acaricides, fungicides and plant growth regulators; they are not compatible with captafol, dinocap, alkaline sprays or, under some conditions, leaf or foliar nutrients (4, 14).

Physical Properties

CAS #: (B.t. variety kurstaki) 68038-71-1
H2O solubility: emulsifies (3); suspendable (10); -- vars. israelensis and kurstaki: insoluble in water (4)
Solubility in other solvents: vars. israelensis and kurstaki: insoluble in organic solvents (4)
Flash point: over 400 degrees, var. kurstaki (3)
Chemical Class/Use: Biological insecticide
There have been reports of air emissions of 0.5 kg of particulates per metric ton of pesticide produced (11)

BASIC MANUFACTURERS

Sandoz Crop Protection Corp.
1300 E. Touhy Ave.
Des Plaines IL 60018

Abbott Laboratories
Chem. and Agric. Prod. Div.
1401 Sheridan Rd.
North Chicago, IL 60064

Review by Basic Manufacturer - Abbott Labs:

Comments solicited: November, 1992
Comments received:

Review by Basic Manufacturer - Sandoz:

Comments solicited: November, 1992
Comments received: December, 1993

REFERENCES

  1. Abbott Laboratories. 1982 (Oct.). Toxicology profile: Dipel, Bacillus thuringiensis insecticide. Chemical and Agricultural Products Division. North Chicago, IL.
  2. Agriculture Canada. 1982. Report of new registration: Bacillus thuringiensis Serotype H14. Food Protection and Inspection Branch. Ottawa, Canada: Agriculture Canada.
  3. Agway, Inc. No date given. Material safety data sheets (on Bacillus thuringiensis formulations). Chemical Division. Syracuse, NY.
  4. Berg, G. L. (ed.). 1986. Farm chemicals handbook. Willoughby, OH: Meister Publishing Company.
  5. Harper, J. D. 1974 (Dec.). Forest insect control with Bacillus thuringiensis. Survey of current knowledge. Agricultural Experimental Station. Auburn University. Auburn, AL: University Printing Services.
  6. Hayes, W. J. 1982. Pesticides studied in man. Baltimore, MD: Williams and Wilkins.
  7. International Minerals and Chemical Corporation. (No date given) Thuricide technical bulletin. Bioferm Division, Microbial Insecticide Department. Wasco, CA.
  8. McEwen, F. L. and G. R. Stephenson. 1979. The use and significance of pesticides in the environment. NY: John Wiley and Sons, Inc.
  9. National Coalition Against the Misuse of Pesticides. 1986 (Dec.) Pesticides and you. Washington, DC.
  10. Nor-Am Chemical Company. 1985. Material safety data sheet: Bacillus thuringiensis. Wilmington, DE.
  11. Sittig, M. 1980. Pesticide manufacturing and toxic materials control encyclopedia. Parkridge, NJ: Noyes Data Corporation.
  12. U.S. Environmental Protection Agency. 1986. Pesticide fact sheet for Bacillus thuringiensis. Fact sheet no. 93. Office of Pesticide Programs. Washington, DC.
  13. Ware, G. W. 1982. Fundamentals of pesticides. A self-instruction guide. Fresno, CA: Thomas Publications.
  14. Worthing, C. R. (ed.). 1983. The pesticide manual: A world compendium. Croydon, England: The British Crop Protection Council.
  15. Meister, R.T. (ed.). 1992. Farm Chemicals Handbook '92. Meister Publishing Company, Willoughby, OH.
  16. Spiegel, J.P. and J.A. Shadduck. 1990. Clearance of Bacillus sphaericus and Bacillus thuringiensis ssp. israelensis from mammals. J. of Economic Entomology 83 (2): 347-355.
  17. Vandenberg, J.D. 1990. Safety of four entomopathogens for caged adult honeybees (Hymenoptera: Apidae). J. of Economic Entomology 83 (3): 756-759.
  18. Dunkle, R.L. and B.S. Shasha. 1989. Response of starch-encapsulated Bacillus thuringiensis containing ultraviolet screens to sunlight. Environmental Entomology 18 (6): 1035-41.
  19. Roe, R.M. et. al. 1991. Vertebrate toxicology of the solubilized parasporal crystalline proteins of Bacillus thuringiensis susp. israelensis in Hodgson, E., R.M. Roe and N. Motoyama (eds.). Reviews in Pesticide Toxicology 1: Pesticides and the Future: Toxicological Studies of Risks and Benefits. North Carolina State Univ., Raleigh, NC.