B.t. and B.t.i.:

Profile Excerpts from the Extension Toxicology Network (Extoxnet)

(Extoxnet is a Pesticide Information Project of Cooperative Extension Offices of Cornell University, Oregon State University, the University of Idaho, and the University of California at Davis and the Institute for Environmental Toxicology, Michigan State University. Major support and funding was provided by the USDA/Extension Service/National Agricultural Pesticide Impact Assessment Program. EXTOXNET primary files maintained and archived at Oregon State University)

 
 

Bacillus thuringiensis

Trade and Other Names: Trade names include Acrobe, Bactospeine, Berliner (variety kurstaki), Certan (variety aizawai), Dipel, Javelin, Leptox, Novabac, Teknar (variety israelensis), Thuricide, and Victory. Bacillus thuringiensis is also known at B.t.

Regulatory Status: This microbial insecticide was originally registered in 1961 as a General Use Pesticide (GUP). It is classified as toxicity class III - slightly toxic. Products containing B.t. bear the Signal Word CAUTION because of its potential to irritate eyes and skin.

Chemical Class: bacterium

Introduction: Bacillus thuringiensis (B.t.) is a naturally-occurring soil bacterium that produces poisons which cause disease in insects. 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. 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 newer strain, B.t. san diego, is effective against certain beetle species and the boll weevil. To be effective, B.t. must be eaten by insects during their feeding stage of development, when they are larvae. B.t. is ineffective against adult insects. More than 150 insects, mostly lepidopterous larvae, are known to be susceptible in some way to B.t.

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. When eaten, the spores and crystals of B.t. act as poisons in the target insects. B.t. is therefore referred to as a stomach poison. B.t. crystals dissolve in the intestine of susceptible insect larvae. They paralyze 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. Typical agricultural formulations include wettable powders, spray concentrates, liquid concentrates, dusts, baits, and time release rings.

Toxicological Effects:

  • Acute toxicity: The dermal LD50 for a formulated B.t. product in rabbits is 6280 mg/kg. A single dermal application of 7200 mg/kg of B.t. was not toxic to rabbits [148]. B.t. is an eye irritant; 100 grams of formulated product applied in each eye of test rabbits caused continuous congestion of the iris as well as redness and swelling [149]. Very slight irritation from inhalation was observed in test animals. This may have been caused by the physical rather than the biological properties of the B.t. formulation tested [8].
  • Chronic toxicity: 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 [8].
  • Reproductive effects: There is no indication that B.t. causes reproductive effects [143].
  • Teratogenic effects: There is no evidence indicating that formulated B.t. can cause birth defects in mammals [143,148].
  • Mutagenic effects: B. thuringiensis appears to have mutagenic potential in plant tissue. There is no evidence of mutagenicity in mammalian species.
  • Carcinogenic effects: It is unlikely that B.t. is carcinogenic.
  • Organ toxicity: There is no evidence of chronic B.t. toxicity in dogs, guinea pigs, rats, humans, or other test animals.
  • Fate in humans and animals: B.t. does not persist in the digestive systems of mammals that ingest it [149].

Ecological Effects:

  • Effects on birds:  Field observations of 74 bird species did not reveal any population changes after aerial spraying of B.t. formulation [148].
  • Effects on aquatic organisms:  Rainbow trout and bluegills exposed for 96 hours to B.t. at concentrations of 560 and 1000 mg/L did not show adverse effects.  Shrimp and mussels may be affected adversely [8].
  • Effects on other organisms: 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 [151]. B.t. applied at rates used for mosquito control may cause the death of some non-target species [8]. 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 [146].

Environmental Fate:

  • Breakdown in soil and groundwater: B.t. is a naturally-occurring pathogen that readily breaks down in the environment. Due to its short biological half-life and its specificity, B.t. is less likely than chemical pesticides to cause field resistance in target insects. B.t. is moderately persistent in soil. Its half-life in suitable conditions is about 4 months [152]. B.t. spores are released into the soil from decomposing dead insects after they have been killed by it. B.t. is rapidly inactivated in soils that have a pH below 5.1 [148]. 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 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 [150].
  • Breakdown in vegetation: B.t. is relatively short-lived on foliage because the ultraviolet (UV) light of the sun destroys it very rapidly. Its half-life under normal sunlight conditions is 3.8 hours [153]. It is not poisonous to plants and has not shown any adverse effect upon seed generation or plant vigor [150].

Exposure Guidelines:

  • ADI: Not Available
  • MCL: Not Available
  • RfD: Not Available
  • PEL: Not Available
  • HA: Not Available
  • TLV: Not Available

References:

References for the information in this PIP can be found in Reference List Number 10

     

Other Notes:

  Erik Kiviat, Science Director of Hudsonia, an ecological research and consulting institute at Bard College, who has worked with health departments concerned with West Nile-like virus in New York state:

"Bti is effective in waters that do not have high organic matter level and its effects are limited to mosquitoes, blackflies, and some other related species of flies with aquatic larvae."

"Bti may be allergenic to some people and should be handled with gloves and the dust not inhaled. Frequent repeated use in wetlands or streams is not recommended."

 

Least Toxic Mosquito Larvae Control: B.t.i. "Donuts" EPA approved for animal water troughs

Date: Mar 7, 2003
Posted by: Ashley Hotz (ahotz@mindspring.com)

The use of a least toxic B.t.i. mosquito larvicide (sometimes called mosquito donuts, mosquito dunks, etc.) has now been expanded to include use as a mosquito larvicide in animal troughs. EPA made this approval official as of 02/24/03. B.t.i. is an excellent mosquito larvicide bacteria that specifically targets mosquito larvae, and Psychodid Flies (a/k/a Filter Flies, a/k/a Drain Moths, a/k/a Hairy Wings). B.t.i. is considered one of the least harmful products because it does not harm non-target species such as mammals, birds, fish and beneficial insects. The product specifically named in the new label approval is for the trade name Summit B.t.i. BRIQUETS, but may include other brands. B.t.i. is the short name for Bacillus thuringiensis israelensis, an effective mosquito larvicide for up to 30 days in temporary bodies of standing water. B.t.i. is NOT recommended by serious biologists for established water eco-systems such as ponds and lakes where natural mosquito predators are well established. The reason is that mosquito larvae are an important food source for these systems and killing the larvae can drastically reduce an important food source for frogs, bats, birds, dragonflies (nymph stage), other beneficial insects, toads and some species of fish -- and cause the entire eco-system to go out of balance. This can result in the need to establish dependence on expensive and labor intensive human interventions for control. Those of us who live near established pond or lake ecosystems, know that after the first week or 10 days of annual Spring mosquito problems that the mosquito predators inevitably find the food source and do their jobs very well.

Page 3 of the revised Summit B.t.i. BRIQUETS (trade name) label states:

"Outdoor Use around the Household to kill Mosquito Larvae: Summit B.t.i. Briquets can be broken into portions for use in many outdoor applications near the household, such as standing water in bird baths, old automobile tires, rain barrels, ponds, abandoned or unused swimming pools (particularly above ground types), ditches, tree holes, roof gutters for collecting rainwater, flower pots, animal watering troughs or any other locations near the household where water collects and remains for periods of time..."

Please be advised that I do not work for Summit and have absolutely no financial interest in this product. My interests are in chemical injury issues and how to prevent chemical injury.

EPA's web page showing the label for the Summit B.t.i. BRIQUET product is outdated as of this date. If you need to verify the newly approved B.t.i. donut briquette use in animal troughs and need a copy of the new label faxed to you, call Summit directly at 800-227-8664 or 410-282-5200 and speak with any company representative who can fax you the new label stamped as of 02/24/03 by the E.P.A.'s Office of Pesticide Programs under the Federal Insecticide, Fungicide, and Rodenticide Act. Summit does not have this new approval status posted on its web site at this time, but plans to do so within the next 30 days. The EPA Registration No. is: 6218-47. EPA Est. No, 6218-MD-2. You may also reach Summit reps via e-mail at info@summitchemical.com and request that a copy of the new label be faxed or mailed to you.

Once again I would like to caution that although B.t.i. manufacturers will sometimes promote B.t.i. as good for use in established ponds with established ecosystems -- that this should NOT be done. Allow me to emphasize as well that it is extremely important to put the "i" at the end of B.t. i. because there is another bacterial insect control product that does not have the "i" after it -- but is only effective for agricultural insect control --not for mosquito larvae.

Thank you for this opportunity to clarify that the product Summit B.t.i. BRIQUETS is currently approved for use in animal troughs can be an effective control for mosquitoes without harming beneficial non-target species.

Ashley Hotz 

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