Mosquito Larvicide - Bti

Bacillus thuringiensis israelensis (Bti) is a naturally occurring soil bacterium that can effectively kill mosquito larvae present in water. It is one of many strains of Bacillus thuringiensis, each having unique toxicity characteristics. Bti is very specific for mosquitoes and black flies, and has some toxicity toward certain other dipterans (including midges). Bti is the primary material used for mosquito control because of its low toxicity to non-target species. Commercially available Bti strains are sold under the trade names Aquabac©, Teknar©, Bactimos© , and Vectobac© . When community mosquito control is needed to reduce mosquito-borne disease, the Department of Health favors use of larvicide applications to the breeding source of mosquitoes. Larvicides are more effective and less toxic than adult mosquito sprays, and the applications are unlikely to result in human exposure.

How does Bti work?

Bti spores that are eaten by mosquito larvae release toxins into the mosquito's gut, causing the larvae to stop eating and die (2). Bti is only effective against actively feeding larvae, and does not affect mosquito pupae or adults. More information on the mosquito life cycle may be found on EPA's Larvicides for Mosquito Control webpage. Products that contain Bti are available in liquid, briquette (small block) and granular formulations. These formulations can be applied to water bodies by hand, with a sprayer, or aerially. Bti breaks down quickly in the environment and may need to be reapplied regularly to obtain adequate mosquito control. Depending on the formulation and environmental conditions, Bti may remain effective from 24 hours to over one month (5).

Where is Bti applied?

Bti may be applied to water bodies where mosquito larvae live. Since mosquitoes generally prefer to breed in standing water, typical locations where Bti may be used include stormwater retention ponds, catch basins and shallow areas of wetlands and lakes that have been identified as a problem source of mosquitoes. The Department of Ecology does not recommend application to most wetlands because natural predators in the ecosystem can control mosquito larvae. If mosquito control is needed, Bti or Bacillus sphaericus products are recommended due to their low toxicity to non-target organisms (5). Federal laws prohibit the application of Bti to reservoirs that contain drinking water (4).

Can I apply Bti to my property?

Some products that contain Bti, such as Mosquito Dunks© and Mosquito Bits©,are available for private residential use in Washington. If using these products, it is important to carefully follow the label instructions and to apply only to waters that will not drain off of the property (e.g., ornamental ponds or other closed systems). If water on private property is connected to or has the potential to reach surface waters of the state, then only a licensed pesticide applicator may apply the product. In addition, the property owner or the licensed pesticide applicator will likely have to obtain a NPDES permit from the Department of Ecology. More information about the permit process is available on the Washington State Department of Ecology's Aquatic Mosquito Control NPDES General Permit webpage.

For most residences a more effective way to control mosquito populations on your property is to reduce standing water where mosquitoes can breed. The Department of Health recommends taking the following steps to reduce mosquito habitat:

The Department of Health also recommends limiting your exposure to mosquitoes by wearing long sleeved clothing and using insect repellent when in areas where you are likely to be bitten.

Is Bti harmful to humans or pets?

No measurable health effects were seen in laboratory animals that ingested large concentrations of Bti. Cases involving human health effects following exposure to Bti are extremely rare. Direct exposure to Bti was shown to cause skin and eye irritation in some animals (3). Cases of eye and skin irritation in humans have also been reported following direct exposure with some Bacillus thuringiensis products (2). Pets are unlikely to experience health effects from exposure to Bti based on the results from numerous studies involving laboratory animals.

Is Bti harmful to the environment?

Bti is nontoxic to mammals, birds, and fish. In laboratory studies, effects were only observed at concentrations much greater than what would occur following a typical application. Few studies, however, have closely examined potential long-term ecological impacts from Bti application. Some studies suggest that continuous application of Bti over a period of 2-3 years to wetlands may result in an overall decrease of biodiversity (3).

What precautions can I take to reduce my exposure to Bti?

Since Bti will generally be applied to areas that are inaccessible to the public, exposure is unlikely. Common sense steps, such as avoiding areas during scheduled larvicide applications, will further reduce the chances for exposure. If applying Bti at home, carefully follow label directions and avoid direct contact of the product with eyes and skin. If irritation to eyes or skin occurs, rinse eyes with tap water and wash skin thoroughly with soap and water for 20 minutes. If symptoms persist, seek medical attention and report the incident to the Washington Poison Center at 1-800-222-1222 or the Washington State Department of Health, Pesticide Program at 1-888-586-9427.

Should I be concerned about other ingredients in Bti formulations?

Secondary or inert ingredients in pesticide formulations do not have to undergo the same stringent testing as active ingredients (i.e., Bti). Some strains of Bacillus thuringiensis have the potential to produce various toxins (exotoxins) that may exhibit toxic symptoms in mammals, however the manufacturing process includes monitoring to prevent these toxins from appearing in products (4). Tests have not shown commercial formulations of Bti larvicides to be more toxic than the isolated active compound. However, it is possible that some individuals may be more sensitive to certain other ingredients. For this reason, the Department of Health recommends that people minimize direct exposure to Bti as much as possible. The EPA's Pesticide Inert Ingredients webpage has more information on inert ingredients used in pesticides.

More Information


  1. Lacey, LA and JP Siegel, 2000. Safety and ecotoxicology of entomopathogenic bacteria in Entomophathogenic Bacteria: From laboratory to field application, pp 253-273. Kluwer Academic, Dordrecht.
  2. National Pesticide Information Center, 2000. Bacillus thuringiensis technical fact sheet.
  3. Siegel, JP and JA Shadduck, 1990. Mammalian Safety of Bacillus thuringiensis israelensis in Bacterial Control of Mosquitoes and Black Flies: Biochemistry, Genetics and Applications of Bacillus thuringiensis israelensis and Bacillus sphaericus, pp 202-217. Editors: Barjac and Sutherland. New Brunswick: Rutgers University Press.
  4. United States Environmental Protection Agency. Registration Eligibility Decision (RED): Bacillus thuringiensis, EPA-738-FR98-004. March 1998.
  5. Washington State Department of Ecology. Aquatic Mosquito Control, National Pollution Discharge Elimination System, Waste Discharge General Permit. Permit no: WAG #992000. Effective Date: May 10, 2002.