Bacillus thuringiensis var. israelensis serotype H-14, also known as Bti, is a strain of bacterium that produces protein crystals, which are toxic to mosquitoes, biting black flies and a few species of midges. These crystals act as a stomach poison, being activated by the alkaline digestive enzymes found within the larval mosquito gut. The gut wall begins to break down (dissolve) within a few hours following ingestion. Within 24-48 hours the mosquito larva has died.

This mosquito control agent has been used successfully in California for more than 20 years. This specific strain of Bti only works on mosquitoes, black flies and a very few species of midges, and does not affect other aquatic organisms when used at Federally and State mandated label rates. Bti does not affect pupal or adult mosquitoes and has no residual effect. This means that a site which routinely produces mosquitoes must be treated on a regular basis to effectively reduce the mosquito population.

This insecticide must be applied to all of the water areas that have active larval mosquitoes and is not a material that can be applied as a preventative measure for anticipated mosquito breeding. Napa County Mosquito Abatement District applies this insecticide routinely to mosquito breeding sites to prevent adult mosquito emergence. The timing of the applications is critical to the effectiveness of this insecticide. Other factors such as density of vegetative cover, temperature, organic content of the water, pH and density of the larval mosquito population readily reduce the effectiveness of this insecticide.