Pesticides Used


Allethrin is a first generation synthetic pyrethroid, which is similar in chemical composition to one of the components found in natural pyrethrin, Cinerin I. This insecticide functions in much the same way as natural pyrethrins by affecting nerve function in insects and is not a cholinesterase inhibitor. Although more stable to heat and light, its persistence in the environment is also limited lasting for at best a few days. Napa County Mosquito Abatement District uses Allethrin on a very limited basis as part of its yellowjacket control program.

Bacillus sphaericus (Bs)

Bacillus sphaericus is a commonly occurring spore-forming bacterium found worldwide in soil and aquatic environments. Certain strains of this bacterium have been found effective against some species of mosquitoes. Like the larvicide Bti (Bacillus thuringiensis var. israelensis serotype H-14) this bacterium forms protein crystals that can destroy the mosquito larva’s gut lining causing death. Unlike Bti, Bacillus sphaericus only affects larval mosquitoes and is virtually non-toxic against black flies. Most Culex spp. and Anopheles spp. mosquitoes may be effectively controlled while many species of Aedes, especially Salt Marsh mosquitoes, are not susceptible.

Bacillus sphaericus is also different from Bti in that it can control mosquito larvae in highly organic aquatic environments, especially wastewater ponds, animal waste ponds and septic ditches. Also in contrast to Bti, Bacillus sphaericus has shown environmental persistence for two to four weeks, with the ability to grow and reproduce, though persistence is low in saline or highly organic environments.

Bacillus thuringiensis var . israelensis (Bti)

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.

BVA-2 Larvicide Oil

BVA-2 is a highly refined petroleum-based product that is used only on the pupal stage of the mosquito. This is a last resort material that is used to prevent adult emergence. It works by changing the surface tension of the water surface, causing both the larval and pupal stages to drown.

Because of its potential to affect other air-breathing aquatic insects, this insecticide is the least preferred mosquito control method used by the District. This insecticide does break down quickly in the environment, persisting as an effective surface film for only a few hours at best. Complete breakdown of this insecticide occurs within a few days. This chemical control strategy can be quite costly both in time and materials.


Methoprene, an insecticide known as an insect growth regulator, is a true analog of a mosquito’s own growth or juvenile hormone. This insecticide works by inducing damaging physical changes to second, third and fourth stage (instar) mosquito larvae preventing the successful emergence of adult mosquitoes from the pupal stage.

Unlike most insecticides, methoprene is not a direct toxin. This insecticide is target specific, affecting only mosquitoes when used at Federally and State mandated label rates for mosquito control. Like the larvicide’s Bti (Bacillus thuringiensis var. israelensis serotype H-14) and Bacillus sphaericus, this insecticide must be applied to all of the water areas that have larval mosquitoes to be effective. Methoprene has a very limited residual effect, being light sensitive and breaking down within three to four days in an aquatic environment. Timing of the applications of Methoprene is critical since this insecticide has no effect on pupal or adult mosquitoes. Therefore, it must be applied on a regular basis to those areas, which have continuous mosquito breeding problems. The effectiveness of Methoprene is limited by the density of vegetative cover, organic content of the water and the density of the larval mosquito population.

Methoprene is routinely used and is applied to those water sites that contain the larval or immature stages of mosquitoes. This material does not affect the pupal or adult stages and thus requires proper timing of the application in order to be effective.


Pyrethrins, extracted from chrysanthemum flowers, are a group of naturally occurring compounds with excellent insecticidal activity. This insecticide was first used as a powder around 1800 and has been used to control mosquitoes since the 1880’s. Pyrethrins are currently used in sprays, powders, mosquito coils and in truck-mounted ultra low volume (ULV) applications.

The toxicity of this insecticide is due to the presence of six chemically related compounds: Pyrethrin I, Pyrethrin II, Cinerin I, Cinerin II, Jasmolin I and Jasmolin II.

Pyrethrins are disruptive to the nerve function of insects by apparently affecting the nerve cell membrane. It is believed this insecticide modifies the cell membrane’s control mechanism so that nervous stimulation remains in a continuous state. This results in intense repetitive activity of both nerve fibers and sensory organs. Following this initial excitatory action, the nervous activity slows, and is followed by a complete synaptic nerve block. The action of pyrethrins can be classified as (1) knockdown, (2) paralysis, and (3) lethality. Pyrethrins act quickly affecting insects within one to a few minutes following exposure.

Natural pyrethrin insecticides are very unstable in the presence of light, moisture and air, breaking down within a few hours into non-detectable residues. No evidence of carcinogenic (cancer causing), teratogenic (birth defect causing), or mutagenic (genetic defect causing) properties have been reported. This insecticide is not a cholinesterase inhibitor, although a few asthmatic and skin reactions have been reported due to the flower impurities in the insecticide (not the pyrethrin molecule itself).

Napa County Mosquito Abatement District uses this insecticide as a last resort for controlling large populations of adult mosquitoes. It is applied using an ultra low volume (ULV) fogging machine, which uses less than one ounce per acre. Pyrethrins are a contact insecticide that can affect other insects and therefore must be used with caution. At mosquito label rates (dictated by Federal and State law) this insecticide has a minimal affect on non-target organisms and no affect to people, pets, birds and other wildlife. The District’s ULV program is limited and is only used when mosquito-borne disease outbreaks occur or when treehole mosquitoes become unbearable. Other restrictions include having an agreement within a community or neighborhood that allows for the use of this mosquito control material.

Pyrethrin is used to control yellowjackets and the Western Treehole Mosquito, a vector of Dog Heartworm. Use of this insecticide would also occur when the District, in conjunction with County public health authorities or the State, has declared a public health emergency. This pesticide comes in both powdered and liquid formulations and is applied with a bulb duster or the use of a truck mounted ultra low volume (ULV) machine that applies 0.87 ounces of liquid per acre by creating an extremely fine mist. Applications with a ULV machine can only occur when the wind is less than five miles per hour and an atmospheric temperature inversion exists at the site of the application.

A list of the pesticides used by the District can be found on our Pesticide Summary page. The pesticides we use are designed to be target specific and cause the least environmental disruption possible when used at the label rates dictated by State and Federal law. For more information you can go to the following links.