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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.