Microbial and Chemical Control

Microbial and Chemical Mosquito Control Materials

Download a fact sheet on aerial applications.
Download a fact sheet on ground-based applications.

Microbial and chemical control is the use of specific microbial and chemical compounds (insect growth regulators and insecticides) that eliminate immature and adult mosquitoes. They are applied when biological and physical control methods are unable to maintain mosquito numbers below a level that is considered tolerable or when emergency control measures are needed to rapidly disrupt or end the transmission of disease to humans. Larvicides target mosquito larvae and pupae in the water. Adulticides are insecticides that reduce adult mosquito populations. All products applied by the District are registered with the California Environmental Protection Agency, and are listed as public health pesticides.

Our primary goal is to protect public health and welfare by managing mosquito populations so they do not present a significant risk to our community. In the event that mosquitoes do pose a significant threat, we will respond by implementing the State of California Mosquito and Mosquito-borne Disease Management Plan.

Materials we use to control mosquitoes fall under two categories: larvicides and adulticides


Larvicides are used to eliminate immature mosquitoes, also known as mosquito larvae. A large proportion of our microbial and chemical control program is spent on controlling mosquito larvae. We have a variety of materials we are able to use for this purpose; among them are microbial insecticides, juvenile growth hormones, and larvicidal films and oils.

Microbial Insecticides

The product known as Bti (Bacillus thuringiensis israeliensis) can be as effective as chemical insecticides. Bti is a bacteria that, when eaten by mosquito larvae, produces a protein crystal that destroys the larvae’s intestinal lining. It has no effect on other aquatic organisms except midges in the same habitat.

Insect Growth Regulators

Methoprene is an insect growth regulator widely used by mosquito control districts. Methoprene mimics a natural juvenile hormone, and when present in the larval habitat it keeps immature insects from maturing into adults. Unable to develop into adults, the mosquitoes die in the pupal stage. Vector control technicians sometimes use methoprene to reach larval sources that would otherwise be difficult to treat.

Larvicidal Films and Oils

Larvicidal films are applied to the surface of standing water. The film reduces the surface tension of the water, making it difficult for mosquito larvae and pupae to attach to the surface and breathe. A significant advantage of using larvicidal films is that mosquitoes cannot develop a resistance, since control is through a physical mode of action.


Adulticides are insecticides that reduce adult mosquito populations. The application method used to target adult mosquitoes is known as ultra-low volume (ULV) spray. A small quantity of the material (between 1/2 to 1 ounce) is atomized into small droplets and broadcast in a fog aimed at where the adult mosquitoes are active. This method can be effective up to 300 feet away with ground applications and even further by aircraft with aerial applications, helping reduce the numbers of biting mosquitoes and the risk of mosquito-transmitted diseases.


The most common material used for ULV spraying are classified as pyrethrins. Pyrethrin is a naturally occurring substance harvested from chrysanthemums, and it degrades within 4 to 6 hours after spraying. Pyrethroids, a synthetic form of pyrethrins, are also commonly used.


Since the use of only one type of pesticide material can lead to pesticide resistance in the mosquito population, organophosphate insecticides are used in rotation with pyrethrins and pyrethroids to help ensure that adulticide applications remain effective.