Mosquito Control

In the lower elevations of Kern County, the mosquito control season usually runs from April through October, depending upon the weather. Mosquitoes must have water to complete their life cycle. Depending upon the species, adult female mosquitoes will lay egg rafts on the surface of water or will lay eggs singly on vegetation (or other surfaces) that are subject to later inundation. Mosquito larvae develop in the aquatic environment until they pupate and emerge an adult mosquito.

The District concentrates on controlling mosquitoes when they are in the aquatic stage because larval control has several benefits. The pesticides used to control mosquito larvae are less toxic to the environment and are highly specific to mosquitoes and, therefore,  do not affect beneficial insects, fish, birds or other mammals. Larvicides are applied to a smaller area than would be required for the treatment of adult mosquitoes because adult mosquitoes spread out and populate a wider area once they emerge. Lastly, controlling mosquitoes in the aquatic stage kills them before they are capable of transmitting disease. The District utilizes several different types of larvicide in its control program. Bacillus thuringiensis israelensis (“Bti”) is a naturally occurring bacteria that is “target specific” to mosquito larvae and does not adversely affect beneficial insects, fish, birds or other mammals. When larvae ingest the bacteria, their stomach lining ruptures which ultimately kills the larvae. Bacillus sphaericus (“Bs”) is another naturally occurring bacteria that is, also, only toxic to mosquito larvae. Altosid (methoprene) is a growth regulator which interferes with normal metamorphosis which prevents the larvae from emerging as adults. Mortality usually occurs in the pupal stage, resulting in no adult mosquitoes to bite and reproduce.  In some situations (such as dairy wastewater lagoons, highly organic or polluted water), effective control requires the use of a larvicidal oil which coats the surface of the water and smothers the mosquito larvae and pupae.

On occasion, the District will make applications of “adulticides” which are insecticides that reduce the adult mosquito population. When mosquito abundance traps indicate the population of mosquitoes in an area are at a level that could pose a threat to the public’s health, the District will apply adulticides by hand-foggers, backpack foggers or vehicle-mounted foggers.

Biological  Control – Mosquito  Fish:

Gambusia affinis, called “Mosquito fish,” are small live-bearing minnows related to the common guppy. Since mosquito fish do not lay eggs, these fish require no special environment, as most other fishes do, for depositing and hatching the eggs. Gambusia females produce eggs that hatch within their bodies. A female is able to produce several broods of young after being fertilized once. New broods are produced at intervals of about six weeks with 50 to 100 young in a single brood. Mosquito fish are effective predators of mosquito larvae and pupae since they are surface feeders. At birth, Gambusia are about 1/4 inch long but they grow rapidly, reaching a maximum size of about three inches. The earliest broods of the season, usually born in April and May, become sexually mature and produce young when six to eight weeks old.

These fish are placed in a variety of permanent and semi-permanent freshwater habitats, including non-maintained swimming pools, livestock watering troughs, ornamental ponds, sumps, reservoirs, duck clubs, and ditches. Mosquito fish are furnished without charge for residents.

Encephalitis  Virus  Surveillance:

Besides mosquito control, another important function the District performs is encephalitis virus surveillance. In the past, St. Louis Encephalitis and Western Equine Encephalitis were viruses that affected the health of residents in Kern County, but West Nile virus is now well established in California and currently is the prevalent encephalitis virus of public health importance.

Encephalitis virus surveillance is conducted in several ways. Since the encephalitis virus in nature is maintained in wild birds, the District participates in the California Department of Public Health’s (CDPH) West Nile virus dead bird program. Dead birds are reported to the CDPH by local residents.  If the Department of Public Health decides to test the bird for the presence of encephalitis virus, the CDPH notifies the District who then picks up and ships the bird to the State Lab for testing. The phone number for the WNV Dead Bird hotline is: 877-968-2473. Dead birds can also be reported online at:

Another method of encephalitis surveillance involves the use of collection traps baited with dry ice (CO2) and gravid traps that are baited with a hay/yeast fermented infusion. These traps collect live adult (female) mosquitoes. Samples of the female mosquitoes that are collected are “pooled” (placed into batches of 10 to 50 per sample) and sent to the State’s Lab and tested for the presence of encephalitis virus. Male mosquitoes do not take blood meals, so they are not important in virus transmission. Since adult mosquito populations can change markedly in response to various factors including water availability, temperature, and other environmental factors, the District deploys more than 100 traps and operates them weekly during the mosquito control season.

Encephalitis  Virus  Protection:

If you are spending time outdoors when mosquitoes are active (especially at dawn and dusk), apply insect repellent according to label directions.

Make sure that doors and windows have tight-fitting screens. Repair or replace screens that have tears or holes.

Eliminate all sources of standing water on your property that can support mosquito breeding. Backyard sources that collect rain or sprinkler water can produce substantial numbers of mosquitoes. Examples of such sources include: used tires, buckets, swimming pools, boat and pool covers, barrels, wheelbarrows, livestock watering troughs and almost any container that will hold water.

Report non-maintained (green) swimming pools to the District. The depressed housing market and home foreclosures have led to many neglected swimming pools. The District inspects and treats thousands of pools every year. One neglected pool can produce thousands of mosquitoes which can affect an entire neighborhood. The District will apply an insecticide that is mosquito-specific and/or will stock the pool with mosquito fish. There is no charge for this service. It is important to know that treating a green pool with an insecticide and/or mosquito fish will not change the water color in the pool – it will remain green, but mosquito-breeding will be controlled. Do not add chlorine to a swimming pool that contains mosquitofish because the chlorine will kill the fish. If the water in a swimming pool contains so much algae or debris that the bottom cannot be seen or if the pool emits an offensive odor, you should contact the appropriate city or county Code Compliance Department.