skip navigation , health centers and clinics , search , accessibility statement , Página en español ,
Contra Costa Health Services
contact us

Home > Health Topics > West Nile Virus > Septic Tanks and Mosquitoes

Septic Tanks and Mosquitoes

You Can Prevent Mosquito Breeding

What to do?

  • Empty or cover receptacles that would otherwise hold water.
  • Fill tree holes with a polymer.
  • Put mosquito fish in permanent ponds.
  • Store old tires inside or cover them.
  • Clean clogged gutters.
  • Manage irrigation water effectively.
  • Report mosquito breeding sites.

Vent Pipes

All vent pipes on the house should be covered with window screening and secured with a hose clamp or a plastic cable tie. When present, vent pipes and inspection ports for your leach field also need screening.

Septic tanks are a common source for mosquito breeding.

The House Mosquito, Culex pipiens thrives in these water sources. All septic tanks must be covered in such a way that will not allow mosquitoes from entering the system. Placing plywood or boards over a tank or manhole cover will not provide adequate coverage. Exposed tanks or manhole lids should be covered with plastic, or a like material and secured in place.

All septic tanks must be covered

Tanks & Lids

Cover exposed tanks or manhole lids with plastic, or a like material and secure in place. Cover the plastic with several inches or more of dirt or sand. Mark the location to make it easy to find later. Those with newly installed septic systems (identified by their PVC plastic risers and lids) may already be adequately sealed without this step. Be certain the lid is fastened and properly secured.

Life Cycle

Mosquitoes have four distinct life stages as seen in the illustration, with the first three stages of Culex (egg-larva-pupa) being spent in the water.

An adult female lays about 150-200 eggs in clusters called rafts, which float on the surface of the water until they hatch in about one to two days.

Females usually prefer to lay eggs in standing, polluted water, such as sewage, street drainage, septic tanks, industrial wastes and backyard sources that include swimming pools, ornamental ponds, cooler drain-water and fouled water in containers. A wide variety of other water sources may also be infested with the aquatic stages of this common mosquito.

The eggs hatch into larvae (wigglers), which then feed on small organic particles and microorganisms in the water. At the end of the larval stage, the mosquito molts and becomes the aquatic pupa (tumbler). The pupa is active only if disturbed, for this is the "resting" stage where the larval form is transformed into the adult. This takes about two days during which time feeding does not occur. When the transformation is completed, the new adult splits the pupal skin and emerges. Under optimum conditions development from egg to adult takes about a week. However, all mosquito developmental times are dependent on the temperature of the water in which they mature.

Culex Pipiens Pipiens
Culex Pipiens Quinquefasciatus

These two subspecies are so similar that they will be treated here as one. They are commonly called "house" mosquitoes because of their often close relationship with humans and their habit of entering into houses and sometimes even breeding in containers indoors.

C. pipiens is a light brown, medium sized mosquito with a blunt-tipped abdomen. There are narrow white bands on the abdominal segments but none on the legs or proboscis (beak).

This is the most widely distributed mosquito species in the world. They are also widespread throughout the U.S. and California.

Contra Costa County home page