Solano County Mosquito Abatement District

MOSQUITO NOTES
Type: California Salt Marsh Mosquito
Scientific Name: Ochlerotatus squamiger

General Information
Life Cycle
Habits (Adult Behavior)
Economic and Medical Importance
Control Methods

GENERAL INFORMATION
Ochlerotatus squamiger is commonly referred to as the California Salt Marsh Mosquito because it breeds exclusively in the salt and brackish marshes along the California coast. It is medium to large in size with a grayish or black coloration. The end segments of the legs have broad white bands. The mixture of dark and light scales on the wings gives them a “salt and pepper” appearance.

This species breeds in marsh waters following extra high tides or rains. It occurs only along the Pacific Coast from Sonoma County to Baja California. It has been one of the major problem mosquito species in the San Francisco Bay area within the recorded history of the area.

LIFE CYCLE
Mosquitoes have four distinct life stages: egg, larval, pupal, and adult as seen in the illustration at the bottom. The female Oc. squamiger deposits its eggs singly (up to 150) on the mud along the edge of receding tide pools. The eggs remain unhatched until the next late fall or winter rains and high tides. Eggs can remain viable for several years and not all will hatch with the next flooding. After such marsh flooding, most eggs may hatch into larvae (wigglers) within a short period of coming into contact with water.

The larvae feed on small organic particles and microorganisms suspended in the water. Feeding takes place either at the bottom or near the water surface. Breathing takes place at the water surface and is accomplished by means of orienting upside-down with the tip of the tail (siphon) pointing up. At the end of the larval stage, the mosquito molts and becomes a pupa (tumbler). The pupa is active only if disturbed, for this is the “resting stage” when the transformation from the larval stage to winged adult takes place. After this transformation has been completed, the adult splits the pupal skin and emerges.

Oc. squamiger is usually single brooded, but two broods may occur under unusual climatic conditions (i.e. El Nino conditions). The larval stage usually matures during the winter and adults emerge from mid February to May. Adult females may live up to three months.

HABITS (ADULT BEHAVIOR)
Female Oc. squamiger are vicious biters, attacking man and other mammals at any time of day, but maximum biting activity occurs at twilight. They may bite actively at night indoors under lights, but normally only bite outdoors. The females are strong fliers, migrating long distances (up to 20 miles or more) in large numbers. Males do not bite, instead they feed on nectar and plant juices. Females may also feed on plant juices, but must obtain a blood meal in order to develop their eggs.

ECONOMIC AND MEDICAL IMPORTANCE
This species has not been known to be commonly involved as a natural vector of disease producing organisms in California. A limited number of this species was found to be infected with West Nile virus during 2004. The vicious biting habits of this species can render areas where it is present virtually uninhabitable for man. Livestock can become agitated, resulting in reduction in feeding and possible injury to frantic animals attempting to escape severe attacks.

CONTROL METHODS
PREVENTION AND CORRECTION:
The most important method of controlling salt marsh mosquitoes is to eliminate or modify the specific areas in the salt marshes where the larvae occur. This may be accomplished by circulation ditching, which permits the water from very high tides or rains to flow back into the bay or ocean.

BIOLOGICAL CONTROL
The most commonly used biological control agent in mosquito control is the mosquitofish (Gambusia affinis). Due to the salinity variance and shallowness of many of the breeding sources for this mosquito, the use of mosquitofish has not been feasible, and other biological control methods have not been developed.

CHEMICAL CONTROL
Problems with drainage or prevention may develop which can make it necessary to use chemical control. Only trained mosquito and vector control personnel should apply chemical control agents. Control agencies have knowledge of the proper compounds and application techniques to assure minimal environmental side effects.

It is important to remember that chemical control provides only temporary relief and is used by mosquito control agencies until other measures can be implemented.

Insect repellents may be useful if it is necessary to be in an area where large numbers of these adults are present. Always read and follow the directions on the label carefully when using a repellent.