Solano County Mosquito Abatement District

MOSQUITO NOTES
Type: Pale Marsh Mosquito
Scientific Name: Ochlerotatus dorsalis

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

GENERAL INFORMATION
Ochlerotatus dorsalis is commonly referred to as the “pale marsh mosquito” because of its whitish-gray appearance. This species breeds in coastal salt marshes, brackish waters of the Sacramento-San Joaquin Delta, and some inland lakes of Northern California. This is a major pest in the San Francisco Bay area. Adults are medium-sized mosquitoes with yellow to straw coloration and a long white band seen from above. The end segments (tarsi) of the legs have broad white bands. The wings have narrow white and dark scales, having a “salt and pepper” appearance.

This species is found throughout most of the United States, excluding the Southeast. In California its distribution is primarily coastal, being common along the western border of the state.

LIFE CYCLE
Mosquitoes have four distinct life stages: egg, larval, pupal, and adult as seen in the illustration at the bottom. The female Oc. dorsalis deposits its eggs singly (up to 150) on the mud along the edge of receding tide pools. Winter is usually passed in the egg stage. The eggs usually hatch during the first warm weather of spring but may hatch as early as January if subsequent re-flooding of the marshes occurs. This often results in multiple generations of mosquitoes emerging during the summer. Eggs can remain viable for several years and do not all hatch with the next flooding. After marsh flooding, most of the eggs hatch into larvae (wigglers) within a short period of time after contact with water. The larvae feed on small organic particles and microorganisms suspended in the water. Feeding may take place at the bottom or at the water surface. The larval stage can last from 5 to 14 days, depending upon the temperature. Breathing takes place at the water surface and is accomplished by means of orienting upside-down with the tip of their tail (siphon) pointing up. At the end of the larval stage, the mosquito molts and becomes the pupa (tumbler). The pupa is active only is disturbed, for this is the resting stage where the larval form is transformed into that of 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 7-10 days. However, all mosquito developmental times are dependent on the temperature and food values of the water in which they develop. Oc. dorsalis can produce continuous broods through the spring and summer having 8-12 generations per year.

HABITS (ADULT BEHAVIOR)
Female Oc. dorsalis are vicious biters, attacking human beings and other mammals at any time of the day or night but are most active toward evening or on calm cloudy days.

Newly emerged adults may remain near their larval habitat for the first 24-48 hours before flying elsewhere. The females are strong fliers, dispersing long distances (up to 20 miles or more); sometimes males accompany the females on these flights. 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 is known to harbor California Encephalitis virus in California. It also has the potential to transmit West Nile virus. The vicious biting habits of this species can render areas where it is present virtually uninhabitable for man. Development of much of the areas in the San Francisco Bay area had to await the control of this species by organized mosquito control agencies. This species can be very annoying to livestock, 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 water 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 durable breeding sources of this mosquito, the use of mosquitofish has not been feasible. Other methods have not yet 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.