Type: Pale Marsh Mosquito
Scientific Name: Ochlerotatus dorsalis
Habits (Adult Behavior)
Economic and Medical Importance
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.
Back To Top
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.
Back To Top
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.
Back To Top
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.
Back To Top
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
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
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.
Back To Top
About Us | Mosquitoes
| West Nile | Control
| Prevention | Links
| Contact | Home Page
Site designed and maintained by KCS Web Creations
© 2005 Solano County Mosquito Abatement District