The Winter Marsh Mosquito (Culiseta inornata) is one of 53 types of mosquitoes that occur in California. One of California’s largest mosquitoes, it reaches a size of 10 mm or almost 1/2″. Adults are a light brown or reddish-brown in color with no unusual or distinctive markings. Populations of this mosquito have been found in most California counties. Total developmental time from egg to adult varies from two to seven weeks depending on water temperature and weather conditions.

Females of this species rest during the summer and become active in the fall after the first rains. Eggs are laid on the surface of rain filled ponds in the fall. Many generations can be produced in one season. This mosquito bites at dusk in the fall and spring and can be aggressive. This is our largest mosquito, measuring approximately 3/8 of an inch in size.

Diseases Transmitted

Winter Marsh Mosquitoes do occasionally create domestic, industrial and agricultural pest problems when they are present in large numbers. No known mosquito-borne diseases have been linked to this mosquito

Life Cycle

Winter Marsh Mosquitoes have four life stages: egg, larva, pupa, and adult. The immature stages need standing water to complete their life cycle.

After an adult female lays her eggs they hatch into larvae (wrigglers), which feed on small organic particles and microorganisms in the water. Feeding occurs when they hang from the water’s surface by the tip of their tail (siphon) or by browsing along the bottom of their habitat. Because they are air breathing organisms they must return to the water’s surface to breathe.

About one to six weeks are required for larval development. 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 two weeks. However, all mosquito developmental times are dependent on the temperature and nutrients of the water in which they mature.

Adult Daily Activity

Adults are present fall, winter and spring entering diapause (a form of hibernation) in the summer as a means of surviving California’s dry weather. Aestivating females are thought to emerge from mammalian burrows and shelters in the fall following decreased temperatures and the first fall rains. Both males and females are strongly attracted to lights. The average life expectancy for adult females varies from 90 to 145 days depending on temperature, humidity and availability of nutrients. Adults usually do not swarm and females freshly emerged from their pupal case are mated by waiting males.

Adult Flight Range

Adults of San Francisco Bay area populations tend to stay close to their breeding sites, usually traveling less than two miles for a blood meal. The maximum recorded flight range is 14 miles. Optimal flight activity occurs on overcast, cool days and an hour before to an hour after dark.

Adult Feeding

Adult female mosquitoes feed primarily on large domestic mammals although San Francisco Bay area populations associated with brackish marshes have been significantly pestiferous to humans.

Eggs and Larvae

Clusters of glued together eggs, also known as egg rafts, are laid directly on the water with a typical egg raft containing 150-200 eggs. Studies have shown that the eggs of this mosquito can tolerate temperatures as low as 17 degrees Fahrenheit, with 50% of the eggs surviving after a 48 hour exposure.

Larval emergence usually occurs within two to three days. Total larval developmental time varies from one to six weeks depending on temperature, weather conditions, competition for space and available nutrients. Larvae are tolerant of moderate organic pollution and have been found developing in water with a salinity of 26 ppt (72% that of seawater).