The Winter Salt Marsh Mosquito (Aedes squamiger) is one of 53 types of mosquitoes that occur in California and is a distinctive black and white mosquito that breeds in California coastal pickle weed tidal and diked marshes. Salt marsh pools that are diluted by winter and early spring rains are especially favored breeding sites. Other sites include the cracked ground of diked wetlands and old dredge disposal sites. This species is a major pest of humans.

This species breeds in the tidal and diked marshes of Napa County. The eggs are laid on plants and muddy areas in the marsh during the spring and hatch as soon as the marsh fills with rainwater in the fall. Adults emerge the following spring and are an aggressive daytime biting mosquito capable of flying more than 15 miles from their breeding site.

Diseases Transmitted

Winter Salt Marsh Mosquitoes are a serious pest problem when they are present in large numbers. California Encephalitis virus has been found in populations of this mosquito although transmission of this virus to humans has not yet been confirmed

Life Cycle

Winter Salt 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.

Larval development varies from one to four months depending on weather conditions with developmental completion occurring near the end of February. 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.

Adult Daily Activity

Adults usually emerge during the last weeks of February through the end of March depending on the amount and timing of the preceding winter rains. Adults usually fly to areas away from their breeding sites, using ravines and natural or man-made waterways from the marshes to the local hills as passage ways. From these passage ways the adults spread laterally into the wind protected areas of the surrounding community. Adults then mate, with the females seeking a blood meal and returning to the marshes to lay their eggs. Mature mosquitoes can live as long as three months depending on temperature, humidity and other climactic factors.

Adult Flight Range

This mosquito readily flies 10-20 miles from its breeding site.

Adult Feeding

Humans and possibly other large mammals are the preferred hosts for this mosquito. Biting activity occurs most often during the daylight hours and at dusk from April through June.

Eggs and Larvae

Eggs are laid on plants and along the muddy margins of ponds close to the water line awaiting submersion by the following years tides and/or rainfall. It should be noted that the eggs can remain viable for many years with only part of any one batch of laid eggs hatching during any single flooding event.

The larval stage can last as little as a few weeks to as long as a few months depending on weather conditions and when the eggs hatched. Optimal larval development occurs in water with salinities of 5-15 ppt (less than 1/2 that of seawater) although complete development can also occur in seawater. Last stage (fourth instar) larvae of this species undergo a late winter diapause resulting in a partial synchronous emergence of the adults.