Culex tarsalis is commonly referred to as the “encephalitis mosquito” because it is the primary vector of encephalitis viruses in the western United States.
Cx. tarsalis is a dark bodied, medium sized mosquito with a predominant whit band on its proboscis (beak) and white bands on the tarsi (feet). It is further characterized by a white stripe on the sides of the rear legs and dark inverted V’s on the under side of a blunt-tipped abdomen. Males resemble females except they have bushy antennae and long palpi on their head and “claspers” on the tip of their abdomen.
This is the most widespread mosquito species in California. It occurs from Mexico into Canada and in the western, central, and southwestern United States. It has been recovered from sea level to about 9,000 feet.
Mosquitoes have four distinct life stages: egg, larval, pupal, and adult as seen in the illustration at the bottom. The first three stages of Culex (egg-larva-pupa) are spent in the water. An adult female lays about 150-200 eggs in clusters called rafts, which float on the surface of the water until they hatch, in about two days. Females usually prefer to lay eggs in clear, standing water sources. These include rain pools, marshes, reservoirs, pools, rice fields, irrigation tailwaters, ditches, and domestic sources. Although this species prefers clear, standing waters, it is sometimes found in vegetation along stream margins and in polluted sources. Maximum populations usually occur in late summer.
The eggs hatch into larvae (wigglers), which then feed on small organic particles and microorganisms in the water. Culex larvae may hang from the tip of their tail (siphon) when they feed or they may feed along the bottom, but they must return to the water surface to breathe. 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 may take 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 usual conditions, development from egg to adult takes about 10 days. However, all mosquito developmental times are dependent on the temperature and nutrients of the water in which they mature.
These mosquitoes may live for two or three weeks in summer, but under cooler conditions the females may live for several months. In areas of moderate climate, larvae may be found in every month of the year, but in areas with cold winters this species usually passes the winter as hibernating females in protected natural or artificial shelters such as cellars, outbuildings, wood piles, caves, culverts, and rodent burrows.
HABITS (ADULT BEHAVIOR)
Females of this species are moderate but common biters of man, attacking at twilight and after dark. Adults will enter dwellings seeking a blood meal, however they seek shelter during the daytime. Although they feed on humans and domestic animals, this species appears to prefer the blood of birds. Males do not bite, but instead feed on nectar and plant juices. Females may also feed on plant juices, but must obtain a blood meal in order to complete egg development. Adult females are commonly found within 7 miles of their breeding habitat, but are capable of flights up to 16 miles.
ECONOMIC AND MEDICAL IMPORTANCE
This species is the most important known vector of Western Equine Encephalitis (WEE) and St. Louis Encephalitis (SLE) viruses in California. California Encephalitis has also been isolated from it. This species was an important vector of West Nile virus in southern California during 2004. It also has a good potential to transmit Venezuelan Equine Encephalitis (VEE) virus if this disease should ever become established in California. From a disease standpoint, this is the most important species of mosquito in the state.
PREVENTION AND CORRECTION
Where possible, the best approach is to prevent mosquitoes from breeding by eliminating or modifying breeding sites. This may be accomplished by such actions as filing, dumping, ditching, or otherwise draining the source. Only rarely is this species found in various containers around the home, but these should be emptied of standing water.
The most commonly used biological control agent in mosquito control is the mosquitofish (Gambusia affinis). Mosquitofish are commonly stocked in sources such as fish ponds, ornamental ponds and watering troughs.
At times, it may become necessary to control populations of Cs. incidens chemicals when other methods of control have failed. 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 should only be used until longer lasting 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.