Southern California is home to roughly two dozen mosquito species, but the ones you’re most likely to encounter fall into two groups: native Culex mosquitoes that have always been here, and invasive Aedes mosquitoes that have established themselves across the region in recent years. Knowing which type is biting you matters because they behave differently, bite at different times of day, and carry different diseases.
Culex: The Native Night Biters
Culex mosquitoes are the species most Southern Californians grew up with. The most common local species are Culex quinquefasciatus (the southern house mosquito) and Culex tarsalis. They’re dull brown, medium-sized, and have blunt-tipped abdomens. You’ll rarely notice them during the day because they feed almost exclusively at dusk and after dark.
Culex mosquitoes are the primary carriers of West Nile virus in California. They also transmit St. Louis encephalitis virus, though cases are far less common. These species breed in stagnant water with organic matter in it: storm drains, neglected swimming pools, birdbaths, and flooded gutters. Females lay their eggs in distinctive floating rafts on the water’s surface, with dozens of eggs glued together in a small, boat-shaped cluster.
Because Culex mosquitoes feed on birds before biting humans, they serve as a bridge for bird-borne viruses. Local vector control districts across Southern California trap and test Culex mosquitoes year-round to monitor West Nile virus activity in real time.
Aedes aegypti: The Yellow Fever Mosquito
Aedes aegypti is an invasive species that has become firmly established across much of Southern California, particularly in Los Angeles, Orange, San Bernardino, and Riverside counties. It’s small, with striking black-and-white banding on its legs and a lyre-shaped pattern of white markings on its back. Unlike Culex mosquitoes, it has a pointed abdomen.
What makes this species so noticeable is its aggressive daytime biting. Even a small number of Aedes aegypti in a yard can make outdoor life miserable. They tend to stay close to where they hatched, rarely flying more than a few hundred feet, so infestations are often hyperlocal. Your neighbor a block away may have no problem while you can’t sit on your patio.
Aedes aegypti is the species global health authorities worry about most because it’s the primary vector for dengue, Zika, chikungunya, and yellow fever. While local transmission of these diseases has been limited in Southern California so far, the presence of an established population means the risk increases whenever an infected traveler returns to the area. Orange County’s vector control district has flagged these mosquitoes as a top concern.
This species lays its eggs singly, not in rafts, on surfaces just above the waterline of small containers. The eggs can survive dry conditions for months and hatch as soon as water reaches them. That means a forgotten saucer under a flowerpot, a bottle cap of rainwater, or even water inside your home (flower vases, lucky bamboo containers) can produce a new generation. A female can go from egg to biting adult in as little as five days under warm conditions.
Aedes albopictus: The Asian Tiger Mosquito
The Asian tiger mosquito is the second invasive Aedes species spreading through Southern California. It looks similar to Aedes aegypti, with black-and-white striped legs, but is distinguished by a single bright white stripe running down the center of its back rather than the lyre-shaped pattern of its cousin.
Aedes albopictus is also a daytime biter and breeds in the same small-container habitats. It can transmit dengue, chikungunya, and Zika, though it’s generally considered a slightly less efficient disease vector than Aedes aegypti. It tolerates cooler temperatures better, which means it can survive at higher elevations and in areas where Aedes aegypti might not thrive. Both species continue to spread into new parts of the state.
How to Tell What’s Biting You
The simplest way to narrow it down is timing. If you’re getting bitten after sunset, a Culex mosquito is the likely culprit. If something is biting your ankles and lower legs in broad daylight, especially while you’re sitting in your yard, that’s almost certainly an Aedes species.
Visually, Aedes mosquitoes are easy to spot once you know what to look for. They’re smaller than most Culex, and the bold black-and-white banding on their legs is distinctive. Culex mosquitoes are a more uniform brown. If you manage to swat one, check the abdomen: blunt and rounded means Culex, tapered to a point means Aedes.
The bite pattern is another clue. Aedes mosquitoes tend to bite low on the body (feet, ankles, calves) and are persistent. They’ll return to bite multiple times if disturbed. Culex mosquitoes are more opportunistic and will bite any exposed skin while you sleep.
When Mosquitoes Are Most Active
Mosquito season in Southern California runs from March through October, with populations peaking between June and September. July and August typically see the highest numbers because sustained heat above 80°F allows females to complete their life cycle in as few as eight to ten days.
Southern California’s mild winters set it apart from most of the country. Cold nights push activity down in October and November in inland areas, but coastal neighborhoods and warmer valleys can see biting mosquitoes well into November. Some species remain active year-round when temperatures stay mild, which is common in much of Los Angeles and San Diego counties. This extended season is one reason invasive Aedes mosquitoes have done so well here.
Reducing Mosquitoes Around Your Home
Both Culex and Aedes mosquitoes need standing water to breed, and the amounts involved are surprisingly small. Aedes mosquitoes will lay eggs inside homes in flower vases, plant saucers, or any container holding even a tiny amount of water. Outdoors, children’s toys, rain barrels, clogged gutters, unused fountains, and potted plant trays all serve as breeding sites. A weekly routine of dumping or treating any standing water you find makes a real difference, since it breaks the breeding cycle before new adults emerge.
For larger water features like backyard ponds, many local vector control districts provide mosquitofish free of charge. These small fish eat mosquito larvae and can keep a pond or horse trough clear without chemicals. San Diego County, Orange County, and the Greater Los Angeles County Vector Control District all offer this service to residents.
Because Aedes aegypti rarely flies far from where it hatched, controlling the breeding sites on your own property has an outsized effect on how many mosquitoes you encounter. This is different from Culex mosquitoes, which often breed in larger water sources like storm drains and may fly a mile or more to find a blood meal, making community-level control more important.
Disease Risk in the Region
West Nile virus is the most significant mosquito-borne disease currently circulating in Southern California. It’s carried by Culex mosquitoes and has been detected in the region every year since arriving in California in 2003. Most people who contract it have no symptoms or experience mild flu-like illness, but a small percentage develop serious neurological complications.
Dengue, Zika, and chikungunya are the diseases associated with the invasive Aedes species. These viruses are not currently circulating locally on a sustained basis, but small clusters of locally transmitted dengue have occurred in parts of California. The established presence of Aedes aegypti across Southern California means the infrastructure for local transmission exists whenever an infected person is bitten by a local mosquito that then bites someone else. Vector control agencies monitor for this scenario continuously.