California’s mosquito season does not have a single fixed end date, as its conclusion is tied directly to local environmental conditions. The decline in mosquito activity is a gradual regional process driven by falling temperatures and the reduction in breeding habitats. This variability means the season can end in early fall in some parts of the state while lingering into the winter months elsewhere.
Defining the End of Mosquito Season
The peak of mosquito activity generally occurs during the late summer months of July and August. The season begins its significant decline as temperatures start to moderate consistently. Active, biting populations typically wind down through October, with the season generally concluding by late October or November. The “end” is defined by a sharp drop in adult mosquitoes seeking blood meals, which is a direct response to cold weather and shorter daylight hours.
Regional Variations Across California
California’s geographical diversity creates microclimates that heavily influence when mosquito activity ceases. Northern California and higher-elevation Central Valley regions experience a faster seasonal end, often seeing a significant drop in populations by mid-October as sustained cold weather arrives sooner. The Central Valley, a major agricultural hub, sees the season end when Culex species, primary vectors for West Nile Virus, initiate overwintering strategies in mid-to-late October. Southern California’s coastal and Inland Empire areas often maintain warm enough temperatures to support a more extended season. Active mosquitoes, particularly the invasive Aedes species, can sometimes be found well into December in these urban environments.
The Environmental Triggers of Season End
The biological mechanism that halts the mosquito season is diapause, a state of reproductive arrest. Adult female mosquitoes, such as Culex tarsalis, cease reproductive development and seek sheltered locations to survive the winter when conditions become unfavorable. The primary trigger for this change is a decrease in both daily temperature and the number of daylight hours, known as photoperiod.
Non-diapausing species generally become inactive and die off as the temperature consistently dips below a threshold of 50°F. This survival mechanism ensures that the next generation of diapausing species can emerge in the spring. A secondary factor contributing to the seasonal end is the reduction of standing water, which limits breeding sites for the aquatic larval stage.
Post-Season Disease Risk
Even as the visible mosquito population decreases in late fall, the risk of disease transmission does not immediately drop to zero. West Nile Virus (WNV) is the most prevalent mosquito-borne illness in California, with cases typically peaking in late summer and early fall. The Culex mosquitoes responsible for transmission often enter diapause already carrying the virus. WNV can persist in these overwintering populations, creating the potential for early-season transmission the following spring. Public health officials continue to monitor for WNV activity well into the cooler months, and residents should maintain vigilance even when biting activity is reduced.