When Does Mosquito Season End in San Antonio?

Two species drive most of the backyard biting pressure in the San Antonio metro. The yellow fever mosquito is a container-breeder, it deposits eggs above the waterline in plant saucers, clogged gutters, bird baths, bottle caps, and corrugated downspout extensions. It is highly adapted to suburban environments and bites during daylight hours, particularly around dawn and dusk.

Culex quinquefasciatus, the southern house mosquito, breeds in stagnant, organically enriched water, poorly maintained rain catchments, low areas that hold water after rain, unmaintained pools, and natural pools in drainage channels. Culex mosquitoes bite primarily at dusk and through the night. Both species remain active as long as temperatures exceed 50°F and standing water is available for breeding.

The primary limiting factor for mosquito activity is temperature. Adult mosquitoes become inactive below approximately 50°F and die or enter diapause when temperatures fall below 40°F for extended periods. San Antonio's average low temperature in January is 39°F, and average lows only consistently fall below 50°F in December and January. This gives the city an effectively year-round mosquito window during warmer-than-average winters.

Rainfall patterns compound the season length. San Antonio's bimodal rainfall pattern (spring rains in April and May, and a secondary rainy period in September and October) creates precisely timed breeding opportunities at the beginning and end of the season. The spring rains initiate the population surge. The fall rains extend it well into November, often delaying the natural population decline that might otherwise occur by mid-October.

June through October represents the period of highest mosquito density in most San Antonio neighborhoods. Summer heat (regularly above 100°F) desiccates container breeding sites quickly, so Aedes aegypti populations fluctuate with rainfall frequency. Unusually dry summers can temporarily reduce Aedes populations, while summers with frequent afternoon thunderstorms produce consistently high breeding conditions.

September and October deserve special attention. Temperatures moderate from summer extremes, outdoor activity resumes after people have retreated indoors through August, and fall rains refill breeding sites. This combination makes the September-October period one of the highest-activity windows of the season despite the cooling trend.

The CDC tracks mosquito-borne illness activity by state and county. Texas regularly reports cases of West Nile virus, which is transmitted by Culex species mosquitoes, primarily in late summer and fall when Culex populations peak. Dengue fever cases are more commonly associated with travel but have been locally transmitted in South Texas in recent years, as the Aedes aegypti range extends northward.

Texas Parks and Wildlife and the Texas Department of State Health Services maintain surveillance programs for mosquito-borne illness in the San Antonio region. Residents who observe unusual illness following mosquito exposure should consult a physician and mention potential mosquito exposure.

Source reduction (eliminating standing water where mosquitoes breed) is the most fundamental control measure and one that homeowners can implement entirely on their own. A complete audit of the yard should cover gutters, plant saucers, pet water bowls, tarps, children's toys, corrugated plastic downspout extensions, bird baths, and any low area of the yard that holds water for more than four days after rain.

Barrier spray treatments applied to vegetation provide residual protection against adult mosquitoes resting on foliage during daylight hours. These treatments are most effective when applied to the underside of large-leaf shrubs, the lower canopy of trees, and shaded vegetation around fence lines, the resting habitats favored by adult mosquitoes during daylight. In San Antonio, live oak canopies and Turk's cap plantings around foundations are common daytime resting sites.