Ask most Texans when mosquito season is and they'll say summer. Ask an entomologist and the answer is more complicated — especially for San Antonio. The combination of a subtropical climate, mild winters, and rainfall patterns that can produce wet stretches at almost any time of year means the San Antonio mosquito calendar does not follow the neat summer arc that people expect. Knowing when the real peaks hit, what drives them, and when you can actually relax helps you plan protection that works.
Quick answer
Mosquito season in San Antonio peaks from April through October, with the worst pressure in the hot, humid months of June through September. Because San Antonio winters are mild, mosquitoes rarely go fully dormant — a warm winter day can bring them back out, and the season resumes hard as soon as spring warms. Treatment started in March pays dividends all summer.
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San Antonio's Climate and Why It Extends the Season
San Antonio sits at the northern edge of a subtropical climate zone. Average winter lows rarely drop below freezing for extended periods, and when they do, warm-ups can follow within days. That mild winter pattern is why the mosquito season here is so much longer than in northern Texas or the Midwest.
Mosquitoes need two conditions to stay active: temperatures above roughly 50 degrees Fahrenheit and available water to breed in. San Antonio frequently satisfies both conditions for ten or more months of the year. That does not mean mosquito pressure is uniform across the calendar — there are clear peaks and genuine lulls — but the season is genuinely longer than most people assume going in.
Peak Season: April Through October
The sustained peak of mosquito activity in San Antonio runs from April through October. Within that window, pressure is typically heaviest from June through September, when heat and humidity are both high and rainfall events keep breeding sites stocked with water.
April and May represent the ramp-up period. Spring rains refill containers, low spots, and drainage areas, and the first major population buildups of the year happen during this stretch. By June the population is in full swing and stays there through the summer. October begins the gradual wind-down, though a warm, wet fall can extend heavy pressure well into November.
- March: first mosquitoes emerge on warm days, breeding begins in earnest
- April to May: population ramps up with spring rain and consistently warm temperatures
- June to September: peak season — heat, humidity, and repeated rain drive fastest breeding
- October to November: activity winds down but continues in warm stretches
- December to February: low activity; mosquitoes present in mild weather, dormant in cold
How Rainfall Creates Population Spikes
Temperature determines the background level of mosquito activity. Rainfall determines the spikes. A significant rain event — enough to refill containers, puddles, and low spots across the yard — can push a mosquito population from manageable to overwhelming within about a week, which is roughly how long it takes for a new generation to go from egg to biting adult in warm weather.
San Antonio's rainfall is notoriously variable. The region can go weeks without meaningful rain and then receive several inches in a short span. Each of those wet periods resets the mosquito population upward. That is why the worst weeks of the year often follow stormy stretches rather than simply the hottest days, and why source reduction — eliminating the standing water before it can produce a new generation — is a continuous effort rather than a one-time task.
Temperature Thresholds: When Mosquitoes Slow Down
Mosquitoes are cold-blooded. Their metabolism, movement, and breeding rate all track with ambient temperature. Below about 60 degrees Fahrenheit, activity slows noticeably. Below 50 degrees, adult mosquitoes become largely inactive. Below 40 degrees for sustained periods, they die or enter a dormant state.
In San Antonio, sustained cold that keeps mosquitoes fully dormant is relatively rare and short-lived. A January cold snap may knock activity down for a week, only for a warm front to bring it back. The winters of 2021 and other years with hard freezes do produce genuine population crashes, but in a typical year the lulls are partial rather than complete.
When to Start and Stop Treatment
The case for starting treatment in March — before the worst of the season — is straightforward. The population is still small in early spring. Treating those first generations interrupts the multiplication cycle before it builds to summer levels. Every female mosquito that does not survive to breed in March is not the source of dozens of offspring in April and May.
When to stop is less clear-cut than it is in northern states. If the October forecast shows continued warm weather, there is real value in extending service into November. A professional can track conditions and adjust the schedule rather than defaulting to an arbitrary end date. Properties that stop treatment too early often find themselves with a late-season surge that continues until the first hard frost.
Practical Takeaways for San Antonio Homeowners
Plan for a long season. If you are thinking of mosquito treatment as a June-to-August service, you are protecting less than half the window during which mosquitoes are active in San Antonio. April and October are real months in the San Antonio mosquito calendar.
Think about rainfall, not just heat. The worst weeks follow the wet ones. After any significant rain event, inspect your yard for standing water and address it before the next generation hatches. This is especially true in late spring and early fall when moderate temperatures mean the breeding cycle is fast but not everyone has their guard up yet.
Start early. The compounding math of mosquito populations means that catching the first spring generation is worth more than treating the same number of adults in July. A treatment in March or early April disrupts multiplication at the point where the gains are largest.
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