Mosquito season typically lasts from early spring through early fall in most of the United States, roughly April to September. But the exact window varies dramatically by region, spanning as few as four months in northern states to nearly year-round along the Gulf Coast and in Hawaii. Temperature, daylight hours, and humidity all determine when mosquitoes appear and when they finally disappear.
Mosquito Season by U.S. Region
Where you live is the single biggest factor in how long you’ll deal with mosquitoes. Here’s what to expect across the country:
- New England, Northern Midwest, Pacific Northwest: Season runs from May through September, with peak activity in July and August.
- Mid-Atlantic, Great Lakes, Northern West: Mosquitoes arrive in April, peak from June through August, and taper off by late September.
- Interior West at higher elevations: The shortest season in the lower 48, running June through September with peaks in July and August.
- South and inland Southwest: Activity starts as early as March, peaks from May through September, and ends around October.
- Gulf Coast, Florida, and Hawaii: Mosquitoes emerge in February, peak from June through October, and persist into November. In parts of southern Florida and Hawaii, they’re active year-round.
In tropical and subtropical regions worldwide, mosquitoes are a constant presence. About half the global population lives in areas where mosquito-borne diseases like malaria and dengue pose a year-round threat, with the heaviest burden falling on Africa and Asia.
What Triggers Mosquitoes to Appear and Disappear
Mosquitoes aren’t simply responding to warm weather. Their seasonal cycle is driven by a combination of temperature, day length, and humidity, with day length playing a surprisingly central role.
Most mosquito species need temperatures above about 50°F (10°C) to become active, with optimal flight occurring around 70°F (21°C). Their immature stages struggle below 60°F (16°C) and above 100°F (38°C), with the sweet spot for larval survival around 79°F (26°C). When temperatures climb too high, mosquito survival actually drops. The species most comfortable in extreme heat still sees its lowest survival rates around 95°F (35°C).
But temperature alone doesn’t shut mosquitoes down for winter. Shortening daylight is the primary signal that tells female mosquitoes to lay special dormant eggs, a process called diapause. These eggs can survive freezing conditions and hatch when conditions improve the following spring. Research has confirmed that short daylight hours alone are enough to trigger this dormancy, while cold temperatures without shorter days are not. This is why mosquitoes don’t simply vanish the first time temperatures dip in early fall. They’re reading the calendar as much as the thermometer.
When Mosquitoes Are Most Active During the Day
Even during peak season, mosquito activity rises and falls throughout the day based on humidity. Mosquitoes are most active when relative humidity is high, generally between 40% and 90%, with activity dropping sharply above 90%. Because humidity peaks during the coolest parts of the night and drops during the warmest afternoon hours, mosquitoes tend to be most aggressive at dawn, dusk, and into the evening. The midday sun that dries out the air also keeps them at bay.
This pattern explains why you can sit outside comfortably at noon in July but get swarmed during a backyard dinner. If you’re planning outdoor time during mosquito season, the hours around sunrise and sunset are when you’ll need the most protection.
The Season Is Getting Longer
Mosquito season in the United States is measurably longer than it was 25 years ago. A 2024 study tracking temperature suitability for West Nile virus transmission found that the mosquito season has expanded by an average of nearly 25 days since 1999. The season now starts about 4 days earlier in spring and, more significantly, extends roughly 20 days later into fall.
That asymmetry matters. The bigger shift is happening at the tail end of the season, meaning mosquitoes are lingering well into October in areas where they once disappeared by mid-September. The effect varies by county, but the overall trend is consistent across the country. Warmer autumn temperatures are giving mosquitoes extra weeks of activity that didn’t exist a generation ago.
What This Means for Planning
If you’re trying to figure out when to start and stop worrying about mosquitoes, use nighttime temperatures as your guide. Once overnight lows consistently stay above 50°F in spring, mosquitoes will follow within a week or two. In fall, they won’t fully disappear until nighttime temperatures regularly drop below that threshold and daylight hours shorten noticeably.
Standing water is the other variable you can control. Mosquitoes lay eggs in as little as a bottle cap’s worth of stagnant water, and larvae can mature in under a week during warm weather. Dumping out flowerpot saucers, bird baths, and clogged gutters throughout the season eliminates breeding sites near your home. This is especially worth doing in the early weeks of spring, when reducing the first generation of mosquitoes can noticeably cut their numbers for the rest of the season.