Mosquito season in the United States typically runs from early spring through fall, though the exact timing depends heavily on where you live. In southern states, mosquitoes can emerge as early as February, while northern regions may not see significant activity until May. The key trigger is temperature: mosquitoes become active when daily temperatures stay between 50°F and 95°F with humidity above 42%.
When Mosquito Season Starts by Region
The country breaks down into roughly five mosquito zones, each with its own timeline. In the Gulf Coast states, Florida, and Hawaii, mosquitoes can appear as early as February and persist through November. Some parts of southern Florida and Hawaii deal with mosquitoes year-round since temperatures rarely drop low enough to shut down activity.
Across the South and inland Southwest, the season typically kicks off in March and peaks from May through September. The Mid-Atlantic, Great Lakes, and northern Western states see mosquitoes arriving in April, with peak biting from June through August. New England, the northern Midwest, and Pacific Northwest have the shortest seasons, generally running from May through September with July and August as peak months. Higher-elevation areas in the interior West get the latest start, often not seeing significant mosquito activity until June.
These windows have been shifting. Research from the London School of Hygiene and Tropical Medicine projects that rising global temperatures could extend transmission seasons by more than a month for some mosquito-borne diseases over the next 50 years. Climate Central’s analysis tracks “mosquito days,” defined as days with temperatures between 50°F and 95°F and average humidity of 42% or higher, and many U.S. cities are already logging more of them each year than they did a few decades ago.
What Makes Mosquitoes Appear
Temperature is the primary switch. Below 50°F, most mosquito species enter a dormant state, essentially hibernating as eggs, larvae, or adults depending on the species. Once temperatures consistently reach the low-to-mid 50s, dormant eggs begin hatching and overwintering adults resume activity. Above 95°F, mosquitoes actually slow down and seek shelter, which is why the hottest desert climates can get a midseason lull.
Humidity matters nearly as much. Mosquitoes lose moisture through their bodies quickly in dry air, so they thrive when relative humidity sits above 42%. This is why the humid Southeast has a longer, more intense season than the arid Southwest despite similar temperatures. Rain plays a role too, but not directly through the moisture it adds to the air. Rainfall creates the standing water mosquitoes need to breed.
How Fast Mosquitoes Multiply
The speed of mosquito reproduction explains why populations seem to explode overnight after a warm rain. Female mosquitoes lay eggs directly on the surface of standing water, sometimes in rafts containing hundreds of eggs at once. Those eggs hatch into larvae within 24 to 48 hours. The larvae live in water for 4 to 14 days depending on temperature, with warmer water producing faster development. After that, pupae spend another 1 to 4 days in the water before emerging as flying adults.
From egg to biting adult, the entire cycle can take as little as a week in warm conditions. This means a forgotten bucket of rainwater, a clogged gutter, or a birdbath left unchanged for 7 to 10 days can produce a fresh generation of mosquitoes in your yard. Species that carry West Nile virus, those in the Culex genus, are especially drawn to any source of stagnant water and will lay egg rafts that hatch in just one to two days.
Peak Biting Times and Disease Risk
Most mosquito species are most active at dawn and dusk, roughly the hour after sunrise and the hour before sunset. Some species, particularly the ones that carry dengue and Zika, bite during the daytime as well. Midday heat usually suppresses activity for most species.
Disease transmission follows a seasonal arc that lags slightly behind mosquito population peaks. West Nile virus, the most common mosquito-borne illness in the continental U.S., circulates from roughly May through October. The virus needs time to amplify in bird populations before mosquitoes pick it up and transmit it to humans, so the highest risk window tends to fall in late summer and early fall, even though mosquito numbers may peak in midsummer. The same general pattern applies to Eastern Equine Encephalitis, which tends to appear in late summer in the eastern states.
Reducing Mosquitoes Around Your Home
Eliminating standing water is the single most effective thing you can do. Walk your property once a week and dump out anything holding water: plant saucers, pet bowls, toys, tarps, old tires, and wheelbarrows. Clean gutters so they drain fully. Change birdbath water every few days. Mosquitoes don’t need much, so even a bottlecap’s worth of water in a discarded container can support larvae.
For water features you can’t drain, like rain barrels or ornamental ponds, mosquito dunks containing a naturally occurring bacteria will kill larvae without harming fish, pets, or wildlife. Keep grass and shrubs trimmed, since adult mosquitoes rest in shaded vegetation during the heat of the day. If you use a fan on a porch or patio, the airflow makes it significantly harder for mosquitoes to land on you, since they’re weak fliers.
Choosing a Repellent That Works
The most reliable repellents use one of three active ingredients: DEET, picaridin, or oil of lemon eucalyptus. All three are registered with the EPA and have strong safety profiles when used as directed. The CDC recommends DEET products with concentrations of 20% or higher. Products above 50% DEET don’t offer additional protection, they just last somewhat longer before you need to reapply.
Concentration directly affects how long you’re protected. Repellents with less than 10% of any active ingredient typically last only one to two hours. A 20% to 30% DEET or picaridin product will cover most people for several hours outdoors. Oil of lemon eucalyptus provides comparable protection but generally needs more frequent reapplication and should not be used on children under three.
Apply repellent to exposed skin and over clothing, not under it. Spray it on your hands first, then rub it onto your face rather than spraying directly. If you’re also wearing sunscreen, apply sunscreen first and repellent second. For clothing-based protection, permethrin-treated shirts and pants repel and kill mosquitoes on contact and remain effective through multiple washes.
What Affects Your Season Length
Several factors determine whether your local season runs three months or nine. Latitude is the biggest, since it controls how early temperatures reach that 50°F threshold and how late they stay above it. Elevation works similarly: mountain towns at 5,000 feet or higher often have a compressed season that doesn’t start until June even if the surrounding lowlands are already dealing with mosquitoes in April.
Rainfall patterns matter for population size more than season length. A wet spring can produce massive early-season hatches, while drought can suppress numbers even during otherwise prime months. Urban heat islands can extend the season slightly in cities compared to surrounding rural areas, since pavement and buildings retain warmth longer into the fall. Proximity to permanent water sources like marshes, ponds, or slow-moving rivers guarantees a local breeding population regardless of yard maintenance.
The first hard freeze, when overnight temperatures drop below 32°F, effectively ends the season in most areas. In regions where freezes are rare or absent, like South Florida and coastal Southern California, mosquito activity simply fluctuates with rainfall and humidity rather than shutting down entirely.