By Shannon Kolarik · Updated November 6, 2026

When Is Mosquito Season? A Month-by-Month Map of Every US County

November

  1. J
  2. F
  3. M
  4. A
  5. M
  6. J
  7. J
  8. A
  9. S
  10. O
  11. N
  12. D

What the colors mean
  • Asleep below 50°F
    Too cold to fly. Adults die or go dormant; eggs wait it out.
  • Low cool, or warm but dry
    A few around, mostly at dusk. Either barely warm enough, or hot with nowhere to breed.
  • Active warm with some rain
    Breeding in whatever water is around and biting most evenings.
  • Peak hot and wet
    Fast breeding, the most bites and the highest disease risk. Reapply hourly outside.

“Is it mosquito season already?” is the question I get most, usually from someone who just got bitten in March. There isn’t one answer. Miami and Minneapolis have different seasons, and so do Dallas and Denver. So we built the map above: every county in the country, scored on temperature, rain and real sightings, month by month.

The year, region by region

  • January – FebruarySouth Florida, the Rio Grande Valley, coastal Louisiana and Hawaii never stopped. Everywhere else is quiet.
  • MarchThe Gulf Coast and Deep South switch on, with low-desert Arizona and Southern California. Texas, Georgia and the Carolinas follow by month’s end.
  • AprilThe Mid-Atlantic, the Ohio Valley, the southern Plains and most of California join. The South is already active.
  • MayThe Northeast, the Upper Midwest, the Northwest and the Rockies thaw. By the end of May nearly every county in the Lower 48 has mosquitoes out.
  • June – AugustPeak across the East, the South and the Plains, and in Alaska, whose whole season fits in these weeks. The West’s dry summer keeps the Central Valley, the Great Basin and the Northwest at low or active; the desert Southwest reaches peak once the monsoon arrives. Most West Nile cases are reported in August and September.5
  • September – OctoberThe North goes quiet first: the mountains and northern tier in September, the Midwest and Northeast through October. The South is still fully active.
  • November – DecemberTexas, the Gulf and the Southeast wind down through November. Florida, Hawaii and the Gulf shore roll straight into next year.

How the map is scored

Three things go into each county’s month. Temperature sets whether mosquitoes can fly at all and how fast they breed: nothing happens below about 50°F, and above it everything speeds up with the heat.1 Rain sets how much water there is to breed in; every mosquito starts in standing water, so a hot month with none is a quiet month. And people’s own sightings tell us when they actually meet mosquitoes.

For each county we take NOAA’s long-term average temperature and rainfall for every month,2 carry over part of the previous month’s rain (puddles don’t reset on the 1st), and combine them into a climate score. Then we fold in roughly 96,000 mosquito sightings that people have logged on iNaturalist,4 state by state and month by month, corrected for how much people are out observing anything at all. The sightings reshape the season to match what people really see, like the Gulf Coast’s spring and fall peaks with a lull in the hottest weeks, without inflating a place that is cool or dry. The result is one of four levels: asleep, low, active or peak. “Low” covers both the cool edges of a season and the dry West, where July is warm enough and there’s nothing to breed in.

50°F

Below it mosquitoes can’t fly. Once nights stay above it, they’re out at dusk; once nights drop back under, the season is over, whatever the afternoon highs say.

NIH studies via Climate Central1

7–10 days

From egg to biting adult. A soaking rain shows up as mosquitoes a week or two later, which is why the map carries part of last month’s rain into this month.

CDC mosquito life cycle3

What the map can’t see: a county average blends the hot valley floor with the cool ridge above it, so a city runs a week or two ahead of its county. And irrigation, pools and clogged gutters make water the rain gauge never records, which is why a “low” desert suburb still has mosquitoes in the yard.

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References
  1. Climate Central. This News Bites: More Mosquito Days, 2020. Mosquito-suitable days are defined as 50–95°F with relative humidity of 42% or more, citing NIH-published studies.
  2. NOAA National Centers for Environmental Information. Climate at a Glance: County Mapping, average temperature and precipitation, 1901–2000 monthly means. Alaska and Hawaii, which the county dataset doesn’t cover, use statewide estimates.
  3. Centers for Disease Control and Prevention. Mosquito life cycle: Culex species. Eggs are laid on standing water in rafts of 100 to 300, and it takes about 7–10 days for an egg to become an adult.
  4. iNaturalist observations of family Culicidae in the United States, verifiable records by month of observation, pulled November 2026 and divided by all observations in the same state and month to correct for observer effort. The activity level is Notch’s own scoring of climate and sightings, not a measured mosquito count.
  5. Centers for Disease Control and Prevention. Data and Maps for West Nile.