When Does Mosquito Season Start? Dates Across the US
It starts when the days hold above 50°F
The common biting mosquitoes in the US, Culex and Aedes, are cold-blooded. Below a mean of about 50°F they barely fly, feed or develop. So a county's season starts the day its average temperature climbs through 50°F in spring, or after the last hard freeze if that comes later. That is why the date slides from late winter on the Gulf Coast to late spring in the northern Plains.
Start month, county by county
| Season starts in | Counties | Share |
|---|---|---|
| January | 94 | 3% |
| February | 271 | 9% |
| March | 884 | 28% |
| April | 1,341 | 43% |
| May | 479 | 15% |
| June | 47 | 2% |
| July | 1 | 0% |
| December | 1 | 0% |
Of 3,118 counties with a real off-season. Another 22 never get one.
Some real counties
| County | Starts | Act by | Ends | Days |
|---|---|---|---|---|
| Fulton County, GA | March 9 | February 23 | November 20 | 256 |
| Cook County, IL | April 20 | April 6 | October 26 | 189 |
| Denver County, CO | April 22 | April 8 | October 18 | 179 |
| Hennepin County, MN | April 25 | April 11 | October 11 | 169 |
| King County, WA | May 8 | April 24 | October 10 | 155 |
Why the date to act is earlier
A mosquito you swat in June hatched from an egg laid one to two weeks before. Once water is warm, egg to biting adult takes about that long. Tipping standing water and dropping Bti larvicide into water you cannot tip works best in the 14 days before the start date, before the first generation flies. Each county page shows that "act by" date. A warm spring can pull everything a couple of weeks earlier than the normal.
Where to go next
See the range: Hennepin County, MN · Cook County, IL · Harris County, TX · Miami-Dade County, FL.
When Do Mosquitoes Go Away? The End of the Season · West Nile Virus by County: How to Read the Case Counts