Mosquito season in Hillsborough County, NH: when it starts and ends

Mosquito months in Hillsborough County

In a normal year mosquito season in Hillsborough County, NH starts around April 30 and ends around October 11, about 23 weeks.

1. Jan
2. Feb
3. Mar
4. Apr
5. May
6. Jun
7. Jul
8. Aug
9. Sep
10. Oct
11. Nov
12. Dec

What to do in Hillsborough County, and when

• Start tipping out standing water weekly by April 16.
• Book any yard treatment for late April, ahead of the July peak.
• Repellent can go away after October 11.
• No West Nile cases since 2016, so bites in Hillsborough County are a nuisance; check window screens.

Hillsborough County against the rest of NH

Hillsborough County has the 4th longest season of 10 NH counties, at 164 days against 170 in Strafford County and 141 in Coos County. Its mean temperature is 46.5°F.

• Rockingham County: April 28 to October 14 (169 days, 2 WNV)
• Belknap County: April 30 to October 13 (166 days, 0 WNV)
• Cheshire County: May 2 to October 10 (161 days, 1 WNV)
• Merrimack County: May 3 to October 9 (159 days, 1 WNV)

West Nile in Hillsborough County

From 1999 to 2025, CDC ArboNET counted 3 human West Nile cases in Hillsborough County (3 neuroinvasive) across 3 years. The worst was 2012 with 1; the latest was 2012.

More: how the dates are worked out · when mosquitoes go away for the winter.