Mosquito season in San Bernardino County, CA: when it starts and ends

Mosquito months in San Bernardino County

In a normal year mosquito season in San Bernardino County, CA starts around January 30 and ends around December 8, about 45 weeks.

What to do in San Bernardino County, and when

Start tipping out standing water weekly by January 16.
Book any yard treatment for late January, ahead of the July peak.
Repellent can go away after December 8.
122 West Nile cases since 2016: you should wear EPA-registered repellent at dusk May to October, and check CA spray notices.

San Bernardino County against the rest of CA

San Bernardino County has the 14th longest season of 58 CA counties, at 312 days against 365 in Los Angeles County and 110 in Mono County. Its mean temperature is 69.5°F.

San Joaquin County: January 27 to December 7 (314 days, 86 WNV)
Solano County: January 28 to December 7 (313 days, 18 WNV)
Sacramento County: January 29 to December 5 (310 days, 131 WNV)
Stanislaus County: January 30 to December 5 (310 days, 180 WNV)

West Nile in San Bernardino County

From 1999 to 2025, CDC ArboNET counted 517 human West Nile cases in San Bernardino County (309 neuroinvasive) across 21 years. The worst was 2004 with 187; the latest was 2024.

See how the dates are worked out, and reading the West Nile counts.