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

Mosquito months in Riverside County

In a normal year mosquito season in Riverside County, CA starts around January 10 and ends around December 31, about 51 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 Riverside County, and when

• Start tipping out standing water weekly by December 27.
• Book any yard treatment for early January, ahead of the July peak.
• Repellent can go away after December 31.
• 110 West Nile cases since 2016: you should wear EPA-registered repellent at dusk May to October, and check CA spray notices.

Riverside County against the rest of CA

Riverside County has the 6th longest season of 58 CA counties, at 355 days against 365 in Los Angeles County and 110 in Mono County. Its mean temperature is 70°F.

• Imperial County: January 2 to December 29 (361 days, 7 WNV)
• Santa Cruz County: January 3 to December 27 (358 days, 1 WNV)
• San Mateo County: December 31 to December 12 (347 days, 5 WNV)
• Santa Clara County: January 18 to December 11 (327 days, 16 WNV)

West Nile in Riverside County

From 1999 to 2025, CDC ArboNET counted 612 human West Nile cases in Riverside County (386 neuroinvasive) across 21 years. The worst was 2015 with 127; the latest was 2025.

More: how the dates are worked out · where mosquitoes never stop.