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

Mosquito months in San Diego County

In a normal year mosquito season in San Diego County, CA starts around March 14 and ends around November 14, about 35 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 San Diego County, and when

• Start tipping out standing water weekly by February 28.
• Book any yard treatment for mid March, ahead of the August peak.
• Repellent can go away after November 14.
• 36 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July to September, and check CA spray notices.

San Diego County against the rest of CA

San Diego County has the 41st longest season of 58 CA counties, at 245 days against 365 in Los Angeles County and 110 in Mono County. Its mean temperature is 60.2°F.

• San Benito County: March 12 to November 22 (255 days, 0 WNV)
• Lake County: March 14 to November 17 (247 days, 10 WNV)
• Inyo County: March 31 to November 12 (226 days, 4 WNV)
• Humboldt County: March 27 to November 7 (225 days, 1 WNV)

West Nile in San Diego County

From 1999 to 2025, CDC ArboNET counted 148 human West Nile cases in San Diego County (110 neuroinvasive) across 17 years. The worst was 2015 with 42; the latest was 2024.

More: how the dates are worked out · reading the West Nile counts.