Mosquito season in New Castle County, DE: when it starts and ends
Mosquito months in New Castle County
In a normal year mosquito season in New Castle County, DE starts around April 4 and ends around November 5, about 31 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in New Castle County, and when
- Start tipping out standing water weekly by March 21.
- Book any yard treatment for early April, ahead of the July peak.
- Repellent can go away after November 5.
- 14 West Nile cases since 2016: you should wear EPA-registered repellent at dusk June to August, and check DE spray notices.
New Castle County against the rest of DE
New Castle County has the 3rd longest season of 3 DE counties, at 215 days against 230 in Sussex County and 215 in New Castle County. Its mean temperature is 55.4°F.
- Sussex County: March 28 to November 13 (230 days, 6 WNV)
- Kent County: March 30 to November 11 (226 days, 4 WNV)
West Nile in New Castle County
From 1999 to 2025, CDC ArboNET counted 36 human West Nile cases in New Castle County (25 neuroinvasive) across 11 years. The worst was 2003 with 8; the latest was 2025.
More: how the dates are worked out · reading the West Nile counts.