Quick answer
Mosquito season in Westchester County typically runs from late April or May, once daytime temperatures hold consistently above the mid-50s°F, through the first hard frost in fall — usually mid-to-late October, though it can run into November in a mild year. Peak pressure is July through August.
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The short answer
Mosquito season in Westchester County runs from roughly late April or May through the first hard frost in fall, usually landing in mid-to-late October. The exact start and end shift a few weeks either way depending on the specific spring and fall weather each year, but the pattern is consistent: activity ramps up as sustained daytime temperatures push past the mid-50s°F, peaks through the hottest, most humid stretch of summer, and shuts down once overnight temperatures drop enough to produce a hard frost.
Why the season isn’t a fixed calendar date
Mosquitoes are temperature- and moisture-driven, not calendar-driven, which is why “mosquito season” is a range rather than a fixed start and end date:
- Spring start: Adult mosquitoes become active once daytime temperatures hold consistently in the mid-50s°F and above, and standing water from spring rain provides breeding sites. A cold, dry spring pushes the start later; a warm, wet one pulls it earlier.
- Summer peak: July and August are consistently the heaviest months in Westchester, driven by warm nighttime temperatures (many species are most active at dusk and through the night) and the humidity that keeps resting vegetation damp.
- Fall decline: Activity tapers as temperatures drop through September and into October, ending abruptly with the first hard frost, which kills off active adults outdoors.
Why Westchester’s suburban lots run a longer, heavier season than a dense urban block
Westchester’s yard sizes matter here. A property with a lawn, garden beds, mature trees, and any low-lying or poorly-drained spot has meaningfully more mosquito habitat than a paved, dense urban block — more shaded resting sites (the underside of leaves, ivy, tall grass) and more opportunities for standing water to collect and go unnoticed (a clogged gutter, a tarp, a low patch of lawn that doesn’t fully drain after a storm). Wooded and pond-adjacent properties — common in towns like Scarsdale and around New Rochelle’s inland waterways — tend to see both an earlier start and a later end to the season than an open, well-drained lot, because the shaded, humid microclimate those features create holds mosquito-friendly conditions longer on both ends of the calendar.
How to time treatment around the season
Because populations rebound quickly as vegetation regrows and new water sources appear, a single early-summer treatment rarely covers the full season. The practical approach:
- Start before the season, not during it. A first treatment in late April or early May, before mosquitoes are already established, is more effective than reacting once a yard is already unusable.
- Retreat on a cycle, not a one-off. A barrier application typically holds two to three weeks before it needs refreshing — see our mosquito control service or the recurring seasonal mosquito treatment program for how that cycle works.
- Address standing water continuously, not once. Gutters, containers, and low spots should be checked through the whole season, not just at the start — a single missed source can restart local breeding within about a week.
- Don’t assume the season is over after one cool week. A single chilly stretch in September doesn’t end the season; wait for an actual hard frost before treating the yard as mosquito-free.
The bottom line
Plan for a season that runs roughly six months in a typical Westchester year — spring through the first hard frost — rather than the handful of peak summer weeks most people mentally associate with “mosquito season.” Properties with wooded edges, ponds, or drainage features should expect the season to run a little longer on both ends than an open, well-maintained lot.