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St. Louis Mosquito Forecast

Today's mosquito activity in St. Louis, the week ahead, and what to do about it. Updated hourly.

Bite Index · Sunday, November 8

Moderate

Moderate mosquito activity in St. Louis today and easing through the week: expect some bites at dusk.

Some bites at dusk and dawn, mostly near creeks, gardens and shade.

Worse than a typical November in St. Louis City (usually around 0).

  • Tonight: a cool 52°F evening, light wind — poor flying weather, so fewer bites than the population alone would suggest.
  • 0.6 in. of rain Oct 15–Oct 28 is now producing adults (cool weather slowed the hatch, so the window is longer than usual).
  • At a two-week average of 58°F, larvae develop slowly and numbers are easing.

The week ahead

Each day's Bite Index, from this week's weather and the rain that is still hatching. Forecast days get less certain as the week goes on.

  1. Today
    21Mod.64°/34°dry
  2. Mon
    17Low63°/37°dry
  3. Tue
    27Mod.70°/45°dry
  4. Wed
    24Mod.72°/53°dry
  5. Thu
    17Low53°/39°0.1 in
  6. Fri
    0Low46°/35°dry
  7. Sat
    0Low52°/36°dry

Protect tonight

Worth repellent if you'll be outside at dusk or dawn.

  1. A botanical or 10% picaridin lotion covers a patio evening; reapply after about two hours.
  2. A fan on the patio does real work: mosquitoes are weak fliers and avoid moving air.
Which repellent, and how much: DEET vs picaridinBest natural repellents

Why St. Louis's mosquitoes do what they do

St. Louis sits at the confluence of the Mississippi and Missouri, with the River des Peres draining the city and hot, humid summers that make it one of the Midwest's worst mosquito towns. The city's Vector Control program traps, larvicides catch basins and sprays; the county around it detects West Nile most summers and recorded cases in 2026.

  • Two great rivers: floodplain backwaters and the River des Peres hold water through the summer.
  • Humid heat: July and August dusk conditions are ideal for flight.
  • Brick-city catch basins: the dense urban storm-drain network breeds the Culex that carry West Nile.
  • Real winter: frosts from late October end the season.

A typical year in St. Louis City

The long-run mosquito index by month from our season map (NOAA climate normals plus effort-corrected sightings). Today's Bite Index is the live version of this; the month you're in is marked.

  1. JJanuary: 0
  2. FFebruary: 0
  3. MMarch: 0
  4. AApril: 23
  5. MMay: 54
  6. JJune: 80
  7. JJuly: 92
  8. AAugust: 93
  9. SSeptember: 80
  10. OOctober: 54
  11. NNovember: 0
  12. DDecember: 0

Local surveillance and West Nile virus

The Bite Index measures nuisance biting, not disease. St. Louis City's health department traps and tests mosquitoes through the season and publishes positive West Nile pools by ZIP code. When your ZIP has an active advisory, follow it regardless of today's number.

How the Bite Index is scored

The index is a model of conditions, not a mosquito count. Four weather factors are combined on a 0–100 scale, then scaled by the season. Every factor and today's value for it is listed here so you can check the number yourself. It uses the same temperature curve and rain logic as our county-level mosquito season map, which has been validated against effort-corrected sighting data across 41 states.

BBreeding pressure

Rain 7–14 days ago (0.6 in. between Oct 15 and Oct 28). Eggs need water to hatch, and at summer temperatures the egg-to-biting-adult cycle takes about a week, so today's mosquitoes are last week's rain. Cool weather stretches the window; hot dry days shrink it as shallow water evaporates.

Weight 40%
Today 31%

DDevelopment

The trailing two-week average temperature, on the same curve as our national season map: nothing below 50°F, full speed at 78–88°F, a small penalty in extreme heat.

Weight 25%
Today 30%

ATonight's activity

Whether the population that exists will actually fly and bite this evening: dusk temperature (65–85°F is ideal), wind (they stop flying around 10 mph), humidity, and rain around sunset.

Weight 25%
Today 16%

GGround truth

A nudge from local trap counts and reports when we have them. For St. Louis this is neutral today; the health department's surveillance is linked above and will feed this factor as we wire it in.

Weight 10%
Today neutral

SSeason gate

A multiplier from the calendar: fully open in the city's season months, half-open in the shoulder months, nearly closed in deep winter unless the week is warm. Below a 50°F weekly average the index is simply 0.

Weight ×
Today 60%

index = 100 × S × (0.40 × (0.2 + 0.8·B) + 0.25 × D + 0.25 × A + 0.10 × (0.5 + 0.5·G))

Bands: Low 0–20 · Moderate 21–40 · High 41–60 · Very High 61–80 · Severe 81–100. Weather is from Open-Meteo (observations and the 7-day forecast), refreshed hourly. The lag and temperature thresholds follow published work on Culex and Aedes development (Rueda et al. 1990; Cornell NRCC degree-day emergence; JAMCA 2024 on rainfall two to four weeks before trap peaks).

Questions people ask about mosquitoes in St. Louis

When is mosquito season in St. Louis?
May through October, peaking in July and August. Frost, usually late October, ends it.
Why is Forest Park so bad at dusk?
Its lagoons, wetlands and shade are textbook Culex and floodwater habitat, and humidity pools under the trees. Parks and river bottoms run well above the citywide index.
Is West Nile virus a risk in St. Louis?
Yes. St. Louis County and the city detect West Nile in mosquito pools most summers and recorded human cases in 2026. Health department advisories are authoritative; follow them over the index.
Where does this forecast's data come from?
Open-Meteo weather for downtown St. Louis, scored by our documented model and compared with the city's season-map climatology. Not a trap count.

Also covers Clayton, University City, Kirkwood, Florissant, Chesterfield, Belleville, Forest Park, the Riverfront: the forecast point is central St. Louis, and conditions within about 30 miles track it closely. Weather fetched Nov 8, 12:38 PM local time.

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