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Mosquito season in the Metro East: when it starts and when it peaks
Mosquito season around Belleville reliably runs from spring into October, but it is really several overlapping seasons. Different mosquitoes take over as the weather changes, and the ones biting in July are not the ones that hatched after the April rains. Knowing which is which explains when bites spike and what actually helps.
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- Written by Darlene Pemberton
- Updated October 1, 2026
- 6 minute read
Around the Metro East, mosquitoes start biting in April or May as nights warm, surge after heavy spring rains, and peak through the humid stretch of July and August, tapering with the first cool weather in October. Floodwater mosquitoes dominate after spring rains, container breeders like the Asian tiger mosquito take over in summer yards, and house mosquitoes that breed in stagnant water drive late-summer West Nile concerns. Removing standing water and treating shaded resting spots does the most good.
Spring: the floodwater wave
The first big wave usually follows the heavy rains of April and May. Floodwater mosquitoes lay eggs in low ground that floods periodically, and those eggs can sit dry for long periods before a soaking rain hatches them in huge numbers. The Mississippi floodplain west of Belleville, creek bottoms and poorly drained lots produce these surges, which can feel sudden: a quiet week, then swarms after a storm.
Summer: the container breeders
By early summer, much of the backyard biting comes from container-breeding mosquitoes, including the Asian tiger mosquito, a black-and-white striped species that bites aggressively during the day as well as at dusk. These breed in small amounts of water close to home: plant saucers, buckets, clogged gutters, tires, toys and tarps. They rarely fly far, so a yard full of tiger mosquitoes is usually producing its own.
- Plant saucers and pots
- Clogged gutters and downspout extensions
- Buckets, toys, tarps and wheelbarrows
- Birdbaths not refreshed twice a week
- Low spots that hold water for days
Late summer: the West Nile stretch
House mosquitoes in the Culex group breed in stagnant, nutrient-rich water such as storm drains, catch basins and neglected pools, and they are the main carriers of West Nile virus in Illinois. Their activity tends to build through late July and August, which is why public health surveillance in the state watches that stretch closely each year. These mosquitoes bite mostly from dusk into the night.
When it ends
Activity falls off as nights cool in late September and October, and the first frosts end the adult season. Some species overwinter as adults in sheltered places like crawl spaces and storm drains, and many overwinter as eggs, so the cycle restarts with the first warm, wet stretch of spring.
What actually helps
The single best habit is a weekly walk around the yard tipping out anything holding water. Professional barrier treatment works on the resting side: mosquitoes spend daylight hours in shaded, humid foliage, and treating those surfaces keeps killing them for several weeks. Fans on the patio help more than candles because mosquitoes are weak fliers.
- Tip standing water weekly, and after every rain
- Refresh birdbaths twice a week
- Keep gutters clear
- Thin dense, shaded shrubs near seating areas
- Use a fan on the patio during gatherings
When to call
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Before you call
Common follow-up questions
Ideally in spring, before populations build. A first treatment in April or May and monthly service through the season keeps resting populations down.
They kill many insects, mostly not mosquitoes. Removing breeding water and treating resting areas does far more.
Illinois records West Nile activity most years, usually peaking in late summer. Reducing bites is the main way households lower their risk.
Darlene Pemberton is the pest control guides editor at Belleville Pest Control. She writes and maintains these homeowner guides from the field notes our technicians bring back from jobs across Belleville and the Metro East, and every guide is checked against what the crews actually see in local homes.
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