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Illinois · Mosquito Season Guide

Is it mosquito season in Illinois?

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II. Illinois Mosquito Season Month by month

Illinois mosquitoes are a summer phenomenon — the season builds through May, peaks in July and August, and tapers off in September. The state has roughly 60 species, but three define the experience for most residents.

Very Low Low Moderate High Very High taller = more active · color = severity

Chart reflects Chicago / northern Illinois baseline. Southern Illinois (Carbondale, Cairo) typically starts 2–3 weeks earlier and finishes 2–3 weeks later at similar peak intensity.

~60
mosquito species recorded in Illinois
2002
year of the deadliest urban WNV outbreak in US history
May–Sep
typical active season statewide
102
counties — all with seasonal mosquito surveillance

Chicago and Cook County: the epicenter

In the summer of 2002, Cook County recorded 884 confirmed human West Nile virus cases and 64 deaths — the first large-scale urban WNV outbreak anywhere in the United States and still one of the deadliest single-season events on record. The primary vector was Culex pipiens, the Northern house mosquito, which breeds in the storm drains, catch basins, and pooled water ubiquitous in dense urban and suburban environments. Cook County has since built one of the most extensive urban mosquito surveillance and larviciding programs in the country, targeting roughly 70,000 catch basins annually — but the underlying biology hasn't changed.

When is it worst?

Peak season runs late June through mid-August. July is typically the worst month statewide, driven by warm temperatures and summer thunderstorm activity. Culex pipiens concentrates around dusk and overnight; Aedes vexans, the floodwater mosquito, emerges in large numbers within days of heavy rain events and bites aggressively during daylight. After a significant storm — particularly along the Illinois and Kankakee river corridors — populations can spike dramatically for two to three weeks.

The floodwater factor

Illinois's flat agricultural landscape, extensive river bottomlands, and tile-drainage systems create ideal conditions for Aedes vexans, the inland floodwater mosquito. It's the most abundant mosquito species in rural Illinois and a significant nuisance across the state after rainfall. Unlike Culex pipiens, floodwater eggs survive dry conditions for years — dormant in soil until inundated. A single flood event can produce enormous numbers within 5–7 days. Residents near the Illinois, Mississippi, Kankakee, and Rock river corridors should expect spikes following any significant rain event throughout the season.

III. Illinois's Three Species to Know
Culex pipiens (Northern house mosquito) macro photograph
Culex pipiens
Northern House Mosquito · ⚠ West Nile Vector

The dominant mosquito in the Chicago metro and across northern Illinois. Breeds prolifically in standing water — storm drains, catch basins, clogged gutters, neglected containers. Feeds primarily at dusk and overnight, preferring birds as hosts but shifting to humans as bird populations decline in late summer. The primary driver of the 2002 Cook County West Nile outbreak. IDPH monitors Cx. pipiens trap counts weekly throughout the season as an early-warning indicator.

Aedes vexans (inland floodwater mosquito) macro photograph
Aedes vexans
Inland Floodwater Mosquito

The most abundant mosquito in rural Illinois and a significant biter statewide. Unlike Culex, it is a daytime and crepuscular biter — and extremely aggressive. Eggs are laid in low-lying areas and remain viable in dry soil for years, hatching en masse when flooded. After a significant rain event, populations can explode within 5–7 days. Not a significant disease vector in Illinois, but responsible for most of the nuisance biting that makes outdoor activity miserable during and after summer storms.

Aedes albopictus (Asian tiger mosquito) macro photograph
Aedes albopictus
Asian Tiger Mosquito

Established in the Chicago metro, the collar counties, and much of downstate Illinois, and continuing to expand northward. Visually distinctive: black with bold white stripes, smaller than Culex. Bites aggressively during the day — particularly in the morning and late afternoon — making it responsible for much of the daytime nuisance biting in suburban yards. Breeds in small containers: bottle caps, saucers, rain gutters, any standing water larger than a teaspoon. Eliminate containers to reduce pressure immediately.

West Nile is an annual presence in Illinois. IDPH publishes weekly WNV surveillance data — mosquito pools, dead birds, and human cases — throughout the season. Cook, DuPage, and Will counties historically see the highest case counts. Check current county-level reports at IDPH →

IV. City-by-City Reference
City Active Season Off-Season Notes Live data
Chicago May – Sep Near-zero Oct–Apr Cook County runs one of the largest urban larviciding programs in the US (~70k catch basins treated annually); 2002 WNV outbreak remains the US urban benchmark; Culex pipiens dominates; Ae. albopictus established in city and suburbs Check live
Springfield May – Sep Off Oct–Apr Central Illinois; Sangamon River corridor amplifies Ae. vexans pressure after rainfall; Culex pipiens present; state capital — IDPH surveillance data often uses Sangamon County as a benchmark Check live
Rockford May – Sep Off Oct–Apr Rock River valley creates reliable floodwater habitat; season slightly shorter than Chicago due to northern latitude; Ae. albopictus present but less dense than Chicago metro Check live
Peoria May – Sep Off Oct–Apr Illinois River bottomlands sustain large Ae. vexans populations; any significant rainfall triggers noticeable spikes; Peoria County runs active aerial spray program in bad years Check live
Champaign May – Sep Off Oct–Apr Flat agricultural landscape and tile drainage maximizes floodwater hatching after storms; University of Illinois campus has extensive standing water in low spots; Culex pipiens and Ae. vexans both present Check live
Joliet May – Sep Off Oct–Apr Will County — historically one of the higher WNV case-count counties in Illinois; Des Plaines River and Kankakee River tributaries provide floodwater habitat; suburban density favors Culex Check live
Aurora May – Sep Off Oct–Apr Fox River corridor; Kane and DuPage counties share aggressive mosquito control programs; Ae. albopictus well established; suburban yards with birdbaths and clogged gutters are primary Culex sources Check live
More State Guides
Florida Year-round activity in the south — 80+ species including invasive Ae. aegypti and the Asian tiger mosquito across all 67 counties Texas ~85 species across four climate zones — Gulf Coast active March through November, with year-round pressure in the Rio Grande Valley California Central Valley is the #1 West Nile corridor in the US — invasive Ae. aegypti and Ae. albopictus now established in 20+ counties Georgia Atlanta metro leads the Southeast in Asian tiger mosquito density — active statewide March through November New York Ground zero for West Nile in the US (1999 Queens outbreak) — NYC metro and Long Island see the highest Northeast seasonal pressure Ohio Lake Erie to the Ohio River — WNV active in Franklin and Cuyahoga counties, with floodwater species surging after summer storms Pennsylvania Philadelphia WNV corridor meets rural EEE risk in western PA — the densest human population in the mid-Atlantic mosquito belt Michigan Site of the 2019 EEE outbreak — the deadliest in decades — 10 human cases and 6 deaths concentrated in SW Michigan Great Lakes wetlands New Jersey Oldest organized county mosquito control in the US (1914) — Pine Barrens EEE corridor and fierce salt marsh Shore species Virginia Tidewater season runs April–October — the longest mid-Atlantic window — with the Great Dismal Swamp as an active EEE hotspot North Carolina The coastal plain runs one of the longest seasons in the Southeast — NC leads the nation in La Crosse encephalitis cases from the Appalachian foothills Maryland Chesapeake Bay watershed creates extraordinary mosquito habitat — tidal marshes, Eastern Shore wetlands, and the DC-Baltimore corridor combine for dense seasonal pressure Tennessee Memphis and the Mississippi River corridor historically record the highest WNV case counts in the mid-South — Ae. albopictus has colonized all 95 counties Indiana Indiana sits at the crossroads of the Great Lakes EEE belt — Lake Michigan wetlands in the north and the Wabash River floodplain drive intense seasonal pressure Louisiana The Gulf Coast's most intense pressure — coastal marshes and summer rains keep Cx. quinquefasciatus and Ae. aegypti active from March through November Alabama Mobile Bay to the Tennessee Valley — Ae. albopictus has reached all 67 counties and peak WNV season runs June through September South Carolina Low Country marshes produce some of the Southeast's densest Anopheles and Culex populations — season runs March through November Minnesota 10,000 lakes means 10,000 breeding sites — the Northwoods sees intense but short July–August peaks, while the Twin Cities face a longer June–September window Wisconsin Great Lakes wetlands and the Mississippi River backwaters drive strong WNV pressure in the Milwaukee corridor — peak season July through August Missouri At the confluence of the Missouri and Mississippi rivers — St. Louis historically ranks among the highest WNV cities in the US — season May through October Arkansas Delta rice-field flooding and the Arkansas River bottomlands create some of the highest rural mosquito pressure in the mid-South — active April through October Washington Western WA stays mild and damp year-round; the Yakima Valley and Columbia Basin face the highest WNV risk in the Pacific Northwest — peak July through September Arizona Monsoon season (July–September) drives sudden mosquito explosions across the Valley of the Sun — WNV detected annually Colorado Front Range WNV corridor stretches Denver to Fort Collins — eastern plains carry the state's highest risk Connecticut Connecticut River valley WNV pressure meets EEE risk in inland freshwater wetlands — active May through October Delaware Chesapeake and Delaware Bay marshes create outsized pressure for the nation's second-smallest state Hawaii Year-round activity at sea level — no native species, but Ae. aegypti and tiger mosquitoes established statewide Idaho Snake River Plain irrigation districts fuel summer WNV pressure — season June through September Iowa Mississippi River corridor and prairie wetlands drive intense June–September pressure — WNV active statewide Kansas Great Plains WNV corridor — Wichita and Kansas City see peak pressure July–August; floodwater species surge after storms Kentucky Ohio River corridor and Kentucky Lake wetlands sustain a long April–October season across all 120 counties Maine Short but intense July–August season — EEE detected in some years in the southern coastal plain Massachusetts EEE fatality rate exceeds 30% — the Plymouth County swamp complex is the active epicenter — season May through October Mississippi Gulf Coast near-year-round pressure meets Delta rice-field flooding for one of the South's most intense mosquito environments Montana Yellowstone and Missouri River corridors drive a compressed June–August season — WNV detected most years in eastern MT Nebraska Platte River valley and eastern Nebraska are among the Great Plains' most active WNV zones — peak July–August Nevada Las Vegas monsoon season drives July–September activity; Reno irrigation districts fuel a parallel northern peak New Hampshire EEE detected most years — the southern lakes and coastal plain are in the active Northeast EEE corridor New Mexico Monsoon season (July–September) creates sudden breeding habitat; WNV circulates in the Rio Grande valley annually North Dakota Red River Valley flooding produces some of the Great Plains' most extreme floodwater mosquito events — peak June–August Oklahoma One of the South-Central's most intense seasons — the Arkansas and Red River corridors sustain populations April through October Oregon Eastern Oregon Columbia Basin is the state's WNV hotspot; western Oregon runs a mild coastal season June through September Rhode Island New England's densest coastal pressure — Narragansett Bay salt marshes and WNV active in all five counties South Dakota Missouri River and James River valleys drive summer WNV activity — eastern SD sees the most intense pressure Utah Salt Lake Valley WNV corridor and irrigation districts sustain a July–September peak — drier areas see limited activity Vermont Champlain Valley runs Vermont's longest season — EEE detected in the Lake Champlain lowlands in some years West Virginia Kanawha and Ohio River valleys sustain a May–October season — Ae. albopictus now established statewide Wyoming The shortest meaningful season of any contiguous US state — North Platte valley WNV window is just 6–8 weeks
West Nile, dengue, and EEE: US disease risk by region → Asian tiger mosquitoes: why you're getting bitten during the day → Eliminating standing water: the highest-leverage yard move →