Today's Mosquito Risk in Colorado: Real-Time Forecast & Activity Map
Check your ZIP code for local mosquito activity, West Nile risk, and prevention insights.
Built from CDC and NOAA data
Mosquito Surveillance Insights
- Annual WNV Cases
- about 50 to 100 confirmed cases annually
- Based on CDC ArboNET surveillance data
- Risk Tier
- Tier 2: High Pressure
- Front Range and Western Slope irrigation drives most WNV transmission; high mountains stay too cold.
- Per mosquito-risk-model regional tier
- Highest-Risk Areas
- Weld, Larimer, Adams, Arapahoe, Boulder (Front Range); Mesa (Western Slope); Pueblo (Arkansas Valley)
Current Situation in Colorado
Colorado carries the highest WNV burden in the Mountain West, with roughly 50 to 100 confirmed human cases annually concentrated along the Front Range from Fort Collins through Denver and Colorado Springs and across the irrigated Western Slope. The state sits in the Plains Cx. tarsalis regime east of the divide, and the agricultural Culex drives transmission across the South Platte and Arkansas River corridors and the irrigated farm grid of Weld, Larimer, and Adams counties. Cx. pipiens fills in across the Denver, Boulder, and Fort Collins urban storm drains. The Western Slope around Grand Junction and the Gunnison Valley carries the same irrigated-alfalfa pattern at lower density. Aedes aegypti and Aedes albopictus are not established. EEE is rare. The Continental Divide, Rocky Mountain National Park, and the high San Juans run too cold for meaningful arboviral pressure. Biting season runs June through September.
Snapshot of the current mosquito situation in Colorado, drawn from CDC ArboNET surveillance and the regional risk model.
- Risk Level
- Moderate
- Peak Activity
- July through September
- Dominant Vector
- Western Encephalitis Mosquito (Culex tarsalis) statewide and dominant WNV vector
Where Mosquitoes Breed in Colorado
Colorado mosquito habitat is built on the Front Range and Western Slope irrigated agriculture and the major river corridors. The South Platte River from Greeley through Denver and on into Nebraska sits at the heart of the agricultural Culex zone, with center pivot and flood irrigation across Weld and Adams counties producing dense Cx. tarsalis populations through summer. The Arkansas River corridor through Pueblo, Lamar, and the Lower Arkansas Valley carries the same pattern. The Western Slope along the Colorado River through Grand Junction and the Gunnison River around Delta and Montrose holds irrigated peach, alfalfa, and pasture habitat that sustains summer Cx. tarsalis. The San Luis Valley around Alamosa carries closed-basin agriculture and seasonal wetlands. The Pawnee National Grassland holds prairie pothole wetlands and steady Aedes vexans hatches. Storm drains across Denver, Aurora, Colorado Springs, Fort Collins, and Boulder carry the urban Cx. pipiens load. Rocky Mountain National Park and the Great Sand Dunes stay too cool and too dry for sustained populations.
Hotspot Counties & Areas
- Denver County (Denver metro)
- El Paso County (Colorado Springs)
- Adams and Arapahoe counties (Aurora, eastern suburbs)
- Larimer County (Fort Collins, South Platte headwaters)
- Weld County (Greeley, irrigated South Platte)
- Boulder County (Boulder)
- Mesa County (Grand Junction, Western Slope)
- Pueblo and Otero counties (Arkansas River Valley)
Seasonal Activity Pattern
Biting starts in early to mid June across the Front Range and the Western Slope as irrigation cycles open up, peaks in late July through August, and tapers through September. WNV cases concentrate in August across Weld, Larimer, Boulder, Adams, Arapahoe, and Mesa counties, with the Greeley and Fort Collins agricultural counties typically leading per-capita rates. Cx. tarsalis populations build through July with the irrigation cycle and stay elevated through August. Cx. pipiens drives Denver and Boulder urban transmission with the same August window. Cool mountain nights hold high-elevation activity down even in midsummer; the high country above 9000 feet stays largely mosquito-free.
Monthly Biting Intensity (Mountain West)
Colorado Mosquito Risk Profile
- WNV burden level
- Moderate
- about 50 to 100 confirmed cases annually
- EEE risk
- Rare
- Peak biting season
- June through September
- Top urban risk areas
- Denver (Denver)
- Colorado Springs (El Paso)
- Aurora (Arapahoe)
- Fort Collins (Larimer)
Signature habitat: Front Range irrigated agriculture, South Platte corridor, Western Slope alfalfa, Arkansas River Valley.
Climate driver: Front Range and Western Slope irrigation drives most WNV transmission; high mountains stay too cold.
Check Your Local Mosquito Risk
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Common Mosquitoes in Colorado
Understanding which mosquitoes are active in Colorado helps you identify threats and take appropriate precautions. Click any card for details.
Northern House Mosquito
Culex pipiens
Risk: West Nile virus, St. Louis encephalitis, dog heartworm.
Key feature: Medium brown body with banded abdomen. Looks nearly identical to Cx. quinquefasciatus.
Active: Late spring through early fall in the north. Adult females overwinter in basements, garages, and storm drains.
West Nile Virus
Vector: Northern House Mosquito
West Nile Virus
The dominant West Nile vector across the northern half of the US. About 80% of infections are asymptomatic.
What to watch for
- Onset typically 2 to 15 days after a bite
- Most cases mild or asymptomatic, small fraction severe
- Neurological signs (severe headache, neck stiffness, confusion) need ER care
Breaking the Cycle in Colorado
The mosquito lifecycle takes about a week. These habits cut local biting pressure faster than anything else, building multiple layers of protection.
Drain the Yard Weekly
Empty buckets, planter saucers, kiddie pools, tarps, and clogged gutters. A week of standing water is one full mosquito generation.
Repel Before Going Out
EPA-registered DEET (20-30%), picaridin (20%), IR3535, or oil of lemon eucalyptus. Apply before going outside, especially at dusk and dawn.
Seal the House
Window and door screens with no tears. AC and porch fans both work. Mosquitoes struggle to fly upwind even at 1-2 mph air speed.
Treat Outdoor Clothing
Permethrin-treated long sleeves and pants for evening yard work, hiking, or fishing. Remains effective through 70+ wash cycles.
If You Develop Symptoms
Most mosquito bites are harmless. A small fraction can transmit illness, and knowing the timeline matters more than panicking at every itch.Five things to know.
Watch for fever
Fever, body aches, headache, or fatigue 2 to 15 days after a bite. West Nile often feels like flu, and most infections never get diagnosed.
Note where and when
A rough record of where and when you got bitten helps your doctor decide what to test for. Date, location, mosquito-heavy areas you visited.
Get help fast for neuro signs
Severe headache, neck stiffness, confusion, vision changes, or sudden muscle weakness can mean neuroinvasive WNV or EEE. Both are medical emergencies.
Pregnant exposure?
If pregnant and bitten in an area with documented Zika or local dengue transmission, mention to your OB. Same goes for travel exposure.
Children and seniors
Higher risk of severe disease from WNV, EEE, and dengue in adults over 60 and children under 5. Early call to the pediatrician or PCP if symptoms appear.
When does mosquito season end in Colorado?
Mosquito activity winds down after the first hard freeze, which in Colorado typically arrives around Oct 2. See the local first-frost window and city-by-city freeze dates.
Colorado first frost datesAlso worried about ticks in Colorado?
Lyme disease risk and tick activity track different seasonal patterns than mosquitoes. See your Colorado tick forecast.
View Colorado tick forecastStay Safe in Colorado
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