Mosquitoes are present in Colorado, but the nuisance and risk they pose vary significantly across the state. The intensity of mosquito activity and associated health concerns depend highly on location, time of year, and annual weather patterns. Understanding these localized factors is necessary to gauge whether mosquitoes will be an issue in a particular area, as the state’s unique climate and geography encourage or discourage their development during warmer months.
Seasonal Peaks and Abundance
The mosquito season in Colorado begins in late April or early May and lasts until the first hard frost, usually in September or October at lower elevations. Mosquito populations are cyclical, tied closely to the availability of standing water, which is influenced by snowpack and summer weather. A strong snowpack followed by a wet spring creates favorable conditions, leading to higher numbers earlier in the season.
Peak mosquito activity occurs from July through September, especially for disease-transmitting species. Warm summer temperatures combined with heavy irrigation, particularly in agricultural areas, provide the stagnant water sources needed for egg-laying and larval development. Warmer temperatures accelerate the mosquito life cycle, allowing development from egg to adult in as little as seven days.
Geographic Variance and Common Species
Mosquito severity varies across Colorado’s diverse landscape, which hosts over 45 documented species. High-density populations are frequently found in plains habitats and irrigated agricultural regions, which provide extensive breeding grounds. These lower-elevation areas, particularly along the Front Range and Western Slope, support the largest mosquito populations.
The two most common types are the Aedes and Culex genera, which have distinct behaviors and breeding preferences. Aedes mosquitoes are often called floodwater mosquitoes because their eggs hatch when depressions are filled by rain or snowmelt; they are known for biting during the day and are considered nuisance species. Culex species prefer permanent water sources like ponds, storm sewers, and neglected containers, and are most active during dusk and dawn. High-elevation mountain zones have a lower abundance of mosquitoes compared to the plains due to cooler temperatures and fewer expansive areas of stagnant water.
Primary Health Concerns in Colorado
The main health concern related to mosquitoes in Colorado is West Nile Virus (WNV). WNV is transmitted to humans through the bite of an infected mosquito, primarily Culex tarsalis and Culex pipiens, which acquire the virus by feeding on infected birds. The majority of human cases are reported in August and September, coinciding with the peak activity of these virus-carrying Culex mosquitoes.
Most WNV infections are asymptomatic, but about 20% of infected individuals develop a fever and other mild, flu-like symptoms. Less than one percent may develop severe neuroinvasive disease such as encephalitis or meningitis, which can be life-threatening. The Colorado Department of Public Health and Environment (CDPHE) conducts surveillance by trapping and testing mosquitoes for the virus from mid-June through late August to monitor the risk. While other mosquito-borne illnesses like Western Equine Encephalitis are present, WNV remains the most consistently monitored public health threat.
Effective Localized Prevention
Protecting yourself from mosquitoes involves personal measures and eliminating breeding sites around the home. Residents are encouraged to follow the “Four D’s” of prevention, which includes using insect repellent containing DEET or picaridin and wearing long sleeves and pants during active times. Avoiding outdoor activity during dusk and dawn, when Culex mosquitoes are most active, reduces the risk of bites.
The most effective localized prevention focuses on eliminating standing water, as only a small amount is needed for mosquitoes to breed. Checking and draining containers at least once a week removes breeding habitats.
Eliminating Breeding Sites
- Birdbaths
- Old tires
- Flowerpots
- Clogged gutters
In agricultural and suburban areas, managing irrigation runoff and repairing leaky outdoor faucets prevents the creation of pools where mosquitoes thrive. Many communities also implement mosquito control programs that involve targeted larvicide application to water sources and adult mosquito spraying when WNV risk is high.