Mosquitoes are consistently present in the Denver Metro area, and residents should take precautions throughout the warmer months. Denver’s high elevation and semi-arid climate do not prevent the establishment of mosquito populations. Although the region is drier than many eastern areas, irrigated green spaces and various sources of standing water create adequate breeding habitats. This requires local public health officials and residents to maintain vigilance against mosquito-borne disease.
Seasonal Presence and Local Habitat
Mosquito activity in Denver is strongly seasonal, typically beginning in late May or early June and continuing until the first hard frost in late September or October. The peak period for mosquito abundance and disease transmission risk generally occurs during the hot mid-summer months, from July through September.
Denver’s overall dryness is counteracted by an abundance of urban water sources that support mosquito life cycles. Breeding habitats include standing water in storm sewers, clogged gutters, and depressions caused by over-watering of lawns. The South Platte River corridor and associated floodplains also provide natural areas for development.
The female mosquito lays eggs in these aquatic environments, which can be as small as a bottle cap filled with water. Since mosquitoes can complete their life cycle from egg to adult in as little as seven days, continuous monitoring of standing water is necessary throughout the active season.
Primary Species in the Denver Area
More than 45 mosquito species are found across Colorado, but the primary public health concern in the Denver region involves the Culex genus. The two most significant species are Culex tarsalis and Culex pipiens, which are the main vectors for disease transmission in the state.
These Culex species are most active during the cooler hours of the day, specifically at dusk and dawn. This biting behavior contrasts with floodwater mosquitoes, such as the Aedes genus, which are known to be more aggressive daytime biters. Limiting outdoor exposure during these peak activity periods is a protective measure, as Culex species are the primary carriers of the most concerning local illness.
Health Risks and Disease Monitoring
The most significant mosquito-borne illness monitored in Colorado is West Nile Virus (WNV), which has been a concern since its introduction in 2002. Most people infected with WNV will be asymptomatic, showing no signs of illness. About 20% of infected individuals develop West Nile fever, a mild febrile illness with symptoms like headache, body aches, joint pain, and fatigue.
A small fraction of those infected, less than 1%, may develop a severe neuroinvasive disease, such as West Nile meningitis or encephalitis. These severe forms involve inflammation of the brain or the membranes surrounding the brain and spinal cord, requiring immediate medical attention. The Colorado Department of Public Health and Environment (CDPHE) actively monitors WNV by trapping adult mosquitoes and testing pools for the virus, often using a rapid RT-PCR multiplex technique.
Local agencies also utilize surveillance programs, sometimes including sentinel chicken flocks, to gauge the level of WNV activity before human cases are reported. The majority of human cases are acquired when mosquito abundance is highest, primarily during August and September.
Effective Personal and Property Protection
Protecting against mosquito bites relies on a two-pronged approach: personal defense and property management. For personal protection, using an EPA-registered insect repellent is effective, with active ingredients such as DEET, Picaridin, oil of lemon eucalyptus, or IR3535. Wearing long-sleeved shirts, long pants, and socks when outdoors, especially during the peak biting hours of dusk and dawn, adds a physical barrier.
Property management focuses on eliminating mosquito breeding sites. This can be summarized by the public health action to “Drain” any standing water around the home. Residents should routinely empty water from containers like bird baths, flower pot saucers, old tires, and buckets.
Ensuring that rain gutters are clean and draining properly prevents the accumulation of stagnant water. Home maintenance, such as repairing tears in window and door screens, will help prevent adult mosquitoes from entering indoor living spaces.