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Mosquito Control Program

Mosquito Control Program

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Mosquito Control Program

West Nile Virus and Mosquito Management

The City of Boulder West Nile Virus Mosquito Management Plan (WNV MMP) was adopted by City Council in March 2006. The main objective of the WNV MMP is to reduce the risk of human West Nile Virus infection while minimizing environmental impacts. There are many species of mosquitoes in Boulder, but only three - Culex tarsalis, Culex pipiens and Culex erythrothorax - can transmit or vector West Nile Virus.

Sixteen adult mosquito traps are placed around the city and monitored weekly from late May through mid-September to determine the numbers of Culex mosquitoes. In addition, community education is essential to preventing the disease and reducing the numbers of the disease-carrying mosquitoes.

What you can do:
You can lower your odds of contracting WNV, by taking steps to avoid mosquito bites.  

Follow the four Ds:

  • Use insect repellent that contains DEET, Picaridin, lemon-eucalyptus oil or use two percent soybean-oil products (more information)
  • Dress in long-sleeves and pants
  • Avoid the outdoors at Dawn and Dusk
  • Drain standing water around homes (pool and hot tub covers, wheelbarrows, tarps, lawn ornaments, flower pots, trash cans, toys, etc.). Change the water in your bird bath every 4-5 days.

For the latest information on human cases of West Nile Virus in Colorado, please refer to Colorado Disease Control and Environmental Epidemiology Division.


Attacking the Source - Killing the Larvae: The city monitors potential mosquito breeding sites city-wide for vector mosquito larvae throughout the entire WNV season (May through September). Whenever Culex larvae are found, the site is treated with a biological insecticide, Bacillus thuringiensis israeliensis (Bti) to kill them. The Bti product is made from a naturally-occurring bacterium and has the least impacts to the environment. It is not toxic to fish or animals, but does kill some other aquatic insects, such as midges and black flies. Bti is very effective at killing mosquito larvae and kills nearly all of them in the treated area. Targeting larvae for treatment is a very effective method of mosquito control.

If a number of factors were to  indicate that West Nile Virus is prevalent enough to warrant a public health emergency, than city staff and City Council could decide to conduct targeted spraying for adult mosquito management.

The Worst Case - A West Nile Virus Outbreak: The presence of mosquito-borne pathogens in Boulder, if detected, will prompt one or more responses or interventions. These interventions can range from continuing existing routine monitoring to, in worst-case scenarios, large-scale application of adulticides. Monitoring data will be used to assess the risk of an outbreak of human disease and the need to apply pesticides in a limited and targeted area to control adult mosquitoes. The control response will depend on a combination of thresholds being met that include, but are not limited to:

  • the overall intensity and persistence of the WNV activity in adult Culex mosquitoes, humans, birds, and non-avian vertebrates;
  • the proximity of WNV activity to human populations within the city;
  • the time of year;
  • vector index level (index of 0.75); and
  • seasonal climate.

Please keep in mind that large-scale spraying has never been necessary before in Boulder's history. If spraying is deemed to be necessary (worst-case scenario) it will be done twice, three nights apart.  Residents will be notified 48 hours in advance by 911 call back, ads in the newspapers, on the city's website ( and on Municipal Channel 8.  Residents can opt out of spraying by calling the city's WNV Hotline at 303-441-3400, Option 4 and leaving their name, address and phone number.

Nuisance Mosquito Management

The City of Boulder treats limited sites to reduce the number of nuisance mosquitoes around several popular recreational areas and the Greenbelt Meadows and San Lazaro neighborhoods. One goal of this program, a cooperative effort between the city and Boulder County, is to make outdoor recreation more comfortable by reducing adult mosquito populations. 

This is achieved by applying a non-toxic mosquito larvicide (Bti) to Sombrero Marsh, Burke II and the Kentucky Property, in addition to wetland areas within Flatirons Golf Course, Boulder Reservoir, Stazio Ball Fields, East Boulder Community Park,  Pleasantview Soccer Fields, Valmont City Park and Christensen Park.

The Parks and Recreation Department will continue to employ other controls, including mosquito magnets and mowing at Stazio Ball Fields and Flatirons Golf Course, to further improve player comfort at these sites.