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Mosquito Control Program

Mosquito Control Program

West Nile virus (WNV) is mosquito-borne disease that can be transmitted to humans by an infected mosquito that contracted the virus from biting an infected bird. More than 50 species of mosquitoes are found in Colorado, and only species of Culex mosquitoes—Culex tarsalis, Culex pipiens and Culex erythrothorax—are vectors (insects that can carry and transmit a disease) of WNV in the region.

Who is at risk of getting WNV?

Anyone can get WNV if bitten by an infected mosquito. Most mosquitoes are not infected, which makes the odds of contracting the virus low. However, if a person does become infected with WNV, most people (around 80 percent) will not become ill or even be aware that they have WNV. About 20 percent of people who are infected with WNV will have symptoms similar to a severe case of the flu. About 1 in 150 people infected with WNV will develop a serious neuroinvasive form of the disease that can cause permanent disability or death. Although the odds of contracting the neuroinvasive form of WNV are low, the consequences can be so devastating that it is important to take steps to prevent it.

What is the City of Boulder doing to protect the community from WNV? 

The most efficient method for controlling mosquitoes is to target larvae. All species of mosquitoes go through an aquatic larval stage and can be eliminated before they emerge as biting adults. The city monitors potential mosquito breeding sites on all city properties 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 the the larvae. The Bti product is made from a naturally-occurring bacterium and has the least environmental impact. Bti is not toxic to fish or animals, but does kill some other aquatic insects, such as midges and black flies. Bti is very effective and kills nearly all mosquito larvae within treated areas.

Adult mosquitoes are monitored from a grid of 16 traps located throughout the city. The mosquitoes are sorted and identified by species. Culex species from designated sentinel traps are sent to the Colorado Department of Public Health and Environment to be tested for the presence of WNV.

What can residents do to protect themselves and the community from WNV?

Mosquito-proof your property
Culex mosquitoes commonly breed in standing water in residential yards, businesses, and other private properties. Any item or depression that can hold water for about a week can breed mosquitoes, including toys, flower pots, wheelbarrows, lawn ornaments, bird baths, tarps, pool covers, trash cans, clogged gutters and downspouts, and other items. Inspect your property and drain anything that is holding water. If there are areas that cannot be drained, mosquito dunks are available at local hardware stores that contain Bti and are safe for home use.

Protect yourself from mosquito bites

  • Avoid going outdoors between dusk and dawn, when Culex are most active.
  • Cover up exposed skin by wearing long sleeves and pants if you are outdoors when mosquitoes are active.
  • Use an effective mosquito repellent. DEET and picardin provide long-lasting protection, depending on the concentration. If you want to avoid DEET, there are natural repellents that are effective, but you must reapply more them more often (once an hour). Choose products with at least 2 percent soybean oil or oil of lemon eucalyptus.

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.

Background information about the Mosquito Control Program

The City of Boulder first implemented the Mosquito Control Program in 2003, in response to an emerging public threat from WNV. At that time, the mosquito control industry typically sprayed insecticides if mosquito traps revealed a count of 100 or more, regardless of whether the trapped mosquitoes were Culex or other species not capable of transmitting WNV. The city did not have a tool to predict where and when WNV human cases could occur, although the technology existed to test mosquitoes for WNV infection. During the peak of WNV in 2003, Colorado, and Boulder County in particular, experienced the highest number of WNV cases in the country. The city hired an ecological consulting firm with aquatic entomology expertise, OtterTail Environmental, Inc., to develop a program that would target Culex mosquitoes and protect the community from both WNV and exposure to toxic chemicals. Another important objective was to protect the environment, particularly fragile wetland ecosystems.

Although it was controversial at the time, the city decided to base mosquito control decisions on the presence of vector mosquitoes—both adult and larvae. Adult mosquitoes from traps were separated into Culex and non-Culex mosquitoes. Culex mosquitoes were tested for WNV. OtterTail developed a “vector index” for the city to help correlate the population density of Culex and the infection rate of the mosquitoes with the presentation and date of onset of WNV human cases. The firm estimated that a vector index level of 0.75 is a predictor of a high likelihood that human cases of WNV will occur. At that time, the Centers for Disease Control (CDC) did not agree with the approach, but later adopted it. The vector index remains the best tool available for targeting mosquito control efforts and predicting WNV outbreaks and is used in multiple communities nationwide.

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