Mosquitoes and West Nile Virus
mosquito larvae, a person applying mosquito repellent, and a bird bath

Mosquitoes and West Nile Virus

Mosquito Control and West Nile Virus Prevention in Boulder County

Mosquitoes are more than a nuisance. They can carry West Nile virus (WNV). Boulder County Public Health works to prevent the transmission of WNV in Boulder County and to address nuisance mosquitoes.

Personal Prevention – the Four Ds

There is no treatment for West Nile virus so it’s important to prevent it. West Nile can cause a debilitating illness in 20% of those infected. Everyone should protect themselves against mosquito bites.

Follow the 4Ds when mosquitoes are most active (June through September).


Use insect repellent that contains at least 10% DEET. If outside during evening, nighttime, and dawn hours, consider the use of an insect repellent containing 10% to 30% DEET (N, N-diethyl- methyl-meta-toluamide). Products with concentrations around 10% are effective for periods of approximately two hours.

Use DEET according to manufacturer’s directions:

  • DEET is not recommended for children under 2 months of age.
  • Do not apply DEET directly to children, and do not apply it to children’s hands. Apply to your own hands, then put it on the child.
  • Apply DEET sparingly on exposed skin; do not apply under clothing.
  • Do not use DEET over cuts, wounds, or irritated skin. Wash all treated skin and clothing after returning indoors.

Find the Repellent that is Right for You


Dress in long sleeves and long pants. If outside during evening, nighttime, and dawn hours when mosquitoes are most active and likely to bite, children and adults should wear protective clothing such as long pants, long-sleeved shirts, and socks.

Dusk and Dawn

Reduce or eliminate outdoor activity between dusk and dawn.

Make sure windows are closed or screened.

Most mosquito species are actively searching for a blood meal in the evening hours from just before dark until 2 to 3 hours after dark. During the daytime the females normally rest in cooler vegetated areas where the humidity is higher and they are protected from drying out. Females will often bite in the daytime if humans or animals invade the wooded areas where they are resting.


Make sure to drain standing water around your home.

  • Remove all discarded tires from your property.
  • Dispose of tin cans, plastic containers, ceramic pots, or similar water-holding containers.
  • Make sure roof gutters drain properly. Clean clogged gutters in the spring and fall.
  • Clean and chlorinate swimming pools, outdoor saunas, and hot tubs. If not in use, keep empty and covered.
  • Drain water from pool covers.
  • Change the water in bird baths at least once a week.
  • Turn over plastic wading pools and wheelbarrows when not in use.
  • Eliminate any standing water that collects on your property.
  • Remind or help neighbors to eliminate breeding sites on their properties.

Protect Your Property from Mosquitoes

What You Can Do As a Property Owner

  • Empty or clean out all water-holding containers in your yard (bird baths, tires, cans, rain barrels, wading pools) – 1 small can of water can hold as many as 10,000 mosquito larvae.
  • Keep your gutters cleaned.
  • Keep ornamental ponds free of weeds, cattails, or other aquatic vegetation.
  • Use anti-larval products (dunks or bits) available at local hardware stores and use them in ponds or standing water.

Adult mosquitoes are most active at dawn and for about an hour after dusk. Plan outdoor activities during warmer daylight hours or later in the evening to avoid adult biting mosquitoes.

  • Keep your lawn mowed as short as possible.
  • Cut back dense vegetation surrounding your yard; this is where mosquitoes hide during the day.
  • Keep ornamental shrubs and bushes pruned to allow more light and air to flow through them.

Farmers and Agricultural Landowners

  • Keep irrigation ditches free-flowing and clear of vegetation and debris.
    Repair ditches to reduce seepage.
  • Minimize standing water to less than four (4) days in fields.
  • Fill tire ruts and hoofprints to avoid puddles.
  • Keep horse and cattle troughs clean by changing the water weekly.
  • Vaccinate horses for Western Equine Encephalitis.

Report a Concern about Mosquitoes in Boulder County

Residents dealing with mosquito issues in various cities can report problems to these contacts.

Within City Limits

Outside City Limits

Spraying & Mosquito Concerns

  • Boulder County Public Health: 303-441-1564
  • COHelp Line: 1-877-462-2911
  • Boulder County Public Health Hotline: 303-441-1460

Monitoring Mosquitoes in Boulder County

Boulder County Public Health (BCPH) keeps an eye on mosquito populations, especially the Culex species to learn how much risk there is for people.

BCPH has traps throughout Boulder County so we can count

  • Total number of mosquitoes
  • Percentage of Culex mosquitoes
  • Percentage of mosquitoes with West Nile virus
  • Number of human cases
  • Number of human deaths

The Culex mosquito is the carrier of West Nile virus in Colorado. Culex mosquitoes are common in Boulder County and take about two weeks to grow in warm weather. Getting rid of standing water can help stop them from multiplying.

The vector index, which measures the risk of the virus spreading, depends on the number of Culex Pipien and Culex Tarsalis mosquitoes trapped and tested. If the vector index reaches 0.75 or more, there might be a need to take action to reduce the number of mosquitoes in Boulder County. We do this by using:

  • Larvicide: A substance that is used to kill mosquito larvae before they become flying mosquitoes. This helps lower the number of mosquitoes that can spread viruses like West Nile.
  • Adultize: Adultize is a method to control the population of flying mosquitoes. This method is taken action to reduce their numbers, especially when the risk of spreading viruses, measured by the vector index, is high in Boulder County.

Learn more about Culex mosquitoes (CDC)

Mosquito Totals in Boulder County

BCPH looks at mosquitoes captured in traps throughout Boulder County and tracks the data for trends.


2023 Total Mosquitoes vs. 5 Year Average

chart showing total mosquito counts over the past five years.

2023 Culex vs. 5 Year Culex Average

five-year trend of Culex mosquitoes in Boulder County.

Vector Index

The vector index helps to figure out the risk of the virus spreading, based on

  • The total number of the species of mosquitoes that can carry West Nile (Culex pipien and Culex tarsalis).
  • The number of infected mosquitoes in an area.
  • The vector index is calculated by determining the average number of infected mosquitoes collected per trap in an area.
  • If it reaches 0.75 or more, there might be a need to do mosquito spraying to lower the risk.

During the mosquito season, we will post updates about mosquito counts here. E.g.,

  • Statewide, 213 positive mosquito pools have been recorded: Adams (2), Arapahoe (7), Boulder (45), Delta (3), Denver (5), Jefferson (1), La Plata (1), Larimer (132), Pueblo (2), Weld (15).
  • Nuisance (non-vector) and Culex (vector) mosquito counts were historically high throughout the 2023 season. Peaking in week 29, with 100,000 total mosquitoes caught, 65% being Culex.
  • Nationwide, there have been 1,153 human cases of West Nile virus (WNV), with 371 cases reported in Colorado.

Learn more about the Culex mosquito lifecycle.

Mosquito Monitoring 2023

Zone 1Zone 2Zone 3
Week Ending OnVector IndexVector IndexVector Index
June 17 (Week 24)
June 24 (Week 25)
July 1 (Week 26)
July 8 (Week 27)0.004.350.00
July 15 (Week 28)
July 22 (Week 29)3.3116.085.44
July 29 (Week 30)14.1013.8210.74
August 5 (Week 31)5.9514.121.72
August 12 (Week 32)
August 19 (Week 33)0.973.200.81
August 26 (Week 34)1.333.871.86
September 2 (Week 35)0.190.500.40
September 9 (Week 36)

Map of Zones Where Mosquitoes Are Trapped

Sentinel Zones for mosquito testing

West Nile Data

Effective Mosquito Management

A comprehensive mosquito management strategy is employed to address the dual challenge of nuisance and potential health risks posed by mosquitoes as carriers of the West Nile virus. The approach is anchored in Integrated Pest Management (IPM), a method designed to reduce mosquito populations effectively while prioritizing environmental friendliness and cost-effectiveness.

This approach focuses on preventing breeding by

  • Eliminating stagnant water
  • Using natural solutions like Bti bacteria
  • Introducing native fathead minnows
  • In cases of high mosquito numbers, permethrin, a synthetic chemical derived from the natural insecticide found in chrysanthemum flowers, is used cautiously to minimize environmental impact.

Boulder County Mosquito Control District (BCMCD)

The Boulder County Mosquito Control District (BCMCD) works to keep residents safe from mosquito-related health issues. Meetings, either virtual or by phone, help everyone stay connected and informed. Annual reports give a peek into ongoing mosquito control efforts.

The Boulder County Mosquito Control District (BCMCD) was established in 1995 by ordinance by the Board of County Commissioners to protect residents from the health risks, annoyance, and discomfort associated with mosquitoes.

Meeting Schedule

The second Wednesday of the month
April – October
6 – 7:30 p.m.

Annual Reports

BCMCD Survey Results

The Boulder County Mosquito Control District surveyed residents in 2011 to gather feedback on mosquito control practices. The responses overwhelmingly indicated that people want mosquito control to continue. Many stated that mosquitoes negatively impacted their ability to enjoy the outdoors.

Residential District

The Boulder County Mosquito District encompasses approximately 180 square miles of eastern Boulder County. Because the residents who live within this district pay for mosquito control services, Boulder County can only provide mosquito control services to those who live within the district.

Sentinel Zones for mosquito testing

Printable Map of Boulder County Mosquito Control District

The Boulder County Mosquito Control District (BCMCD) Program is overseen by a citizens’ advisory board appointed by the Boulder County Commissioners.

Key Components of Boulder County Mosquito Control

  • More than 94% of the program is biological and physical control of mosquito larvae before adult emergence.
  • Adult Mosquito Control comprises less than 6% of the program.

Services that are available, upon request, to those living within the Boulder County Mosquito Control District include:

  • Answering questions and providing information on mosquito control.
  • Investigating reports of mosquitoes or standing water.
  • Excluding district residents’ properties from spraying, if requested.
  • Notifying district residents before spraying in their neighborhoods.
  • Providing spray schedules.

Integrated Pest Management

The board promotes an integrated pest management program, collaborates with agencies to control mosquito-borne diseases, educates residents in the BCMCD to help identify and reduce mosquito breeding sites around their homes and neighborhoods, and advises the County Commissioners and Boulder County Public Health on mosquito control activities and needs.

Environmentally Responsible

The Boulder County Mosquito Control Program (BCMCP) is one of the most environmentally conscious programs in the nation. Mosquito control services in Boulder County are contracted to a company, Vector Disease Control International (VDCI).

VDCI utilizes an integrated pest management approach to safely and effectively reduce mosquito populations. The National Cancer Institute has shown that all mosquito control materials used in Boulder County are non-carcinogenic and are registered for use by the Environmental Protection Agency (EPA).

Educational Materials and Clinical Guidelines

West Nile virus (WNV) poses a significant health concern in Colorado. Approximately 20% of those infected may experience severe symptoms, ranging from fever and fatigue to more serious conditions like meningitis or paralysis.

These guidelines are provided to assist health care providers and educators with the recognition and diagnosis of illness due to West Nile virus (WNV).

Please feel free to download, reproduce, and distribute these materials.

One Bite Campaign

The One Bite campaign was developed in 2003 when Boulder County had an extreme number of people who contracted West Nile virus. A significant number of residents died or had long-term or permanent complications.

One Bite Poster

Flyers for Distributing at Events

Flyer for Mailings

Contact Us

Vector Control Program

Main: 303-441-1564
Submit a question


3450 Broadway
Map and Directions

Boulder County Public Health website