Mosquito Spraying Schedules

A spray schedule for municipalities in Larimer County can be found here. The City of Fort Collins has a subscription notification service to alert residents of spraying. Text Alerts from the City of Fort Collins: To receive a text message alert, simply text FCWNV to 888-777.

  • Image 1: West Nile Virus

West Nile virus (WNV) is a mosquito-borne virus that can potentially cause serious illness. West Nile virus is considered to be permanent in Colorado and infections can be expected each year.

Risk and the Vector index

People of any age who spend a lot of time outdoors are more likely to be bitten by an infected mosquito and therefore develop West Nile disease. People over 50 years of age and those who have serious diseases and immune disorders are more likely than younger patients to have the most severe forms of the disease.

The vector index (VI) measures the level of WNV infected mosquitoes in an area, and is the best available predictor of human risk of disease. The VI is a number calculated by combining measures of the abundance of Culex mosquitoes (the species that carries WNV) and the rate of infection in those mosquitoes. Published investigations of large West Nile outbreaks have shown that once a vector index is over .5, the risk of humans getting infected with West Nile virus increases.


Signs of illness may appear anywhere from 3 - 14 days after a person is bitten by an infected mosquito.

About one in 150 people infected with WNV will develop severe illness. The severe symptoms can include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis. These symptoms may last several weeks, and neurological effects may be permanent.

About 20 percent of the people who become infected have milder illness such as fever, headache, body aches, nausea, vomiting, and sometimes swollen lymph glands or a skin rash on the chest, stomach and back. Symptoms can last anywhere from a few days to several weeks.

Approximately 80 percent of people who are infected with WNV will not show any symptoms at all.


WNV is transmitted to mosquitoes after they bite infected birds. You cannot catch the virus from another person.


Thousands of people in Colorado were infected with West Nile virus in 2003, the worst year in Larimer County. Sixty-three people died. Many people who became seriously ill are continuing to recover while others suffered permanent disabilities.

Infected mosquitoes have been trapped in Larimer County each year since 2003.

More Information About West Nile Virus

  • Ensure greenbelts and all common areas have good drainage and tall grasses are cut
  • Promptly repair broken sprinkler heads
  • Report ponding in nearby irrigation canals after the water has been turned off
  • Add larva-killing Bti* granules or “dunks” to standing water in places not treated by the city’s contractor
  • Clean gutters to allow proper drainage
  • Clear storm drains to prevent standing water
  • Stock ponds and basins with fathead minnows that eat mosquito larvae
  • Change water in birdbaths at least weekly (Twice a week in hot weather)
  • Clean and maintain swimming pools
  • Prevent boats from holding rain water. Store upside down, or sprinkle Bti* granules where water collects
  • Ensure outdoor toys, playground equipment, & household items don’t collect water


Larimer County Department of Health and Environment works closely with the local mosquito control contractors as well as the local cities and Colorado State University to monitor trap data and assess human risk during each week of West Nile season. Trap data is published each week on our website. This information is what helps determine a spray recommendation throughout the season.

Vector index

The vector index (VI) measures the level of WNV infected mosquitoes in an area, and is the best available predictor of human risk of disease. The VI is a number calculated by combining measures of the abundance of Culex mosquitoes (the species that carries WNV) and the rate of infection in those mosquitoes. Published investigations of large West Nile outbreaks have shown that once a vector index is over .5, the risk of humans getting infected with West Nile virus increases.

WNV spraying for Public Health

The decision to spray an area in order to control adult mosquitoes for reasons of public health is based on monitoring for West Nile virus in mosquito populations.

Public health recommends spraying to control adult mosquitoes when the risk index in an area exceeds 0.5 – the level above which human cases are expected to occur.

The city of Fort Collins-operated WNV management program is restricted by city policy to consider spraying only when the Risk Index (Vector index) is at or over 0 .75. A significant lag time (approximately 4 weeks) exists between when a person is infected and when their case is reported to the Larimer County Department of Health and Environment. Spraying decisions are based on elevated numbers of infected mosquitoes. 

If and when Larimer county sprays, it does so to target areas at the highest risk of West Nile infection.

Nuisance mosquitoes

Many of the mosquitoes you see may be what are called nuisance mosquitoes.

These do not pose a public health risk and are really just what they are called, a nuisance. The level of infected Culex mosquitoes varies in different areas of the county in different weeks. Trap data is used to determine whether the mosquitos collected every week are carrying the virus. There can be an increase of nuisance mosquitoes in an area without risk of the virus. We monitor that with the mosquito control contractors each week.

Private spraying

Some cities, towns, homeowners associations, individual property owners, and businesses have made the decision to spray in order to control both nuisance and WNV infected mosquitoes. Those efforts are independent of Larimer County.

HOA and County spraying

Homeowner’s associations may decide to have their neighborhoods sprayed to control adult mosquitoes independently of any City or County actions. In this case, the cost of spraying is covered by the requesting HOA, and double spraying is avoided through communication between mosquito control contractors. Colorado Mosquito Control will contact the HOAs in the area that do their own mosquito spraying to ensure they are not being sprayed more often than intended.

Mosquito control on County lands

Until Larimer County voters choose to create a mosquito control district and fund it, there is no funding available for broader mosquito efforts. At this time, mosquito control efforts are focused on protecting population centers, and control measures (larviciding and adulticiding) are paid for by municipalities (Windsor, Timnath, Loveland, Fort Collins), as is true throughout most of Colorado.


The City of Fort Collins has a subscription notification services to alert residents of spraying. Text Alerts: To receive a text message alert, simply text FCWNV to 888-777

Larimer County does not manage a mosquito control program at this time. Many local cities contract with Vector Disease Control International (VDCI) to manage their specific programs.

We are fortunate to have a partnership with Vector Disease Control International, to help monitor and mitigate mosquitos in Larimer County.

Vector Disease Control International conducts a local mosquito surveillance program. VDCI maps potential mosquito breeding sites and utilizes light traps to monitor adult mosquito activity as the season progresses.

If you have standing water to report - contact the following depending upon your location.

  • Loveland: 970-278-9977
  • Fort Collins: (866) 691-3987
  • LaPorte (southern area only): (866) 691-3987
  • Timnath - (877-276-4306) VCDI vendor - see Timnath.org mosquito control
  • Windsor: 970-674-5400 - see Windsor.gov mosquito control
  • Wellington: 970-568-3381
  • Berthoud: 970-278-9977
  • Unincorporated Larimer County: Call either (866) 691-3987 or 498-6775 to determine if you are within the City of Loveland's or City of Fort Collins' Larval Control Buffer Zone

When you are choosing a repellent, use a mosquito repellent that has been proven to be effective against West Nile Virus-carrying mosquitoes. DEET, Picaridin, Oil of Lemon Eucalyptus (also called p-menthane-3,8-diol or PMD) and IR3535 are good choices.

Find the repellent that is right for you.


  • Apply repellent only to exposed skin, never under clothing.
  • Never use repellent over cuts or irritated skin.
  • Do not apply to eyes and mouth.
  • Never let children handle repellents.
  • Do not spray in an enclosed area.
  • When you come back inside, wash the repellent off thoroughly.
  • Always follow directions listed on the label.
  • Don't over-apply repellent. Use only what you need and what is recommended on the label.\
  • Do not use a repellent that is combined with sunscreen. The sunscreen will need to be reapplied more often than the repellent so you might end up using too much repellent

When using repellent on a child:

  • Always read the label to determine if it is safe for children.
  • Follow application directions for children.
  • Avoid applying to the child's eyes, mouth, and palms of hands. If a child is still sucking a thumb or mouthing fingers often, don't apply repellent to the child's hands at all.
  • Always have an adult apply repellent to a child.
  • Keep repellents out of reach of children.
  • Only apply to exposed skin.
  • Do not over-apply. Use only what your child needs.
  • Do not use a repellent that is combined with sunscreen into one product. However, using separate repellent and separate sunscreen products at the same time is an acceptable practice.
  • If a child develops a rash or other apparent allergic reaction from an insect repellent, stop using the repellent, wash it off with mild soap and water, and call a local poison control center for further guidance.

Don't use:

  • A repellent with 30% or higher concentration of DEET on children.
  • Repellents containing oil of lemon eucalyptus on children under three. Oil of lemon eucalyptus has not been significantly tested on children of that age.
  • Any repellent on infants younger than two months.

DEET expiration

DEET is very stable and is effective indefinitely as a repellent. For this reason, the federal government doesn't require an expiration date on product labels. But manufacturers of repellents say that the feel, smell and appearance of their products may change after about three years. This does not reduce the DEET's ability to repel mosquitoes and ticks but may make the product less appealing to users. If you're not sure about your particular product, contact the manufacturer.

Caring for yourself if you have West Nile Virus (WNV)

West Nile virus infection is diagnosed by a blood test which generally does not test positive until a week or more after you have become ill.

Testing will tell you whether or not you have antibodies to West Nile virus. You develop antibodies against West Nile virus as your body fights off the infection. Having antibodies against West Nile means it's unlikely you'll be infected again.

Testing will also rule out other diseases that might have the same symptoms.

Mild to moderate WNV illness improves on its own, and people do not necessarily need to seek medical attention for this infection though they may choose to do so.

If you develop symptoms of severe WNV illness, such as unusually severe headaches or confusion, seek medical attention immediately. Severe WNV illness usually requires hospitalization. Pregnant women and nursing mothers are encouraged to talk to their doctor if they develop symptoms that could be WNV.

What to expect during your illness

  • WNV illness, if it occurs, will appear from 3-14 days after a bite from an infected mosquito
  • Up to 30% of infected people will have West Nile Fever (a flu or mono-like form of the illness). Most people who are infected have slight or no symptoms.
  • 1 in 150 infected people get a very serious form of WNV (encephalitis, meningitis) with fever, fatigue, paralysis, disorientation and tremors
  • Drink plenty of fluids.
  • Treat headaches/muscle pain with non-prescription pain relievers (aspirin, ibuprofen, etc) according to product directions. It is very important not to exceed the recommended dose.
  • Consult your health care provider if you are not getting sufficient pain relief. Rest to allow adequate time for the body to heal. This will most likely mean time off from work.
  • Recurrences of symptoms and delayed recovery are more likely with insufficient fluid replacement and not enough rest.
  • There is no vaccine or cure for WNV in humans at this time

When to call your health care provider

  • If you are experiencing a severe headache, stiff neck, or a high fever (over 103 degrees)
  • If you have severe pain, confusion, delirium, tremors, convulsions, profound muscle weakness or paralysis
  • If you have 2 or more days of vomiting or cannot keep fluids down.
  • If you have not improved in 3-6 days

Blood donation

If you have been infected with West Nile Virus, do not donate blood or organs until you have recovered

People who have been diagnosed with West Nile virus confirmed by a positive laboratory test should not donate blood for 120 days from the start of their symptoms or their laboratory diagnosis, whichever is later.

All donated blood is tested for active West Nile virus infection.

Link to Surveillance Data Map                                     Local Spray Schedule 

Larimer County works with the local cities and towns to monitor West Nile Virus.

Trap data helps the Larimer County Department of Health and Environment monitor the culex mosquito count in the area. Culex mosquitoes can carry West Nile Virus, so watching these numbers help recognize risk in the area.

In 2024, there will be enhanced surveillance and additional traps placed in the Town of Berthoud. Read more here

Domestic pets including dogs, cats, birds, horses, alpacas and others can get West Nile virus from mosquito bites.

If a pet is infected with WNV, most will not become ill or show only mild symptoms and are expected to fully recover from the disease. Dead birds do not infect pets that come into contact with them.

There is no documented WNV transmission from pet animals to humans.

West Nile virus infections in unvaccinated horses may cause severe symptoms and up to one-third of unvaccinated horses die.

Protect your horses from West Nile virus by vaccinating them.

Horses require 2 vaccinations within a three to six week period to prevent infection. Previously vaccinated horses require one booster shot annually. Ask your veterinarian about protecting your horses from West Nile virus.

  1. Drain - Mosquitoes breed in water! Drain any standing water in your yard each week. Bird baths, clogged gutters and kiddie pools are common breeding sites. 
  2. Dress - Wear lightweight, long-sleeved shirts and long pants while outdoors. Spray or treat clothing with insect repellent since mosquitoes may bite through clothing. 
  3. Defend - Apply insect repellent sparingly to exposed skin. Use an approved repellent according to its label. During the peak WNV season (mid-June through August) infected mosquitoes can be found all along the Front Range so use repellent where you live, work and recreate.
  4. Dawn/Dusk - The best way to avoid West Nile virus is to prevent mosquito bites. Stay indoors if possible, during peak mosquito biting hours (generally from dusk to early dawn; sunset to 1.5 hours after sunset appear to be the most active "feeding time" for the species that carry West Nile virus).

Environmental Health