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