Action Thresholds

How do we decide when to spray for mosquitoes?

The Williston Vector Control District uses a management system called “Integrated Vector Management” or IVM for short to manage the mosquito population in Williams County.

Spray determinations are based on several factors with the most important being the quantity of mosquitoes measured using our system of traps located throughout the County. Other factors include disease detection (West Nile Virus, St. Louis Encephalitis Virus, Western Equine Encephalitis Virus) within a sampled pool of mosquitoes, the time of year, and the number of service requests.

Using these factors and other sources of data, we have created a set of action thresholds. An action threshold (Figure 1) is the point at which some intervention must take place in order to reduce the negative impact a pest species, in this case mosquitoes, has on a community.

Action thresholds act as a guide for when an intervention, such as an adulticide or larvicide treatment, needs to be applied. Thresholds can be adjusted as the data and the community changes, and in some cases they can change weekly.

Graph showing simulated data for vector control action thresholds

Figure 1: Graph demonstrating what an action threshold is and the idealized impact an intervention should have on the pest population. Data are simulated.

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