Mosquito Control Awareness Week: Valent Biosciences Field Cage Trial Results in Mosquito Insecticide Resistance
West Nile Virus is the major mosquito-borne disease in the state of Wyoming. One element of our District's Integrated Mosquito Management Plan (IMM) is the use of adulticides for the control of adult mosquitoes. Currently, there are limited modes of action available for their use as adulticides. This has resulted in an alarming degree of insecticide resistance in mosquitoes, particularly in vector species such as Culex tarsalis. Explore how abundance of West Nile Virus has exploded in recent years, the results of our field cage trials with Valent Biosciences and evidence of the resistant populations in Teton County, how resistance develops in mosquitoes, the types of resistance, and how our team prevents, detects, and manages resistance once found.
West Nile Virus in Wyoming
Incidents of West Nile Virus have exploded in recent years! We have, also, had almost three times the number of human cases in 2023 than we have had in any year in the past 10 years. Most of the cases from 2023 that were interviewed reported no travel outside of Wyoming, so we know they were exposed in the state.
The primary solution to controlling infectious adult mosquitoes during a West Nile Virus outbreak is adulticides. Controlling infectious and pesticide-resistant adult mosquitoes during an outbreak means that adulticides may not work.
Resistance Development
What is insecticide resistance? It is a heritable change in the sensitivity of an insect population to an insecticide. This results in repeated failure of an insecticide to achieve the expected level of control.
In a first generation, resistance development is rare, naturally occurring, pre-adapted insects with resistance genes. With these genes, pre-adapted insects can survive applications.
In later generations, pre-adapted insects survive and reproduce, passing on pre-adapted genes. Offspring are, also, capable of surviving pesticide applications. Eventually, after subsequent generations and pesticide applications, a larger majority of the population is resistant. This results in pesticide application failure in the field.
Mechanisms of Resistance
Target site insecticide resistance means that an insect develops a genetic change at the specific spot on its body where an insecticide is supposed to work, making the insecticide less effective because it can no longer properly bind to that site, essentially like a key that no longer fits in a lock due to a slight change in the lock's shape.
Detection
Our work in detecting the presence or absence of insecticide resistance lies in characterizing the resistance mechanism and its severity. Detection is done primarily through bioassays via lab work and field cage trials while characterizing resistance is done by combining bioassays with synergist tests and microplate assays.
Resistance in Wyoming
The Culex tarsalis mosquito is the predominant WNV vector in Wyoming. Our TCWP Resistance Testing Program includes larval collections (reared to adults in an insectary), CDC bottle bioassays, microplate assays, and field cage trials. The sampling locations included 2 Jackson ranches. In the fall of 2024, our team partnered with Valent Biosciences to complete field cage trials, the results of the trials are below.
Metabolic & target-site resistance for mosquitoes was present at both sites to two classes of insecticides. The field application failure was really bad! This has never been documented in Culex tarsalis in our region.
Prevention & Management
This means we have reached an action threshold locally. An action threshold is a specific point at which a mosquito population reaches a level that necessitates control measures. This is what brings our District's Integrated Mosquito Management Plan into play. Our team will continue larviciding treatments with a wider range of products to help prevent biting mosquitoes. We will limit residual products with a greater selection potential for pre-adapted resistant insects and set action thresholds by spray based on mosquito abundance only.
Non-insecticidal methods
We will continue to utilize our Integrated Mosquito Management Plan that brings together several methods to prevent mosquito-borne diseases and illness, like West Nile Virus, in Teton County. This includes: physical interventions with habitat manipulation, biological interventions with microbial larvicides and promoting healthy ponds & mosquito predators, cultural methods by community outreach & education, and more.
Get Involved
While our team continues our work both in the lab and out in the field to learn about the behavior of mosquitoes regionally and to inform the public, there are small interventions we can all take to support the work of mosquito abatement. Remember, the best way to reduce mosquito populations and protect public health from mosquitoes is to prevent standing water on your property. Thank you for being an advocate for community wellness!