How do we detect adulticide resistance?
There are a variety of assays that are used to assess resistance in mosquito populations. These include laboratory, semi-field, and field assays that can reveal both the genotypic and phenotypic response of mosquitoes. Some of these assays are briefly described below.
The CDC bottle bioassay is a commonly used resistance assay that assess the phenotypic response of mosquitoes to various active ingredients. In this assay, bottles are treated with a known concentration of insecticide. Populations are categorized as susceptible, developing resistance, or resistant based on their percent mortality at a diagnostic time. Based on how the mosquitoes are categorized, the CDC recommends next steps as well. A more detailed description of the CDC bottle bioassay can be found in their CONUS manual.
* Mechanism testing options: enzymes, molecular assays, CDC bottle bioassay with inhibitors.
** Intensity testing (strength of the resistance mechanism) can be done by looking at mortality at 120 minutes or by running bottles with 1X, 2X, 5X, and 10X the diagnostic dosage of insecticide.
The CDC suggested algorithm for further testing depending on the level of resistance detected in the CDC bottle bioassay.
For populations that are classified as developing resistance, the underlying mechanism can be investigated, field assays can be conducted, or both can be done for the clearest picture of resistance. Mechanism testing can be done using molecular assays, enzyme testing, or by adding inhibitors to the CDC bottle bioassay.
Molecular methods can detect mutations in:
- GABA receptor
- Sodium channels (kdr)
Enzyme testing can detect increased activity in:
Common inhibitors used with the CDC bottle bioassay are:
- Piperonyl butoxide (PBO)
- S.S.S-tributlyphosphorotrithioate (DEF)
- Diethyl maleate (DEM)
Field testing can be conducted and will provide the clearest picture of what would happen in an actual adulticide application. These tests are usually done by placing caged mosquitoes 100, 200, and 300 ft back from the line of the spray application. The insecticide is then applied at operationally relevant speeds and application rate and the mosquitoes are monitored for mortality over time. An example of a field assay setup can be seen below.