Adulticide Resistance

Insecticides are a useful tool for controlling a variety of insects, including mosquitoes, and have been used for hundreds of years. However, over time, resistance can develop to those insecticides, rendering them less effective than they once were. Resistance to insecticides greatly threatens the ability of vector control professionals to protect public health. Here you will find information on how you can prevent, detect, and manage resistance in mosquito populations. 

Types of Resistance

There are different ways in which a mosquito can become resistant to an insecticide. These mechanisms are metabolic resistance, target-site resistance, penetration resistance, and behavioral resistance.  

Metabolic Resistance

Metabolic resistance is the most common mechanism of insecticide resistance. When a toxin enters an insect’s body, enzymes are used to break down the toxin. Resistant mosquitoes may have elevated levels or more efficient enzymes that are better at binding to and breaking down the insecticides. This mechanism can affect the insect’s resistance to multiple chemical classes and is considered to have a broad spectrum of activity.  

adulticide resistance mosquito

Target-Site Resistance

Target-site resistance is characterized by an alteration in the target-site of the insecticide. This modification to the binding site means that the insecticide’s effect is greatly diminished.  

Penetration Resistance

When an insecticide contacts an insect, it can be absorbed through the cuticle and enter the body. Penetration resistance is characterized by a decreased ability of the insecticide to make it through the cuticle. These cuticular barriers result in slower absorption of the insecticide and a decreased ability of the insecticide to penetrate into the insect’s body.  

adulticide resistance

Behavioral Resistance

Behavioral resistance is a change in the normal activity of an insect that ultimately results in less or no exposure to the insecticide. This avoidance increases the insect’s chances of survival. An example has been documented with Anopheles mosquitoes that prefer to rest outdoors to avoid the residual sprays on the interior of homes.

Adulticide Insecticide Classes for Public Health Mosquito Control in the US 

Only two chemical classes are labeled for wide-area mosquito control use in the United States. These are pyrethroids and pyrethrins and organophosphates.  

Pyrethroids and Pyrethrins
(Group 3A)

Pyrethroids and pyrethrins fall into Group 3A of IRAC’s mode of action classification. They are sodium channel modulators and affect the nervous system of the insect. Active ingredients in this chemical class impact the nervous system by disrupting the normal activity of the voltage-gated sodium channel in the insect. This causes overexcitation and eventually death in the insect. Examples of pyrethroids used in mosquito control include permethrin, deltamethrin, and etofenprox. 

(Group 1B)

Organophosphates fall into Group 1B of IRAC’s mode of action classificationThey are acetylcholinesterase inhibitors and affect the nervous system of the insect. Active ingredients in this chemical class inhibit the cholinesterase enzyme from breaking down acetylcholine. This results in over stimulation of the insect’s nervous system causing muscle twitching, convulsions, paralysis, and eventually death. Examples of organophosphates used in mosquito control include naled and malathion.  

Resistance: What is it? Why is it important? 

‘A heritable change in the sensitivity of a pest population that is reflected in the repeated failure of a product to achieve the expected level of control when used according to the label recommendations for that pest species’ – Insecticide Resistance Action Committee, IRAC

Preventing, detecting, and managing insecticide resistance is important because the public health mosquito control community has a limited toolbox of adulticides available to them. With only two classes available, pyrethroids and organophosphates, the ability to rotate is extremely constrained. If mosquitoes develop resistance to either of these classes, the ability to rotate effectively is also diminished. Therefore, being good stewards of the chemistries currently available and preventing or slowing the development of resistance is critical to the future of mosquito control. 

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) 
  • AChE 

Enzyme testing can detect increased activity in: 

  • Esterases 
  • Glutathione-S-transferase 
  • P450 

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. 

What can we do about resistance? 

Rotate your chemistries and make sure integrated mosquito management is emphasized in your program!  

Understanding the susceptibility profiles of your local mosquito populations will help you treat with the best product for your specific area. Once you’ve determined what products are effective against your local mosquitoes, you can establish a rotation schedule that balances organophosphate and pyrethroid use. Throughout the mosquito season, you can monitor the effectiveness of your treatments based on pre- and post-trap counts in treated areas. Reducing pressure from adulticides also aids in preventing resistance. This includes emphasis on your larviciding program, source reduction, and physical control. 

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