Combatting Mosquito Insecticide Resistance – Rotation and Integrated Mosquito Management: Northwest

The topic of insecticide resistance has received increasing amounts of attention within the operational mosquito control industry. As a result, assays are being conducted in many areas to identify and monitor resistance so that it can be swiftly and efficaciously addressed.

Below are a couple of examples of documentation of resistance in the northwest region of the US in Culex pipiens and Aedes vexans. These studies indicate that there is resistance in these species to multiple active ingredients.

However, populations classified as ‘susceptible’ and ‘developing resistance’ are still prevalent and preventing further resistance from developing is critical to preserving the chemistries available to us. This involves continuing to utilize an Integrated Mosquito Management approach that emphasizes the balanced use of larvicides and adulticides, while monitoring for resistance.


  1. An integrated approach to mosquito control is the best way to mitigate and prevent the development of insecticide resistance.
  2. Using larvicides to target the larval stage when they are restricted to a concentrated area allows us to attack them when they are in a known and discrete area.
  3. Larvicides offer greater rotational flexibility as there are three insecticide classes with multiple active ingredients.
  4. Rotation of adulticide chemical classes is also critical (i.e. between a pyrethroid and organophosphate) and is the recommended rotational strategy from the Insecticide Resistance Action Committee (IRAC).
  5. Your balance of larviciding and adulticiding efforts, as well as your rotation intervals will be determined by the response and susceptibility of your local populations.

Operational Recommendations

The use of larvicides and adulticides for mosquito control falls into the larger picture of integrated mosquito management. In addition to having a plan for adulticide rotation, emphasis on your larvicide program can aid in resistance management. Larvicides have a greater number of active ingredients to choose from within three classes and some even provide intrinsic resistance management, reducing the likelihood of developing resistance to larvicides. Application of larvicides and combatting mosquito populations while they are confined in a single habitat, and are not biting/flying adults, will reduce populations and alleviate the stress on adulticides (and your constituents).

Like adulticides, overuse or sublethal dosing of larvicides can cause mosquito populations to develop tolerance and eventually resistance to some active ingredients. Lysinibacillus sphaericus, Spinosad, and (S)-methoprene have all been documented in the literature to have caused resistance in populations. However, the “magic bullet,” or Bacillus thuringiensis israelensis, has been used to combat resistance to these products and eventually return mosquito populations to larvicide susceptibility. According to the EPA, there is no evidence of documented resistance to Bti in its 30+ years of use, which is likely due to the synergistic nature of the four protein toxins.

If through your routine surveillance or resistance monitoring, you believe there is a need to rotate between your pyrethroids and organophosphate chemistries, rotation can be as simple as switching truck routes. From your fleet of ground-adulticiding vehicles, set up ½ with your preferred synthetic pyrethroid product and the other ½ with an organophosphate product. On an as needed basis as indicated by your surveillance and monitoring, switch the routes that spray trucks are applying. Routes that were being treated with the synthetic pyrethroid product will now be treated with the organophosphate and vice versa until your next rotation occurs. This method also doesn’t require re-calibration. Alternatively, you can rotate your entire fleet between a pyrethroid and organophosphate, but this requires re-calibration every time you are switching between products in your sprayers.

Fyfanon Dual Tank

The efficacy of any rotational program for both larvicides and adulticides should be supplemented with resistance monitoring and field validation through field trials whenever possible to ensure you are maintaining maximum efficacy. Resistance monitoring can be done through the strategic monitoring of your mosquito trap counts, laboratory assays, or field assays. If you have questions about monitoring for resistance in your area, please contact us at

Benefits of Resistance Management

  • Rotating insecticides will delay the onset of insecticide resistance and preserve the effectiveness of your larvicides and adulticides.
  • If following the operational recommendations above, adulticide rotation does not necessarily require mid-season recalibration.
  • Reliance on alternative methods of mosquito control, such as larviciding, physical, and biological control, alleviates pressure put on mosquitoes from adulticide use.
  • Emphasizing all parts of integrated mosquito management ensures your program achieves the highest levels of control while preserving product chemistries.


Richards, S., J.G. Balanay, M. Fields, and K. Vandock. 2017. Baseline insecticide susceptibility screening against six active ingredients for Culex and Aedes (Diptera: Culicidae) mosquitoes in the United States. J Med Entomol. 54(3): 682-695.

Dunbar, M.W., A. Bachmann, and A.J. Varenhorst. 2018. Reduced insecticide susceptibility in Aedes vexans (Diptera: Culicidae) where agricultural pest management overlaps with mosquito abatement. J Med Entomol. 55(3): 741-751.

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