Insecticide Resistance

Insecticides and the Integrated Pest Management Approach

• Reduce/remove egg laying sites

• Biological control (e.g., mosquito fish)

• Larvicides

• Adulticides (last line of defense when other options have been exhausted)

Insecticides and Surveillance

• Surveillance based targeted mosquito control should be practiced.

• Potentially dangerous (vector) populations controlled.

• In post-disaster (hurricane/flooding) when high numbers of mosquitoes impede recovery workers, increase chance of pathogen transmission, etc.

Mosquitoes have short life cycles (7-8 days from egg to adult), they can ______.

adapt to environmental pressures over time.

• Active ingredients used in insecticides act in different ways (different modes of action) to kill mosquitoes.

• Currently widespread reliance on pyrethroid adulticides in mosquito control.

If mosquitoes are exposed to doses of insecticides that are too weak to kill them, they may _______.

develop resistance to those active ingredients.

• Surveillance-based targeted control very important.

• Is the product that is being used working?

Potential Sources of Insecticide Exposure

• Mosquito larvae and adults may be exposed to insecticides used in agriculture. 

• Homeowners may apply insecticide products (for mosquitoes and other pests) to their properties and this can influence insecticide resistance/susceptibility.

• Private/public mosquito control (and other pest control) programs.

Pesticide resistance can _______.

mean many things

• Behavioral changes where mosquitoes avoid insecticide-treated areas.

• Metabolic changes in the mosquito that help it survive insecticide exposure.

• Genetic changes or adaptations that prevent the insecticide from acting on its target within a mosquito.

• Physical changes that lessen penetration of insecticides into the mosquito’s body.

How is resistance tested?

• Mosquito control programs should check for resistance/susceptibility periodically during the mosquito season to ensure the most effective control measures are used.

• This is especially important if control benchmarks are not being met.

CDC Bottle Bioassay used to ______.

monitor insecticide resistance to adulticides.

• Treatment (glass) bottles coated with appropriate diagnostic dose of insecticide made in acetone.

  • Diagnostic dose and time determined based on susceptible populations.

• Control bottles coated with acetone only.

Methods in lab for CDC Bottle Bioassay

• 1 mL of each pure active ingredient stock/bottle. These are diluted by acetone placed in the bottles.

• Inside of bottles coated.

• At least 3 control bottles with only 1 mL acetone used in each assay.

• Caps removed and bottles placed on bottle roller until contents evaporated (1 - 2 minutes)

• Uncapped bottles placed into dark drawer and used within 24 h.

• Live mosquitoes transferred to bottles.

• Similar ages of mosquitoes is best since resistance can vary with age.

• CDC bottle bioassays conducted.

• Mosquito mortality  recorded at 11 time points using the “CDC bottle bioassay data recording form”. Go to the complete 2 hours unless all the mosquitoes in the bottle have died.

CDC insecticide resistance testing guidelines - Susceptible

≥ 97% of mosquitoes dead at the diagnostic time

CDC insecticide resistance testing guidelines - Developing Resistance

90-96 % of mosquitoes dead at the diagnostic time

CDC insecticide resistance testing guidelines - Resistant

< 90 % of mosquitoes dead at the diagnostic time

Differences observed in _______.

resistance profiles between genera

• Aedes spp. generally more susceptible than Culex.

Mapping of Resistant Populations

• In 2016, CDC investigators at the Dengue Branch in Puerto Rico carried out resistance tests and mapped resistance to several Active ingredents.

• Example of map for one Active ingredients.

• MosquitoNET has been set up by the CDC and will be used by Epidemiology and Laboratory Capacity program recipients to monitor mosquito collections and insecticide resistance in the United States

The World Health Organization Pesticide Evaluation Scheme (WHOPES) is ______.

another method for evaluating insecticide resistance.

• WHO Cone method for evaluating bed nets, permethrin-treated clothing, etc.

Some studies introduce insecticide (adulticides) solutions into ______.

larval rearing water to assess resistance (proxy for assessing adult resistance).

• Similar methods can be used to evaluate larvicides for resistance.

Wind tunnel

• Live mosquitoes transferred to circular mesh cages and placed into wind tunnel within a chemical hood. 

• Background air for unidirectional air flow through wind tunnel.

• Formulated insecticide product (Biomist® 3+15) aerosolized via Blaustein Atomizing Module (BLAM).

• Mosquitoes exposed in the wind tunnel for 10 s.

Evaluation of control measures should be an _______.

integral component of a mosquito control program.

• If expertise/funding is not available for resistance testing, programs should periodically rotate insecticide active ingredients.

• New active ingredients are being developed with novel modes of action.

• Manufacturers develop insecticide active ingredients and formulations.

  • Products evaluated and registered by the EPA

Summary

• Only the most effective insecticides should be used for targeted control. 

• Surveillance of insecticide resistance enhances the ability of control programs to protect public health.

• We expect variation in susceptibility or resistance between mosquito species, populations, and for the same populations within and/or between seasons.

Wind Tunnel Study Example

• Two Culex populations classified as resistant in bottle bioassays with Pernetherin were susceptible in wind tunnel.

• Wind tunnel exposure resulted in a 100% mortality in all mosquito populations tested. 

Wind tunnels mimic how ______.

aersol spraying works in the field

CDC light traps _________.

capture mosquitoes and allow us to test them for insecticide resistance

WHO Cone method

used for evaluating bed nets, permethrin-treated clothing, etc.

• cut out material into size of a petri dish and place under cone and add live mosquitoes

• count over a period of time to see how long mosquitoes last