Do Mosquito Misting Systems Contribute to Resistance? Joseph M. Conlon Technical Advisor American Mosquito Control Association

What is Resistance? The ability, in a population of insects, to tolerate doses of a pesticide that would be lethal to the majority of the individuals in a normal population of the same species, and developed as a result of the selection pressure exerted by the insecticide. The ability, in a population of insects, to tolerate doses of a pesticide that would be lethal to the majority of the individuals in a normal population of the same species, and developed as a result of the selection pressure exerted by the insecticide.

Resistance Mechanisms Pyrethroids – act by binding with sodium channel protein, causing nerve to be permanently switched on: “kdr” Pyrethroids – act by binding with sodium channel protein, causing nerve to be permanently switched on: “kdr” Persistent activation of sodium channels by delaying normal voltage-dependent deactivation Persistent activation of sodium channels by delaying normal voltage-dependent deactivation Increase in permeability due to lesions produced by pyrethroids Increase in permeability due to lesions produced by pyrethroids Resistance due to altered sodium channel protein Resistance due to altered sodium channel protein

Resistance Mechanisms Esterases Esterases Bind and sequester Bind and sequester Culex resistance Culex resistance Amplification/over expression of genes encoding xenobiotic metabolizing enzymes Amplification/over expression of genes encoding xenobiotic metabolizing enzymes Low fitness cost compared to gene structure alteration Low fitness cost compared to gene structure alteration Anopheline resistance Anopheline resistance kdr – reduction in site sensitivity of the voltage-gated sodium channels to binding of insecticides kdr – reduction in site sensitivity of the voltage-gated sodium channels to binding of insecticides Fitness cost - genes occur at low frequency unless selected for Fitness cost - genes occur at low frequency unless selected for

Resistance Mechanisms Monooxygenase – P450 gene binds molecular oxygen and introduces the oxygen molecule into substrate Monooxygenase – P450 gene binds molecular oxygen and introduces the oxygen molecule into substrate Elevated in Anopheles and C. quinquefasciatus Elevated in Anopheles and C. quinquefasciatus Primary mechanism in pyrethrum resistance Primary mechanism in pyrethrum resistance High fitness cost due to gene alteration High fitness cost due to gene alteration Glutathione S-transferase (GST) Glutathione S-transferase (GST) Binds the insecticide to glutathione, making it more water soluble Binds the insecticide to glutathione, making it more water soluble More easily excreted More easily excreted

Resistance Management Management by Moderation - spare portion of susceptible genotypes Management by Moderation - spare portion of susceptible genotypes Low dose Low dose Non-persistent chemicals – no slow release Non-persistent chemicals – no slow release Leave certain generations untreated Leave certain generations untreated Preserving refugia Preserving refugia Apply locally Apply locally High thresholds High thresholds

Resistance Management Management by Saturation – overwhelm defense mechanisms by high dosages Management by Saturation – overwhelm defense mechanisms by high dosages Apply dosages sufficiently high to be lethal to susceptible as well as heterozygous-resistant Apply dosages sufficiently high to be lethal to susceptible as well as heterozygous-resistant Prevents buildup of homozygous resistant Prevents buildup of homozygous resistant Use of synergists Use of synergists May not work with OPs May not work with OPs

Resistance Management Lower selection pressure Lower selection pressure Remove selective advantage – increase fitness cost Remove selective advantage – increase fitness cost IPM IPM

Factors Affecting Resistance Management Genetic Genetic Frequency/dominance of resistant alleles - How extensive is resistance Frequency/dominance of resistant alleles - How extensive is resistance Past selection by other chemicals Past selection by other chemicals Fitness factors – competitive disadvantage Fitness factors – competitive disadvantage Biotic Biotic Generations/year Generations/year Offspring/generation Offspring/generation Single/multiple matings Single/multiple matings Behavior/ecological Behavior/ecological Migration, mobility, isolation Migration, mobility, isolation Monophagy/polyphagy – feeding on many hosts reduces exposure Monophagy/polyphagy – feeding on many hosts reduces exposure Chemical Chemical Mode of action Mode of action Relationship to previously used chemicals Relationship to previously used chemicals Persistence Persistence Application Application Rate, frequency and size of area Rate, frequency and size of area Life stage – treat only one stage Life stage – treat only one stage Application threshold – lower thresholds yield quicker resistance Application threshold – lower thresholds yield quicker resistance

Resistance in Mosquitoes Harris County Texas – resmethrin (Scourge©) Harris County Texas – resmethrin (Scourge©) lbs AI/acre – no control lbs AI/acre – no control lbs AI/acre – 40% control lbs AI/acre – 40% control Recovered resmethrin mortality at lbs AI/acre within one year of malathion rotation Recovered resmethrin mortality at lbs AI/acre within one year of malathion rotation California California C. tarsalis – resmethrin, permethrin and pyrethrum C. tarsalis – resmethrin, permethrin and pyrethrum Contra Costa, Fresno, Merced, Riverside, Sacramento Contra Costa, Fresno, Merced, Riverside, Sacramento C. pipiens – pyrethrum, permethrin, deltamethrin and lambda-cyhalothrin C. pipiens – pyrethrum, permethrin, deltamethrin and lambda-cyhalothrin Marin Marin

Do MMS Contribute To Resistance? Sublethal dosing – increases selection pressure Sublethal dosing – increases selection pressure 90% control in heavily vegetated areas may require 2x-3x maximum label rate 90% control in heavily vegetated areas may require 2x-3x maximum label rate Resistance not monitored Resistance not monitored No rotation of insecticides No rotation of insecticides May kill natural predators in foliage May kill natural predators in foliage Saturation in one area promotes resistance Saturation in one area promotes resistance Lessens incentive for IPM implementation Lessens incentive for IPM implementation