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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. Resistance Mechanisms 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 Increase in permeability due to lesions produced by pyrethroids Resistance due to altered sodium channel protein Resistance Mechanisms Esterases Bind and sequester Amplification/over expression of genes encoding xenobiotic metabolizing enzymes Culex resistance Low fitness cost compared to gene structure alteration Anopheline resistance 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 Resistance Mechanisms Monooxygenase – P450 gene binds molecular oxygen and introduces the oxygen molecule into substrate Elevated in Anopheles and C. quinquefasciatus Primary mechanism in pyrethrum resistance High fitness cost due to gene alteration Glutathione S-transferase (GST) Binds the insecticide to glutathione, making it more water soluble More easily excreted Resistance Management Management by Moderation - spare portion of susceptible genotypes Low dose Non-persistent chemicals – no slow release Leave certain generations untreated Preserving refugia Apply locally High thresholds Resistance Management Management by Saturation – overwhelm defense mechanisms by high dosages Apply dosages sufficiently high to be lethal to susceptible as well as heterozygous-resistant Prevents buildup of homozygous resistant Use of synergists May not work with OPs Resistance Management Lower selection pressure Remove selective advantage – increase fitness cost IPM Factors Affecting Resistance Management Genetic Biotic Migration, mobility, isolation Monophagy/polyphagy – feeding on many hosts reduces exposure Chemical Generations/year Offspring/generation Single/multiple matings Behavior/ecological Frequency/dominance of resistant alleles - How extensive is resistance Past selection by other chemicals Fitness factors – competitive disadvantage Mode of action Relationship to previously used chemicals Persistence Application Rate, frequency and size of area Life stage – treat only one stage Application threshold – lower thresholds yield quicker resistance Resistance in Mosquitoes Harris County Texas – resmethrin (Scourge©) 0.003 lbs AI/acre – no control 0.007 lbs AI/acre – 40% control Recovered resmethrin mortality at 0.007 lbs AI/acre within one year of malathion rotation California C. tarsalis – resmethrin, permethrin and pyrethrum Contra Costa, Fresno, Merced, Riverside, Sacramento C. pipiens – pyrethrum, permethrin, deltamethrin and lambda-cyhalothrin Marin Do MMS Contribute To Resistance? Sublethal dosing – increases selection pressure 90% control in heavily vegetated areas may require 2x-3x maximum label rate Resistance not monitored No rotation of insecticides May kill natural predators in foliage Saturation in one area promotes resistance Lessens incentive for IPM implementation