Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)



First Advisor

C. Lamar Meek


Significant resistance to malathion and resmethrin was found in insecticide-treated field populations of adult Culex quinquefasciatus , Say, compared with a laboratory and non-treated field strain. Resistance to both insecticides varied during the active control season and was associated with the frequency of insecticide application. Geographic variations in resistance levels were noted, and significant differences occurred between separate populations within the same mosquito control districts. Four model substrates were assessed for their suitability in detecting elevated esterase activity associated with malathion resistance. Esterase activities towards all substrates were significantly higher in field stains than a laboratory strain. Esterase activities toward the widely used substrates, alpha and beta-naphthyl acetates, were equivalent or significantly higher than activities toward the alternative substrates alpha and beta-naphthyl propionates, suggesting that the naphthyl acetates provide a more sensitive measure of esterase activities in mosquito populations. Synergist and biochemical assays were used to study mechanisms associated with malathion and resmethrin resistance in field populations of Cx. quinquefasciatus. Pretreatment with an esterase inhibitor (DEF) significantly reduced malathion resistance. Resmethrin resistance was completely suppressed in mosquitoes pretreated with the oxidase inhibitor, piperonyl butoxide. Results of biochemical assays suggest the involvement of enhanced esterase and oxidase activities in malathion and resmethrin resistance, respectively. Additionally, malathion resistance was associated with reduced inhibition of acetylcholinesterase (AChE), the target site of organophosphate and carbamate insecticides. Selection with malathion resulted in a significant increase in esterase activities and reduced AChE inhibition. The effects of developmental stage, sex, age, and blood feeding on expression of malathion and resmethrin resistance was also evaluated. Significant variations in resistance and expression of resistance mechanisms occurred between larvae and adults and male and female adults. In female Cx. quinquefasciatus , malathion resistance declined with age and was associated with a decrease in esterase activity. Resmethrin resistance also declined with age, but was not related to a reduction in peroxidase activity. Results of this study indicate that life history characteristics can have significant effects on the expression of insecticide resistance and suggests that their impact on field control should be further evaluated.