Date of Award


Degree Name

Master of Science



First Advisor

Lydy, Michael

Second Advisor

Kwasek, Karolina

Third Advisor

Heist, Edward


Hyalella azteca are non-target epibenthic amphipods that have developed resistance to pyrethroid and organophosphate insecticides due to single amino acid substitutions in the voltage gated sodium channel and the acetylcholinesterase-1 gene, respectively. Aquatic systems are often contaminated with several different types of insecticides, therefore there is a possibility that H. azteca have also developed resistance to other classes of insecticides. The aims of the current study were to verify that pyrethroid- and organophosphate-resistant H. azteca have retained their resistance after being cultured for 5 years (Escondido Creek population) and 9 years (Mosher Slough population) in pesticide-free water, to determine if these populations have crossresistance to carbaryl (carbamate) and 4,4’-dichlorodiphenyltrichloroethane (DDT; organochlorine), and check if previous field exposure to fipronil (phenylpyrazole) and imidacloprid (neonicotinoid) caused resistance in our pyrethroid- and organophosphate-resistant H. azteca populations. Escondido Creek and Mosher Slough H. azteca populations both maintained high tolerances for bifenthrin due to L925I and I936F amino acid substitutions.Resistance was also found for chlorpyrifos in the Escondido Creek and Mosher Slough populations with lower genotype frequencies of the G119S substitution, indicating that additional factors may be responsible for organophosphate resistance in this study. Mosher Slough H. azteca were moderately resistant to DDT, and Escondido Creek and Mosher Slough H. azteca were moderately resistant to carbaryl, suggesting cross-resistance. No differences were observed in LC50 values across the three populations of H. azteca for fipronil and imidacloprid, and this is possibly due to the lack of exposure to toxic concentrations of these insecticides in the field and lack of similar modes of action to pyrethroids and organophosphates. Resistance is known to be associated with fitness costs that can place insecticide-resistant populations at risk for decline through decreased survival and reduced fecundity.




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