Date of Award


Document Type


Degree Name

Master of Science (M.S.)


Biological Sciences

First Advisor

Tara Thiemann

First Committee Member

John Mayberry

Second Committee Member

Geoff Lin-Cereghino


Culex tarsalis, a vector for diseases like West Nile virus, is evolving resistance to pyrethroid and organophosphate insecticides used for agriculture and vector control in Northern California. Common mechanisms of resistance in other Culex species include the target-site mutations kdr and ace-1 and increased levels of detoxification enzymes (esterases, GSTs, and oxidases). This study contained two parts: Pyrethroid Individual study and an Organophosphate Preliminary study. For the Pyrethroid Individual study, the goal was to compare the prevalence of kdr mutations and detoxification enzymes between pyrethroid susceptible and resistant Cx. tarsalis individuals in Northern California. For the Organophosphate Preliminary study, the goal was to survey current organophosphate resistance in Northern California Cx. tarsalis populations while identifying possible resistance mechanisms: ace-1 and variations in detoxification enzymes between populations and individuals. Individual mosquitoes previously categorized by resistance status from CDC bottle bioassays with permethrin or naled were prepared for both molecular and enzymatic testing by separating the legs of a mosquito from the remaining body. Legs were used to test for the presence of kdr and ace-1 mutation by qPCR and PCR respectively and confirmed by DNA sequencing while the bodies were used to test for levels of detoxification enzymes. A subset of individuals in the Organophosphate Preliminary study forwent bottle bioassays and were tested directly to make comparisons between populations without the stress of going into bottle bioassays. For the Pyrethroid Individual study, the greater number of F alleles present at the kdr target-site as well as increased levels of GST significantly increased survival when exposed to permethrin. Individuals with 2 F alleles and an active GST level greater than or equal to 0.052 ug/ml showed a higher survival rate than either mechanism independently demonstrating resistance to pyrethroids in Cx. tarsalis is likely the result of multiple resistance mechanisms acting collectively. For the Organophosphate Preliminary study, currently little resistance to naled (3%) was observed in Northern California Cx. tarsalis populations. No ace-1 mutations were found in the 208 individuals tested, however three did possess silent mutations at the target-site, suggesting surveillance is needed in the future. Between the small sample size and little resistance found in this preliminary study, there was no correlation between levels of detoxification enzymes from resistant and susceptible individuals. Due to differences in the biology of Cx. tarsalis and other mechanisms of resistance not looked at in this study it is hard to quantify definitive resistance mechanisms in Cx. tarsalis. From this study it appears that kdr mutation (L1014F) and increased levels of GST likely contribute at some level to resistance to pyrethroid insecticides in Cx. tarsalis.



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