Esterase Mediated Insecticide Resistance in the Southern House Mosquito, Culex quinquefasciatus

Abstract

Much is known about the development of insecticide resistance associated with targeted application against insect populations. However, off-target selection by applications of insecticides in agricultural and residential sites also impacts development of insecticide resistance and is understudied. Similarly, the impact of selecting one life stage of mosquitoes on the insecticide susceptibility of different life stages is largely unknown. The first part of this study shows that susceptibility to chlorantraniliprole, which is applied in rice and sugarcane fields in LA, decreased (5.7 to12-fold) in populations of Culex quinquefasciatus collected from near these fields compared to a reference field strain. In addition, application of bifenthrin by commercial application on an individual residence increased the resistance frequencies to bifenthrin in five nearby residential sites, where resistance frequencies between larvae and adult were highly correlated (R2=0.92) suggesting that selection of adults also confers resistance to larvae. The second part of this study focused on measurement of esterase activities associated with insecticide resistance. Most such studies have measured esterases, one of the major metabolic mechanisms of insecticide resistance, using the model substrate, α-naphthyl acetate (α-NA), which has a chemical structure vastly different than the insecticide, malathion. Therefore, the second objective of this study was to validate measurement of esterase activity using model substates through comparison with esterase hydrolyzing malathion. These results validated the use of α-NA as a model substrate to measure esterase activity in the lab. Additionally, malathion resistance in C. quinquefasciatus was highly associated with esterase activity, both with α-NA or malathion. This shows that esterase activity is a suitable biomarker for malathion resistance in adult C. quinquefasciatus. The third part of this study has examined the association between expression of EST-3 gene encoding an esterase enzyme and malathion resistance in C. quinquefasciatus. A positive correlation was found between malathion resistance and expression of EST-3 in field-collected C. quinquefasciatus. Similarly, high expression of EST-3 was found in individuals surviving application of a diagnostic concentration of malathion compared to individuals that was killed at this dose in both lab and field-strains of C. quinquefasciatus.