Susceptibility of the southern house mosquito, Culex quinquefasciatus, in East Baton Rouge Parish to larval insecticides

Abstract

Mosquito control districts in Louisiana focus their efforts on Culex quinquefasciatus, the primary vector of West Nile virus in the southern United States, with rigorous larvicide treatments. However, the development of resistant populations of Cx. quinquefasciatus in response to extensive insecticide application has been demonstrated repeatedly. Examining changes in insecticide susceptibility and larvicide efficacy in real world scenarios can help inform mosquito control districts as to whether or not their treatments are killing mosquitoes. We hypothesized that frequent larvicide applications for the control of mosquitoes in East Baton Rouge Parish had lowered susceptibility of wild Cx. quinquefasciatus to insecticides, and that treatment in real-world septic water conditions negatively impacts larvicide efficacy. Larvicide susceptibility and efficacy in septic-water were measured using the larvicides Bacillus sphaericus, spinosad, and temephos. Culex quinquefasciatus populations were sampled from sites in three Parishes where frequencies of insecticide applications varied, and frequencies of resistance and efficacy were measured relative to a susceptible reference colony. Five-fold resistance to the organophosphate temephos was detected at one site in East Baton Rouge Parish in the spring of 2016, which increased to ten-fold resistance by the end of the mosquito season. Activities of esterases were found to be elevated in wild, temephos-resistant mosquitoes, indicating the potential role of these enzymes as a mechanism of resistance. Water quality did not appear to play a significant role in the efficacy of the larvicides used in this study. The results of this study provide a baseline of comparison for future measurements of susceptibility in Cx. quinquefasciatus in Louisiana, and may help inform local mosquito control districts as to the effectiveness and sustainability of their insecticide programs.