High Resistance to Bacillus sphaericus and Susceptibility to Other Common Pesticides in Culex pipiens (Diptera: Culicidae) from Salt Lake City, UT

: Biorational mosquito larvicides based on microbial organisms and insect growth regulators (IGRs) have played a vital role in integrated mosquito control, particularly since the invasion of West Nile virus to the United States in 1999. Products that are formulated with technical powder of the bacterium, Bacillus sphaericus Neide (recently Lysinibacillus sphaericus Meyer and Neide), are among the ones that have been extensively applied to combat Culex and other mosquito species. Due to the simplicity of the binary toxins, resistance to this pesticide in laboratory and field populations of Culex pipiens L. complex has occurred globally since 1994. A Cx. pipiens population with a high level of resistance to B. sphaericus (VectoLex WDG) was identified in Salt Lake City, UT, in September 2016. The resistance ratios in this population were 20,780.0- and 23,926.9-fold at LC50 and LC90, respectively, when compared with a susceptible population of a laboratory reference colony of the same species. This B. sphaericus-resistant population remained mostly susceptible to other commonly used pesticides to control arthropods of public health and urban significance, including ones based on microbial organisms (Bacillus thuringiensis subsp. israelensis, spinosad, spinetoram, abamectin), IGRs (pyriproxyfen, methoprene, diflubenzuron, novaluron), organophosphate (temephos), neonicotinoid (imidacloprid), phenylpyrazole (fipronil), oxadiazine (indoxacarb), and pyrethroid (permethrin). Results are discussed according to the modes of action of the pesticides tested, and suggestions are made to manage B. sphaericus-resistant mosquito populations.