Prevention of resistance to spinosad by a combination of spinosad and Bacillus thuringiensis subsp. israelensis in Culex quinquefasciatus (Diptera: Culicidae).

Background: Spinosad consists of spinosyn A and spinosyn D that are produced by the soil-dwelling actinomycete Saccharopolyspora spinosa. It has been used to control a wide variety of arthropod pests of economic importance. Formulations of spinosad have been used to control larval mosquitoes since approximately 2010. However, the target site- and metabolism-based resistance to this neurotoxin has been reported since 2000 in agricultural pests The current studies aim to further evaluate resistance risk and develop resistance prevention tactics in the southern house mosquito Culex quinquefasciatus.

Results: The first comparison group involved selection at lethal concentration, 75% (LC) of technical spinosad for 30 generations, resulting in baseline resistance ratios (RRs) of 51.1-fold at LC and 45.4-fold at LC in Cx. quinquefasciatus. However, under the same conditions, selection by a combination of spinosad and Bacillus thuringiensis subsp. israelensis (B.t.i.), negated resistance development to spinosad, RRs ranging 1.00-1.75-fold at LC and 0.83-1.76-fold at LC. At the same time, the selected population remained susceptible to the combination throughout the selection process, RRs fluctuating 0.74-1.38-fold at LC and 0.63-1.23-fold at LC.

Conclusion: Combination of spinosad and B.t.i. negates resistance development to spinosad, as opposed to spinosad alone in Cx. quinquefasciatus. Moreover, the species tested does not develop resistance to this combination upon repeated exposures, implying the potential for further developing this combination as a viable product for larval mosquito control. © 2024 Society of Chemical Industry.