First report of field-derived Drosophila suzukii resistance to pyrethroids in California

Calendar-based applications of pyrethroids, organophosphates, and spinosyns have been the main approach against the invasive spotted wing drosophila, Drosophila suzukii (Matsumura), a serious pest of berry and cherry crops in California. Since field-derived resistance to spinosad has already been documented in California caneberry production and concern for development of resistance to other insecticide classes exists among growers, we studied the susceptibility of D. suzukii populations collected from different caneberry and strawberry fields to two pyrethroids, bifenthrin (Type I) and zeta-cypermethrin (Type II), in 2019 and 2020. Flies from all sites showed high levels of resistance to both pyrethroids, and males were generally more susceptible than females in all assays. The LC₅₀ values for flies collected from caneberries in 2019 ranged from 4.5 to 5.2 mg l^-1 with RR₅₀s ranging from 7.5 to 8.7-fold. For flies collected from strawberries in 2020, the LC₅₀ values ranged from 19.0 to 36.1 mg l^-1 and from 30.3 to 90.7 mg l^-1 for zeta-cypermethrin and bifenthrin, respectively. The RR₅₀ values varied from 19.0 to 36.1-fold for zeta-cypermethrin, and from 15.9-47.7-fold for bifenthrin. This study is the first report of field-derived pyrethroid resistance in D. suzukii that appears to be widespread in major California berry production areas. This resistance presents a challenge to the control of D. suzukii. Implementing informed insecticide resistance management practices would be critical to prolong the efficacy of products currently available for management of this pest.