Piperonyl Butoxide Efficacy as a Synergist of Zeta-Cypermethrin in Resistant Drosophila suzukii Populations

Spotted-wing Drosophila (Drosophila suzukii), an economically important invasive but widely distributed pest, has developed resistance to multiple insecticide classes, threatening its management in commercial soft fruit production. This study evaluated the synergism of piperonyl butoxide (PBO) with zeta-cypermethrin in two field-collected resistant California populations and a susceptible population with bioassays. Female flies from the two resistant populations exhibited 55-fold and 25-fold resistance, respectively, compared to the susceptible population. PBO co-application significantly enhanced insecticide efficacy in both resistant populations, with synergism ratios of 6.51 and 4.06. However, complete susceptibility at label rates of the insecticide was not restored, indicating that other resistance mechanisms may also be present in these populations. PBO–pyrethroid combinations show promise for improving field efficacy against resistant populations, though they should be integrated with insecticide rotation and other integrated pest management tactics for sustainable resistance management.