Stability of resistance in spotted-wing drosophila populations from California

The invasive spotted-wing drosophila, Drosophila suzukii, has become one of the most economic pests of berry crops and stone fruit worldwide. In the United States, as in other countries, commercial production heavily relies on chemical control, and broad-spectrum insecticides are the primary control options for D. suzukii management. Frequent applications of spinosad and pyrethroids in California berry productions has already resulted in resistance development in this pest. We assessed the stability of resistance in populations of D. suzukii collected from strawberry fields in California. For this purpose, field populations were reared in the laboratory for multiple generations without exposure to insecticides. An initial dose-response bioassay with spinosad and zeta-cypermethrin was conducted with F₁ flies to determine the baseline susceptibility of these populations and repeated at F₁₁ to determine any changes in susceptibilities over 10 generations. In a separate study, susceptibility of a field population collected from caneberries was initially assessed against zeta-cypermethrin at F₁, after which larvae were exposed to a sub-lethal concentration of zeta-cypermethrin for 18 generations. The insecticide exposure was terminated for one generation to build of fly populations and a dose-response bioassay was conducted at F₂₀ to investigate changes in susceptibility over 18 generations of laboratory selection. The susceptibility of this population to the discriminating dose of zeta-cypermethrin was then evaluated every two generations after selection termination to determine if susceptibility will be restored. The overall results show significant increase in resistance with selection and stability of resistance following selection.