Temporal Variation in Insecticide Susceptibility of Field-Derived Drosophila suzukii from UK Soft-Fruit Farms

Drosophila suzukii is a globally invasive pest that lays eggs in ripening fruit, causing degradation through larval feeding and significant economic losses. Control of D. suzukii relies on integrated pest management, combining cultural, biological and chemical methods, with insecticide applications serving as a final line of defence. This study investigated whether regular insecticide use in United Kingdom (UK) soft-fruit crops resulted in reduced between- and within-year susceptibility of D. suzukii to three commonly used insecticides. Field-collected D. suzukii were used to establish laboratory strains from summer collections in 2019, 2020 and 2021, and autumn collections in 2020. Strains were reared for 8–10 generations before exposure to a range of concentrations of cyantraniliprole, lambda-cyhalothrin and spinosad in mortality bioassays. No consistent directional increase in LC50 was observed across the three sampling years. However, more than half of the strains established from autumn 2020 collections showed reduced susceptibility to some insecticides compared with strains from the same locations collected four months earlier, with a 1.4–2.1-fold increase in LC50. These results indicate generally stable susceptibility in UK populations while highlighting the value of continued resistance monitoring.