Evaluation of insecticide efficacy and insecticide adaptive response in Italian populations of Drosophila suzukii

Monitoring sensitivity to insecticides is crucial to prevent outbreaks of invasive pests characterized by high reproductive and adaptive potential such as the Drosophila suzukii (Matsumura) (Diptera Drosophilidae). The aim of the present study was to investigate the possible appearance of resistance to cyantraniliprole, deltamethrin and spinosad. Field trials on commercial sweet cherry orchards in Northern Italy showed that two out of six strains were not fully controlled using cyantraniliprole and deltamethrin, while spinosad was thoroughly effective. At the bioassay, two populations showed a decrease in deltamethrin and cyantraniliprole susceptibility (LC50 values 12.7-21.0 and 3.4-5.8 times higher than those from the untreated populations, respectively). Biochemical analyses revealed that low resistance to the pesticides was associated with high monooxygenase and carboxylesterase activities (range 2.68-4.37-and 1.97-2.73 times higher than in the wild population). A dose-dependent increase in cytochrome P450 monooxygenase Cyp12d1 and ryanodine receptor gene expression was found when a strain with low resistance to cyantraniliprole in field trials was treated with increasing dosages of the diamide in bioassays. No mutations were detected in voltage-gated sodium channel and ryanodine receptor genes, which accounted for the reduction in pyrethroid and diamide susceptibility in other pests. After 8 generations of selection, starting from a susceptible population, the LC50 values of cyantraniliprole and deltamethrin were increased 2.2 and 25.0 fold, respectively, compared with the unselected colony. In contrast, no selection was possible for spinosad. Our study would suggest that spotted wing drosophila, upon continued selective pressure, are more prone to develop low resistance to cyantraniliprole and deltamethrin than spinosad. The adaptive response relies on detoxifying activities of monooxygenases and increased Cyp12d1 and ryanodine receptor gene expression.