Laboratory Selection and Assessment of Resistance Risk in Drosophila suzukii (Diptera: Drosophilidae) to Spinosad and Malathion

Drosophila suzukii (Matsumura) is one of the most economically important pests of soft-skinned fruits worldwide. Repeated insecticide applications commonly used to prevent fruit infestations increase the risk of resistance development in D. suzukii. Assessment of resistance risk in D. suzukii using artificial selection can be valuable in developing proactive resistance management strategies to retain susceptibility in the field populations. Here, we artificially selected a colony of field-collected D. suzukii for resistance against spinosad and malathion. A quantitative genetic approach was then used to estimate realized heritability (h2) of resistance and predict the rates of resistance development. After 10 and 11 generations of selection, resistance to spinosad and malathion in D. suzukii females significantly increased by 7.55- and 2.23-fold, respectively. Based on the predicted rates of resistance development, assuming h2 = 0.14 (mean h2 of spinosad resistance in this study) and 90% of population was killed at each generation, 10-fold increase in LC50 of D. suzukii females would be expected in nine generations for spinosad. However, 10-fold increase in LC50 of D. suzukii females for malathion would be expected in 37 generations, assuming h2 = 0.08 (mean h2 of malathion resistance) and 90% of population was killed at each generation. These results indicate that the risk of resistance in D. suzukii populations exists against both spinosad and malathion. However, resistance would develop faster against spinosad as compared to malathion. Thus, resistance management strategies should be implemented proactively to maintain the effectiveness of these insecticides to control D. suzukii.