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

SIMPLE SUMMARY: Continuous insecticide applications used to prevent fruit infestations by spotted-wing drosophila (SWD), an invasive pest of soft-skinned fruits worldwide, can elevate the risk of resistance development in D. Suzukii field populations. However, proactive assessment of resistance risk using laboratory selection provides valuable information for development of sustainable resistance management strategies for SWD. After 10 and 11 generations of artificial selection of a colony of field-collected SWD for resistance against spinosad and malathion, a 7.55- and 2.23-fold resistance to spinosad and malathion was realized. A quantitative genetic approach used to estimate realized heritability (h(2)) of resistance shows that the risk of resistance in SWD populations exists against both spinosad and malathion, and a faster rate of resistance development is expected against spinosad. However, timely implementation of resistance management strategies can slow the development of resistance and prolong effective life of these insecticides against D. suzukii. ABSTRACT: 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 (h(2)) 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 h(2) = 0.14 (mean h(2) of spinosad resistance in this study) and 90% of population was killed at each generation, 10-fold increase in LC(50) of D. suzukii females would be expected in nine generations for spinosad. However, 10-fold increase in LC(50) of D. suzukii females for malathion would be expected in 37 generations, assuming h(2) = 0.08 (mean h(2) 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.