Population genomics of Drosophila suzukii reveal longitudinal population structure and signals of migrations in and out of the continental United States

Drosophila suzukii, or spotted-wing drosophila, is now an established pest in many parts of the world, causing significant damage to numerous fruit crop industries. Native to East Asia, D. suzukii infestations started in the United States (U.S.) a decade ago, occupying a wide range of climates. To better understand invasion ecology of this pest, knowledge of past migration events, population structure, and genetic diversity is needed. In this study, we sequenced whole genomes of 237 individual flies collected across the continental U.S., as well as several sites in Europe, Brazil, and Asia, to identify and analyze hundreds of thousands of genetic markers. We observed strong population structure between Western and Eastern U.S. populations, but no evidence of any population structure between different latitudes within the continental U.S., suggesting there is no broad-scale adaptations occurring in response to differences in winter climates. We detect admixture from Hawaii to the Western U.S. and from the Eastern U.S. to Europe, in agreement with previously identified introduction routes inferred from microsatellite analysis. We also detect potential signals of admixture from the Western U.S. back to Asia, which could have important implications for shipping and quarantine policies for exported agriculture. We anticipate this large genomic dataset will spur future research into the genomic adaptations underlying D. suzukii pest activity and development of novel control methods for this agricultural pes

Evaluation of Wild Flora Surrounding Blueberry Fields as Viable Hosts of Drosophila suzukii (Matsumura, 1931) (Diptera: Drosophilidae) in Georgia

Drosophila suzukii, an economically important pest of small and thin-skinned fruits, has caused annual crop losses up to 20% in the state of Georgia’s multimillion-dollar blueberry industry. The known host range of D. suzukii is large, yet the breadth of uncultivated and wild plants that can serve as alternative hosts in the southeastern United States is still not fully understood. Establishing comprehensive lists of non-crop D. suzukii hosts in woodlands near blueberry production will assist in the creation of more sustainable integrated pest management (IPM) strategies. Objectives of this study were to determine viability of wild fruiting plant species to this pest based on survivorship to adulthood and assess D. suzukii short-range preference between cultivated blueberries and wild fruit. Laboratory choice and no-choice assays were performed to determine if D. suzukii could complete its development on wild fruits sampled from the field. Results from our no-choice assays indicated that multiple species of wild fruits surveyed in Georgia were viable D. suzukii hosts including blackberry species, deerberry, hillside blueberry, common pokeweed, beautyberry, elderberry, evergreen blueberry, and large gallberry. Yet, none of these hosts were preferred by adult female D. suzukii as ovipositional substrates when compared to cultivated blueberries. However, these uncultivated species have the potential to sustain D. suzukii populations pre- and post-harvest season. This information can help farmers do more targeted management of these viable alternative hosts from wooded areas surrounding blueberry fields in order to minimize D. suzukii populations

Table_1_Validation of vacuum extraction protocol for larval sampling of spotted-wing drosophila in blueberries.xlsx

For spotted-wing drosophila, Drosophila suzukii Matsumura, the standard larval sampling protocol from the fruits involves immersing the fruits in hot water, salt, or sugar solution followed by enumerating the larvae that exit from the fruit. In this study, using the fruit samples collected from commercial blueberry farms, we validated the efficacy of a novel D. suzukii larval sampling technique, the vacuum extraction protocol. The larval extraction efficacy of the vacuum extraction method was compared with the salt extraction method using paired fruit samples collected from three different fields for four consecutive weeks. Overall, larval counts detected through the vacuum extraction were significantly higher than those detected through salt extraction in two out of three blueberry fields. Additionally, among the five blueberry fruit sample volumes tested (118, 177, 237, 355, and 473 mL), we identified 355 mL (12 fl. oz.) as the optimum fruit sample volume for vacuum extraction that maximized sampling precision for unit sampling cost. Since D. suzukii larvae exited from fruits after vacuum extraction remain alive and active, a delay in separating the larvae from the sample with coffee filter extraction may result in larval movement back into the fruit flesh. However, compared with immediate processing, delaying coffee filter extraction up to 60 minutes after vacuum incubating the fruit samples at -98 kPa for 60 minutes did not significantly reduce the larval count. Overall, the results of validation trials indicate that vacuum extraction is a promising sampling method for growers to detect D. suzukii larval infestation in blueberries.</p

Diel periodicity of <i>Drosophila suzukii</i> (Diptera: Drosophilidae) under field conditions

<div><p><i>Drosophila suzukii</i> Matsumura (Diptera: Drosophilidae), an economically important pest of blueberry and other thin-skinned fruits, persists and prolifically reproduces under seemingly lethal climatic conditions in the field. However, behavioral and physiological mechanisms employed by <i>D</i>. <i>suzukii</i> to tolerate such extreme climatic conditions in the field are unknown. The primary objective of this project was to investigate diel periodicity of <i>D</i>. <i>suzukii</i> and their reproductive success under field conditions as related by climatic factors such as temperature and relative humidity. Results show that <i>D</i>. <i>suzukii</i> reproductive success was significantly higher during the night (including dawn and dusk periods) than the day in terms of oviposition, pupation, adult eclosion, and the number of progeny per female. Female <i>D</i>. <i>suzukii</i> reproductive success was not significantly different between specific regions of a blueberry bush in relation to the amount of shade provided by the canopy. Our studies indicate that <i>D</i>. <i>suzukii</i> flight activity is crepuscular and is sensitive to fluctuations in temperature and relative humidity. Results also suggest that the majority of fly activity during peak hours is concentrated in areas around the border and within the center of blueberry orchards with little activity in the surrounding wooded areas. These findings suggest that <i>D</i>. <i>suzukii</i> prefers microclimate with mild temperatures and high humidity, and does not function well when exposed to direct sunlight with extreme heat. The authors propose that <i>D</i>. <i>suzukii</i> management strategies should be implemented during the early morning and immediately before darkness to maximize efficacy.</p></div

Satellite imagery of the blueberry orchard in Bacon County, GA shows orchard layout with trap location and relative distance between each trap.

<p>Traps were spaced 75 meters apart between each location and 50 meters apart within each location.</p

Percent pupation.

<p>Percent pupation (± SEM) of flies caged during the 12-hour periods designated as day or night including dawn and dusk (top), and F1 progeny per female (± SEM) during the day or night (bottom). Different letters signify statistical differences among means (LSMEANS test).</p

Ratio of females to total fly captures.

<p>Ratio of females to total fly captures.</p

Mean number of eggs per female (± SEM) by time of day and chamber location.

<p>Different letters signify statistical differences among means (LSMEANS test).</p

Mean percent pupation.

<p>Mean percent pupation (± SEM) by time of day (top) and mean progeny per female (± SEM) by time of day (bottom). Different letters signify statistical differences among means (LSMEANS test).</p

Mean number of spotted wing drosophila captured at different times of day and location near the orchard in 2016.

<p>Different letters signify statistical differences among locations within times of day (LSMEANS test).</p