Abiotic and biotic factors influencing the spatio-temporal dispersal and population dynamics of Drosophila suzukii, the Spotted Wing Drosophila, in Southwest Germany

The Spotted Wing Drosophila, Drosophila suzukii Matsumura, is an invasive polyphagous insect pest. Indigenous to Southeast Asia it overcame its natural barriers in 2008 and has spread to southern Europe (Italy and Spain) and North America (California, USA). Through its adaptability to different climate conditions and the annidation to a hitherto vacant ecological niche, it was able to establish and spread rapidly through Europe and the Americas (South America, North America). In the following, it was first recorded in Germany in 2011. Since summer 2014, it is known from the entire federal territory. The mild winter of 2013/2014 supported D. suzukii to finally establish in south-western Germany, the same year when enormous crop loss occurred for the first time. In anticipation of a forecasted immigration of D. suzukii to Germany, the Institute for plant Protection in Fruit Crops and Viticulture of the Julius Kühn-Institut (JKI) started to establish a monitoring program in southwest Germany in 2011. In late summer of the same year, the first individuals were captured. In the following years the monitoring program was enlarged. Traps were installed at sites believed to be suitable habitats such as forest edges, forests, and open agricultural landscape as well as in fruit crops and vineyards. Based on this monitoring, the distribution and flight activity of D. suzukii was examined and statistically analysed along the years. On the basis of the obtained monitoring data, the year was divided into five „Drosophila seasons“: early spring, late spring, summer, autumn and winter. Statistical analysis of the data revealed that both hot and dry phases in late spring and summer and low precipitation during these seasons are negatively correlated with the activity of D. suzukii. Frost days in winter were also negatively correlated with the flight activity and thus the survival rate of the individuals. As part of the monitoring, additional traps were installed in the canopy of various tree species at different forested sites. By examination of the canopy of pine trees (Pinus sylvestris), especially in winter and spring, it was found that crowndwelling mistletoe (Viscum album) represents one of the first host plants in early spring for D. suzukii in Southwest Germany. In laboratory assays V. album was identified as a host for nutrition and development. The identification of additional host plants, especially in winter and spring, proved to be difficult, thus, a method to analyse ingested food resources molecularly was adapted for the first time to D. suzukii, a sponging-feeding Brachycera, to facilitate future analysis of the nutrition spectrum. The optimized molecular nutrition analysis promises to identify new winter retreats and resources, which may be helpful for developing management strategies. The results presented here will improve the timing of future management strategies through increased knowledge of activity and food intake of D. suzukii.