2017 Field Collections
We visited four locations in North Carolina where large wild blackberry stands (>15 canes/10m) were established: Southern Nantahala Forest in Macon County; Cherokee National Forest in Avery County, and Joyce Kilmer –Slickrock Wilderness Area (JKWA) and along the Cherohala Skyway in Graham County (Supplementary Table S1). Required permits to sample in these places were obtained from the appropriate agencies. Sampling sites were determined by location and density of blackberry plants and separated by a distance of at least 1 km. Fruit collection began when fruit appeared almost full size (subjectively determined by drupelet size) but were still green. Sites were resampled every 2-3 weeks until no ripe fruit were available.
At each site, blackberry plants within a radius of 10 m were sampled for fruit at the following ripeness stages: green, blush (reddish green), red, purple, and ripe. Two research station plantings of cultivated blackberries were sampled during the same week as wild collections, but only ripe fruit were collected at research farms after the first visit due to low fruit set that year. Up to 20 fruit of each stage were sampled at each site as available, grouped in breathable bags, and transported to the lab in a cooler (4ºC). Fruits were collectively weighed by sample group and examined under a dissecting microscope for the number of D. suzukii eggs laid per berry. Drosophila suzukii eggs were distinguished from other potential fruit-infesting flies by observing and counting the number of respiratory filaments per oviposition site (Hauser et al. 2011). Although Drosophila melanogaster and D. simulans eggs also possess only two filaments per egg, we collected fruits before they were susceptible to oviposition by these two species. Other plant species growing adjacent to blackberry plants with ripe fruit that appeared susceptible toD. suzukii were collected at random and similarly checked for infestation. All plant species were identified using Weakley (2006).