Discussion:
Reconstructing the invasion pathways for non-native pests represents a critically important step for the prevention of additional introductions (Le Roux & Wieczorek, 2009), and can help the implementation of biosecurity programs for particular pest species and their relatives by identifying the most likely entry routes (e.g., Maxwell, Vettraino, Eschen, & Andjic, 2014). Moreover, reconstruction of the invasion history is crucial for effective biological control, as it enables the identification of regions from which to evaluate natural enemies (e.g., Goolsby et al., 2006). For evolutionary and ecological studies of factors that influence the probability of establishment of invasive species and how non-native species adapt to their introduced environments and ecosystems, (Allendorf & Lundquist, 2003; Dlugosch & Parker, 2008; Lavergne & Molofsky, 2007), identifying the number of introductions of an invasive organism is critical (Sakai et al., 2001). Unfortunately, reconstructing the invasion histories of non-native organisms can both computationally demanding and/or biologically untenable due to the combined effects of genetic bottlenecks, hybridization, and rapid evolution (Buhk & Thielsch, 2015; Ficetola, Bonin, & Miaud, 2008; McEvoy, Higgs, Coombs, Karçetin, & Starcevich, 2012; Mesgaran et al., 2016; Prentis, Wilson, Dormontt, Richardson, & Lowe, 2008). Using a combination of Bayesian clustering and approximate Bayesian computation methods, we find that invasive populations of winter moth in Nova Scotia, British Columbia, and the northeastern United States were all introduced separately from Central Europe, and that the invasive population in Oregon was introduced from Western Europe (Figures 3 and 4). Distance analyses of the coefficient of membership assignments from our Bayesian clustering analyses, suggest that the invasive population in Nova Scotia is most closely related to a population of winter moth in Orleans, France, and that the invasive population in British Columbia is most closely related to a population of winter moth in Uggvallen, Sweden (Figure 3), representing potential source localities for both Canadian populations.
The relationships of the American populations were less clear based on distance analyses, however, with the Oregon population being closely related to a large number of populations from the British Isles and northern Fennoscandia, and the northeastern United States population being unrelated to any sampled European population. In a recent study of winter moth in Fennoscandia, we determined that the population there was introduced to the region ~2,000 ya from the British Isles, either via dispersal across the North Sea or by human-mediated dispersal (Andersen et al. In Press). Therefore, without finer-scale genomic analyses (e.g., genotype-by-sequencing), we will unlikely be able to determine which of these two locations (British Isles or northern Fennoscandia) are the source of the Oregon population. The northeastern United States population, in contrast to all other sampled invasive populations, was clearly distinct from other winter moth populations, including the invasive population in Nova Scotia that can be found only a short distance away across the Bay of Fundy. While it is possible that we have yet to sample the source of this invasive population, given our extensive sampling of the native distribution of winter moth, we believe this is unlikely. For example, the only regions that we did not sample from which winter moth has previously been reported are Japan, Taiwan, and the Russian far-east (Troubridge & Fitzpatrick, 1993), and in those locations, records of winter moth are most likely misidentifications of different species that are native to those regions (Nakajima, 1991). In addition, our inclusion of an unsampled “ghost” population in our DiyABC analyses to represent a possible extra-European source was not supported in any tournament (P ≤ 0.01 in all analyses). Lastly, as part of ongoing efforts to study the evolution of the genus Operophtera , we have worked with collaborators in both the Russian far-east and in Japan to collect samples using traps baited with the winter moth sex-pheromone, and based on preliminary DNA-barcoding results all samples that were collected in these regions have been native Operophtera species (NPH unpublished data). Therefore, we believe that the genetic distinctness of the northeastern United States population is likely the result of a strong genetic bottleneck and/or rapid-local adaptation following its introduction from Central Europe.