Introduction:
Hybridization is a driver of speciation and evolutionary trajectories across the tree of life (Allendorf, Leary, Spruell, & Wenburg, 2001; Costedoat, Pech, Chappaz, & Gilles, 2007; Harrison & Larson, 2014; Mallet, 2005). While numerous pre- and post-zygotic barriers exist in most natural ecosystems to reduce genetic exchanges between species, human-mediated disturbance and climate change have led to increased hybridization rates across a diversity of taxonomic groups (Gomez, Gonzalez-Megias, Lorite, Abdelaziz, & Perfectti, 2015; Hegarty, 2012; Larson, Tinghitella, & Taylor, 2019). The accidental introduction of non-native organisms to novel habitats has further increased these rates by uniting previously disjunct species or genetically distinct populations (Chown et al., 2015; Havill et al., 2012; Havill et al., 2021; Michaelides, While, Bell, & Uller, 2013). In addition to illuminating factors that may be important in invasion ecology, studying the real-time formation of hybrid zones between native and non-native species may provide a type of natural laboratory, providing important insights into how other well-established hybrid zones may have formed and settled over evolutionary timescales. As such, recent work has highlighted the importance of studying newly-formed hybrid zones for understanding speciation and the preservation of species boundaries (Johannesson, Le Moan, Perini, & Andre, 2020; Larson et al., 2019).
These natural laboratories are particularly important because most documented hybrid zones have likely existed for thousands of years and formed following the movement of species in response to long-term processes such as changing climates during the Quaternary climatic oscillations (e.g., Ryan et al., 2018; Ryan et al., 2017; Scriber, 2011; Taylor, Larson, & Harrison, 2015). Natural hybrid zones frequently have a clinal structure, with a narrow, linear geographic zone of admixture where phenotypic and genetic states change across a gradient between parent species (Barton & Hewitt, 1985; Endler, 1977). In contrast, most documented hybrid zones created in contemporary settings between introduced and native species have a mosaic structure (see Harrison & Rand, 1989), with zones of genetic exchange spread across the landscape in a patchy and non-linear fashion (e.g. Cordeiro et al., 2020; Havill et al., 2012). Therefore, additional examples of newly formed clinal hybrid zones are needed to better understand the evolutionary and ecological processes that shape these temporally and spatially dynamic regions of secondary contact.
Species of moths and butterflies (Insecta: Lepidoptera) have provided some of the most stunning examples of the diversity of interactions resulting from hybridization (e.g., Ipekdal, Burban, Saune, Battisti, & Kerdelhue, 2020; Lucek, Butlin, & Patsiou, 2020; Ryan et al., 2018; Ryan et al., 2017; Scriber, 2011). Here we explore the formation of a hybrid zone in northeastern North America between the introduced European winter moth, Operophtera brumata L. (Lepidoptera: Geometridae) and the native Bruce spanworm, O. bruceata Hulst. Winter moth is native to western Eurasia and North Africa (Ferguson, 1978) and originally became established in North America in Nova Scotia in the 1930s, where it was identified as a major pest in apple orchards and oak-dominated hardwood forests (Embree, 1966, 1967). Subsequently, populations were identified in Oregon as a pest in hazelnut (filbert) orchards in the 1950s (Kimberling, Miller, & Penrose, 1986), British Columbia as a pest in apple orchards and of urban trees in the 1970s (Gillespie, Wratten, Cruickshank, Wiseman, & Gibbs, 1978), and most recently in the northeastern United States (hereafter, the “Northeast”) as a pest of blueberries, cranberries, and many native deciduous trees in the 1990’s (Elkinton et al., 2010; Elkinton, Liebhold, Boettner, & Sremac, 2014). Each of these regions were likely the result of independent invasions from Europe (Andersen, Havill, Caccone, & Elkinton, 2021), and while successful biological control programs have reduced the abundance and economic impacts of this important pest in each invaded region (Elkinton, Boettner, Liebhold, & Gwiazdowski, 2015; Elkinton, Boettner, & Broadley, 2021; Kimberling et al., 1986; Roland & Embree, 1995), populations of winter moth continue to persist at low densities in each location. Previous work in this system has shown that winter moth and Bruce spanworm hybridize readily in the field (Andersen et al., 2019; Elkinton et al., 2010, 2014; Havill et al., 2017). Additionally, in the Northeast it has been documented that the proportions of individuals of winter moth versus Bruce spanworm can be modeled using logistic regression, with populations proximate to Boston, Massachusetts being nearly 100% winter moth, and populations in western Massachusetts being nearly 100% Bruce spanworm (Elkinton et al., 2014). This gradient in winter moth and Bruce spanworm population densities in the Northeast therefore raises the possibility that a clinal hybrid zone may exist in this region, making it one of the first documented cases of this type of hybrid zone between an introduced and a native species.
We explored the spatial and temporal dynamics of the hybrid zone between the invasive winter moth and native Bruce spanworm by collecting moths with pheromone traps along two transects that crossed the leading edge of winter moth spread in the Northeast region. One of these transects was located along Route 2 in Massachusetts (hereafter the “Massachusetts transect”) and was sampled over a 12-year period (from 2007 to 2018) where a gradient across decreasing extreme cold winter temperatures has been hypothesized to limit the distribution of winter moth (Elkinton, Lance, Boettner, Khrimian, & Leva, 2011). The second transect is located along the coast of southern Connecticut following Route 1 (hereafter the “Connecticut transect”) and was sampled over a 3-year period (2016-2018). This second transect was added so that we could compare the role of winter temperatures on the geographic location of the hybrid zone as temperatures along the Connecticut transect are milder than at any point along the Massachusetts transect, and geographic settling would therefore be independent of low winter temperatures. With these data, we explore: 1) the structure and movement of the hybrid zone in the Northeast, 2) changes in the rate of hybridization over time, and 3) the impacts of environmental gradients and population densities in the regulation of the hybrid zone.