Increased populations of EAB in its native range enhanced invasion risk
Propagule pressure is perhaps the most important factor influencing the establishment of invasive species and increases with the size of species pools and higher pest populations from source regions (Brockerhoffet al . 2014). Exotic tree species have been planted widely throughout the world as ornamental trees, for reforestation, to increase timber production, and in response to climate change; however, they may also facilitate biological invasion by associated pests (Ennos et al . 2019). With more extensive plantings of non-Asian ash trees in northern China since the 1970s, EAB outbreaks have become increasingly severe since the 1990s, thus elevating the risk of accidental introduction of EAB to other non-native ranges. For similar reasons, the widespread planting of exotic poplar trees since the 1950s in China might have also contributed to increases in propagule pressure of the Asian longhorned beetle (Anoplophora glabripennis (Motschulsky)) for accidental introduction to North America and Europe (Zhao et al . 2007; Hu et al . 2009).
Humans are important facilitators of forest insect pest spread, increasing both the frequency and severity of infestations through expanding global trade and domestic commerce (Koch & Smith 2010). The abundance of native woodboring beetles captured in traps near exporting ports was associated with nearby surrounding forests while non-native species captured within ports was associated with the volume of imports indicating that propagule pressure at exporting ports can serve as a source of species that can be potentially moved with exports (Rassatiet al . 2018). The spread of EAB is characterized by stratified dispersal which includes both short-distance natural spread and long-distance human-assisted movement (Siegert et al . 2015). Most EAB adults disperse less than 100 m per flight when their host trees are close together (Barlow et al . 2014) but are capable of flying up to 7.2 km over a 4-day period in flight mills (Taylor et al . 2010). Estimated radial spread rates of infestations in North America have reached at least 13 km/year (Siegert et al . 2015). Long-distance dispersal of up to hundreds of kilometers usually result from human-assisted movement of host material including nursery stock, unprocessed logs, firewood, and branches trimmed from infested trees (Siegert et al . 2015). In North America, EAB has spread rapidly due to transportation of infested ash firewood and the abundance of susceptible host trees including green, white and velvet ash that are widely planted in south-central North America (MacFarlane & Meyer 2005; Herms & McCullough 2014). On the other hand, few susceptible exotic non-Asian ash trees have been introduced and planted in the central and southern regions of China and human activities such as transportation of ash firewood are not common. Consequently, differences in spread and distribution of EAB in North America and China may be related to the distributions of susceptible host plants and human activities.
In conclusion, our study demonstrated that widespread planting of non-Asian ash trees in northern China was significantly correlated to increasing outbreaks of EAB, ultimately resulting in accidental introduction of this invasive species to North America via commercial trade. The distribution, occurrence, and frequency of EAB outbreaks from 1960 to 2019 in China also suggest that native insect pests must reach a threshold population density before outbreaks occur on non-native host trees. Further, EAB infestation level was positively correlated with northern latitudes where more non-Asian ash trees were planted. Increased populations of pests in their native range along with increasing international trade may elevate the risk of invasion to non-occurrence regions. Historical analyses of the spatiotemporal dynamics and outbreaks of native insect pests can provide useful information on effective management in both native and invaded regions.