Borered to death: EAB invasion likely in all northeastern counties by 2030

Paper Citation

Ward, Samuel F., Songlin Fei, and Andrew M. Liebhold. 2020. “Temporal Dynamics and Drivers of Landscape-Level Spread by Emerald Ash Borer.” Journal of Applied Ecology 57 (6): 1020–30. 

Summary by Emma van der Heide, edited by Nikki Read and Bethany Bradley

Summary 

Invasive insects often disperse both under their own power and via human-assisted, long-distance jumps that elevate rates of range expansion. Predictive models of range expansion are useful in setting up barriers, containment zones, and management plans, but accurately predicting spread in species that are capable of long-distance jumps is challenging. Large datasets on prior range expansion can therefore be useful to retroactively ascertain the factors that promote invasion spread and make predictions about how the species will continue to spread in the future. 

In this paper, the authors used 20 years’ worth of Emerald Ash Borer (Agrilus planipennis, EAB) data to model the rate of range expansion and the frequency and length of long-distance jumps, which are typically due to human activity such as moving infested firewood. They produced a county-level forecast of invasion risk across the contiguous United States, incorporating information about human population density, climate, and proximity to invaded areas.

Not surprisingly, Ward et al. (2020) found that uninvaded counties are most likely to become invaded when they have high human population densities and are close to areas that have already been invaded by EAB. Most new infestations were recorded within ~50 km of previous infestations, but longer-distance dispersal events of >100 km were not uncommon, occurring about 30% of the time.  There was also little relationship between invasion risk and climate, suggesting that dispersal limitation rather than climate is the main limiter of EAB invasion. In the Northeast, most counties have already been invaded and spread is expected to be slower in the coming years as EAB invasion reaches a saturation point.

Take Home Points 

  • Emerald Ash Borer is more likely to invade counties that are close to previously invaded areas, have high human population density, and high densities of both ash and non-ash trees. 

  • In the Northeast, almost all counties have a 90% probability of invasion by EAB before 2030.

Management Implications

  • Most high-risk counties in the eastern United States have already been invaded by Emerald Ash Borer, meaning that the Northeast is likely to be a source of new invasions in other parts of the U.S. and Canada. EAB outreach and education in the Northeast should therefore focus on curbing activities, such as firewood transport, that are likely to result in long-distance jumps. 

  • Many invasive bark- and wood-boring insects spread cryptically via movement of infested firewood or packing material. Because the mechanisms of invasion in these species are similar to those of EAB, managers of future invasions should expect similar risk factors to be associated with spread.

Related Papers

Some EAB work has focused on the impact of climate change on biocontrol agents. Find summaries of this work here and here.

Keywords 

Range expansion, Emerald Ash Borer (Agrilus planipennis), Human-mediated dispersal

Range ExpansionNortheast RISCCrange expansion, Emerald Ash Borer, human-mediated dispersal