Objective 1: To identify factors and variables related to the spread of Hemlock Woolly Adelgid using academic literature.
Mean winter temperature
Since the Hemlock Woolly Adelgid has no natural predators in North America, cold temperature is the greatest limiting factor in spreading northward (McAvoy et al., 2017). When temperatures reach between -20℃ and -25℃ for at least several hours, populations are significantly reduced and spread to new trees is slowed; at -30℃ to -35℃ enough Hemlock Woolly Adelgid die off so that spread is halted (Paradis et al., 2007). Furthermore, it is found that they are not able to survive when mean winter temperatures are at -10℃ (Paradis et al., 2007). Therefore winter temperature is an essential constraint and factor when looking at the spread of this invasive species.
Eastern Hemlock density
Since the Hemlock Woolly Adelgid exclusively feeds on Eastern Hemlock in Eastern North America, the potential spread of the insect is limited by the range of Eastern Hemlock trees (Evans and Gregoire, 2006). Furthermore, rates of spread are influenced by the density of Eastern Hemlock, as Hemlock Woolly Adelgid spreads faster in more dense patches (Morin et al., 2009).
Distance from current Hemlock Woolly Adelgid distribution
Hemlock Woolly Adelgid is an aphid-like insect species that is spread through wind, vectors such as birds, deer, and the movement of affected wood. Therefore the most vulnerable trees are those in close proximity to currently affected trees. Past studies have found Hemlock Woolly Adelgid to spread northward at a rate of approximately 12.5 km/yr (Evans and Gregoire, 2006).
Distance from roads and road type
Roadways have been shown to be a significant contributor to the spread of pests by increasing connectivity and exposure to Hemlock Woolly Adelgid’s primary vectors (Koch et al., 2006). Roads along forests create an open space allowing for increased and more rapid spread throughout the forest (Koch et al., 2006). Major roadways can also act as flyways for bird migration causing long-distance spread (Koch et al., 2006). Finally, human activity along roads also contributes to the accidental spread of the pest, as insect travel to unaffected hemlock patches can be accelerated by human movement (Morin et al., 2009).
Table 1: Data sources