Colony size and initial establishment of hemlock woolly adelgid (Adelges tsugae)

A central and ongoing question in invasion ecology is why establishment success in a new environment varies so markedly among and within species. In many low-density populations of invading species, establishment success can be affected by stochasticity and Allee effects, which can arise due to the inability to find suitable mates, saturate natural enemies, and overcome host plant defenses in sparse populations. The hemlock woolly adelgid, Adelges tsugae, is a non-native species that attacks and kills eastern North American species of hemlock; however, there is little information regarding its invasion success at low densities. Because A. tsugae is parthenogenic and natural enemies are rare or absent in expanding populations, many of the documented causes of an Allee effect in insect populations are not relevant to its invasion dynamics. To study the initial, low-density dynamics of A. tsugae, we inoculated hemlock trees with varying densities of A. tsugae over two years and observed a positive relationship between density and colonization success. In some cases with an initial density of one ovisac per tree, we observed successful establishment, development, and the initiation of a subsequent generation. Understanding the drivers of initial establishment success could be useful in developing more effective management strategies against this non-native pest.