Interactions between invasive herbivores: Adelges tsugae, Fiorinia externa, and their impact on eastern hemlock growth and foliar chemistry

Background/Question/Methods

The rapid population growth rates and high population densities of invasive species can lead to strong competition between invasive herbivores with a variety of resultant effects on native foodwebs. On the eastern coast of the US, eastern hemlock (Tsuga canadensis) forests are threatened by two invasive herbivores: the elongate hemlock scale Fiorinia externa (EHS) and the hemlock woolly adelgid Adelges tsugae (HWA). EHS was introduced into New York City in 1908 and began to rapidly spread northward into southern New England in the mid-1980s. EHS feeds on needle mesophyll and can cause discoloration and premature needle drop; however, mortality is rare and only occurs in already stressed trees. Infestation of HWA has more severe consequences for hemlock: high HWA densities can kill mature hemlocks in as little as four years.

Despite predictions that HWA would largely extirpate hemlocks in southern New England by the late 1990s, healthy hemlock stands still exist in areas where HWA has been present for >15 years. One possible explanation of the slower rate of hemlock mortality in these areas is the range expansion of EHS. Although recent surveys have shown hemlock mortality to be more strongly related to HWA density than to EHS density, the interactions between these two species remain largely unexplored.

Results/Conclusions

We present the results of research examining the interaction(s) between these two herbivores and their native host plant. In spring 2007, we inoculated previously-uninfested hemlock saplings with HWA only, EHS only, both, or neither herbivore species. After four months, we measured the impact of each herbivore on the population density of the other species as well as their individual and combined effects on foliar chemistry. Although EHS densities were lower in the presence of HWA, EHS had no impact on HWA density. HWA-infested foliage (in both the HWA and HWA+EHS treatments) was lower in %N and had a higher C:N ratio than uninfested foliage. In contrast, the EHS and control treatments did not differ in %N and C:N ratio. These findings suggest a competitive advantage of HWA over EHS, a finding that seems at odds with the fact that EHS continues spreading northward into HWA-dominated hemlock forests.