Hemlock woolly adelgid alters fine root bacterial abundance and mycorrhizal associations in eastern hemlock

While the impact of aboveground herbivores on plant biomass and fitness has received considerable attention, there has been far less research on the corresponding belowground impacts. The belowground effects of aboveground feeding may be particularly noticeable for invasive and/or outbreaking herbivore species that reach high densities and can cause major damage and sometimes death. The hemlock woolly adelgid, Adelges tsugae, is an invasive pest on the eastern seaboard of the United States that feeds on a native shade-tolerant conifer, the eastern hemlock Tsuga canadensis. Trees rapidly decline and die following infestation, and the invasion of this insect has devastated hemlock populations from Georgia in the south to Maine in the north. Despite their substantial impact on tree health, we are unaware of any research into the adelgid's effect on hemlock roots and the surrounding rhizosphere. We report the results of research assessing ectomycorrhizal root colonization, rhizosphere bacterial abundance, and root C:N ratios of infested and uninfested T. canadensis. We found that adelgid infestation decreased the percentage of root material colonized by ectomycorrhizal fungi by more than 67%. Rhizosphere bacterial abundance on fine roots was 25% lower on adelgid-infested versus uninfested trees, and roots of adelgid-infested trees contained significantly less carbon. Our results demonstrate that aboveground adelgid infestation can affect hemlock root composition and alter belowground interactions with ectomycorrhizal fungi and bacteria. This information demonstrates that above-belowground linkages can transmit the impact of herbivory far from the site of localized damage.