Effects of individual and multiple invasive herbivores on nutritional chemistry in eastern hemlock foliage

Background/Question/Methods

Plants are often attacked by multiple enemies in nature. The effect(s) of attacks that are simultaneous and/or sequential in time may not be predictable on the basis of information from single herbivore-plant interactions. The unpredictability of multiple interactions can be even greater in the case of alien species that lack a co-evolutionary history with their shared host. This study investigated how two invasive alien species (hemlock woolly adelgid, Adelges tsugae, and elongate hemlock scale, Fiorinia externa) alter foliage quality (amino acid concentration and composition) when they feed individually and jointly on their shared host, the eastern hemlock (Tsuga canadensis). A long-term study was set-up with initially uninfested hemlock saplings. Controlled herbivory treatments were applied once a year over a period of 3 years. Joint herbivory treatments consisted of simultaneous or sequential infestations by the two herbivores; data from the sequential infestations treatments allowed us to test whether priority effects altered the two herbivores’ effect on their host.

Results/Conclusions

Even though both herbivores belonged to the same feeding guild, they had very different impacts on hemlock physiology. The hemlock woolly adelgid had a dramatic impact on free amino acid concentration and composition. Total amino acid concentration was 3.3 times higher than control trees after one year of individual adelgid infestation and 1.6 times higher after three years of adelgid attack. Conversely, the individual presence of elongate scale had a negligible effect on plant nutritional chemistry. In the simultaneous infestation treatment, the presence of the scale amplified the adelgid’s effect on amino acid concentration. Prior infestation by scale reduced the impact of adelgid in the sequential herbivory treatment, however, suggesting that priority effects may play an important role in the adelgid-scale-hemlock interaction. The changes we observed in hemlock amino acid concentrations suggest that adelgid infestation prompts shifts in resource allocation. Whether this change is caused by the adelgid’s direct manipulation of source-sink gradients or indirectly, perhaps by water stress caused by HWA-induced anatomical changes in the xylem, remains unknown. Understanding how the interaction of invasive species and their interaction(s) change eastern hemlock physiology is crucial in assessing the impact of multiple pests and for the development of appropriate management strategies.