Invasive species often have detrimental effects on the native communities they invade. In the eastern US, two invasive hemipteran herbivores, the elongate hemlock scale Fiorinia externa (‘EHS’) and the hemlock woolly adelgid Adelges tsugae (‘HWA’) both specialize on a common native host, the eastern hemlock Tsuga canadensis. HWA can kill mature hemlock trees in as little as four years, and is considered a severe threat to hemlock forests in its invaded range. In light of the fact that Tsuga species in HWA's native range appear largely unaffected by this insect, it has been hypothesized that the adelgid causes a hypersensitive response ('phytotoxicity') in eastern hemlocks. If correct, this phytotoxicity may explain why HWA has a more detrimental effect on plant growth and survival than would be expected on the basis of resource depletion alone. In contrast, EHS only marginally reduces hemlock growth and appears capable of killing only severely-stressed hemlock trees. Prior research has shown competition between sap-feeding herbivores increases as plant nutritional quality decreases; we present the results of work assessing whether interspecific competition occurs between HWA and EHS, and the effect of this interaction of host plant foliar chemistry.

Results/Conclusions

In spring 2007, we inoculated previously-uninfested hemlock saplings with HWA only, EHS only, both, or neither herbivore species. In October 2007, March 2008, and October 2008, 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, the presence of EHS did not significantly decrease HWA density. The HWA-only foliage was significantly lower in %N and had a higher C:N ratio than all other treatments. The EHS-only, HWA&EHS, and control treatments did not differ in %N, %C or the C:N ratio. Interestingly, resource depletion (measured as %N) in the HWA-only treatment was significantly higher than in the combined HWA&EHS treatment. Our findings suggest that the presence of multiple herbivore species in this system do not lead to more detrimental impacts than single herbivore impacts.