Print PageEastern hemlock (Tsuga canadensis) is a slow growing, shade tolerant tree that creates unique microclimates essential for native wildlife. The hemlock woolly adelgid (HWA; Adelges tsugae) is an invasive pest that is rapidly decimating eastern hemlock in large parts of the hemlock’s range. Another invasive pest, the elongate hemlock scale (EHS; Fiorinia externa) often co-occurs in the same stands where the HWA is present. Recent research has suggested that EHS presence can ameliorate the HWA’s negative effect on tree growth.
Resource depletion has often been regarded as the cause of mortality in HWA-infested hemlocks. In this study, we investigated how HWA and EHS affect resource availability by measuring starch concentration after individual, multiple, and sequential attack by both herbivores. We hypothesized that if hemlock mortality is due to resource depletion, HWA feeding would decrease starch while EHS should have a minimal to positive effect on starch levels.
Results/Conclusions
Contrary to our hypothesis, HWA increased starch content after one year of infestation. Since HWA feeding reduces new growth, an increase in starch might be explained by a reallocation of resources from growth to storage. Because HWA feeds on reserves in ray parenchyma cells, the HWA-induced starch accumulation could be the result of host manipulation. In agreement with our hypothesis, EHS did not alter starch concentrations. Trees infested simultaneously with both insects showed intermediate starch concentrations. In the sequential attack (EHS followed by HWA), the starch concentration was similar to control trees. This was likely explained by a reduction in HWA density in the presence of EHS. A lower HWA density likely results in less resource diversion to storage (to support HWA feeding) and more towards tissue growth. Overall, our results suggest that the herbivores differ substantially in their impact on tree physiology and resource allocation, a difference that may affect tree survival.
