The impact of co-occurring invasive insects on eastern hemlock health and chemistry

Background/Question/Methods

Throughout most of eastern New England, eastern hemlock (Tsuga canadensis) has been experiencing widespread decline due to the invasion of two invasive insects, the hemlock woolly adelgid (HWA; Adelges tsugae) and the elongate hemlock scale (EHS; Fiorinia externa). HWA causes foliar damage, crown loss and mortality, while EHS usually causes needle discoloration but rarely leads to hemlock mortality. Understanding individual tree responses to the combined impacts of these insects is especially important as these insects may interact in unpredictable ways. Our objective was to investigate how the presence of these two invasive insects affects twig, needle, and fallen litter chemistry and new branch growth across forested sites containing the following insect densities: high HWA, high EHS, HWA and EHS, and no infestation (control). Three hemlock stands of each treatment were selected from a large set of stands examined across southern New England over the last 15 years. Within each site we examined 3 small hemlock trees to conduct foliar and litter analyses, insect density determinations, and branch growth measurements. Based on earlier studies, we hypothesized that insect feeding would lead to reduced branch growth and elevated nitrogen content in twigs and needles compared to uninfested trees. 

Results/Conclusions

Results suggest that new shoot growth was significantly lower on trees with both HWA and EHS (1.5 cm) compared to uninfested trees (> 3cm). The percentages of branches that produced new growth was significantly lower (40-50%) on HWA infested trees compared to EHS infested and uninfested trees (75 and 80%, respectively). Branch growth and percent of branches producing new growth declined with increasing HWA density but not EHS density. Foliar nitrogen content was significantly higher in infested compared to uninfested trees, but did not differ among insect treatments. Twigs that were fed on by HWA alone or in combination with EHS had significantly higher nitrogen content than EHS-infested or uninfested trees. Nitrogen content of forest floor litter did not differ among treatment types but contained higher N content than live, intact needles. Carbon content of twigs, needles, and litter did not differ among insect treatment. An analysis of insect by-products suggests these insects are also contributing nutrients to these ecosystems, as nitrogen content of EHS wax covers was just over 6% while HWA wool was around 2%. These byproducts and pest-induced changes in twig and foliar chemistry will likely affect subsequent ecosystem processes like decomposition, and should be examined further.