Invasive Species

The Hemlock Woolly Adelgid (Adelges tsugae)

The hemlock woolly adelgid (HWA) is a destructive, introduced pest of forest and ornamental hemlock trees (Tsuga) in the eastern United States.  The HWA is native to Japan where it is a common, but innocuous inhabitant of forest and ornamental hemlock and spruce.  Host resistance and several arthropod predators are believed to help regulate HWA populations in Asia.  The HWA was first observed in western North America in the early 1920’s in British Columbia and eastern North America some 30 years later in Richmond, Virginia.  In 1931, the Eastern Hemlock (Tsuga canadensis) was adopted as state tree of Pennsylvania.  This research is an inter-disciplinary effort that combines the expertise of biologists, entomologists, and chemists to save the Eastern Hemlock from elimination.  Collaborators have included researchers at the University of Tennessee, Virginia Tech, the National Park Service (NPS), the U.S. Forest Service (USFS; Hamden, CT and Morgantown, WV), Longwood Gardens (Kennett Square, PA), and the Morris Arboretum of the University of Pennsylvania (Philadelphia, PA).  In this research, we have sought to explore two questions to reduce HWA populations on Eastern Hemlock; 1) is there a chemical communication between the HWA and its host hemlock worldwide? and 2) what is the efficacy of current insecticidal treatments against the HWA?

In order to understand the chemical communication between the HWA and hemlock species we have developed a headspace solid-phase microextraction/gas chromatography/mass spectrometry (SPME/GC/MS) technique.  We analyzed the volatile chemicals in the needles of seven of the world’s nine species of hemlock.  Principal component analysis (PCA) was applied to the volatile chemical signatures for each hemlock species to elucidate similarities and differences as they relate to the resistance/susceptibility of the hemlock hosts.  The statistics tell us that chemically the Eastern Hemlock is very different from other hemlock species and this chemical fingerprint may serve as an attractant to HWA or be unable to deter HWA colonization.  Further research examined how the chemicals varied as the hemlock foliage matured and this variation was found to be statistically linked to the settling and feeding preference of the HWA.  This may imply that the lifecycle of the HWA has adopted itself to the life cycle of the host tree.  Lastly, we have evaluated Eastern Hemlock cultivars for the presence of chemicals associated with host resistance and susceptibility.

We have also investigated the use of imidacloprid (a chloronicotinic insecticide) against the HWA.  In collaboration with the USFS, NPS, and others we have developed a liquid chromatography tandem mass spectrometry (LC/MS/MS) method quantify imidacloprid and imidacloprid metabolites in treated hemlock trees.  In conjunction with forestry bioassays, we have statistically analyzed the insecticide levels in hemlock needles to quantify the parent imidacloprid molecule and major metabolites that are also efficacious and persists longer in the hemlock tree.  Thus, while the parent insecticide is effective, greater efficacy may be obtained from other imidacloprid-metabolites that have great bearing on control, re-treatment, and non-target impacts of the insecticide.  Recently, imidacloprid has received worldwide attention in the media due to purported links to colony collapse disorder (CCD) in honeybee colonies and its use has been banned in Europe.  In one aspect, we are working to preserve North American hemlock trees and also working to understand how this may impact non-target effects on native pollinators.

Funding Sources:

  • National Park Service, Cooperative Ecosystems Study Unit (through the University of Tennessee) “Hemlock Woolly Adelgid Insecticide Treatment Assessment in Great Smoky Mountains National Park” , Funding Period: 10/2010-10/2013, Amount: $189,000.
  • U.S. Department of Agriculture, U.S. Forest Service “Evaluation of imidacloprid and metabolites in hemlock treated with imidacloprid”, Funding Period: 7/2004-6/2010, Amount: $110,144.
  • U.S. Department of Agriculture, U.S. Forest Service “Measurement of phytochemicals in Tsuga cultivars and an evaluation of their role in host resistance to the hemlock woolly adelgid (Adelges tsugae)”, Funding Period: 7/2005-9/2007), Amount: $75,000.