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The Hemlock Woolly Adelgid

Hemlock Woolly Adelgid

The Hemlock Woolly Adelgid (HWA) is an invasive species of aphid-like insect that attacks hemlock trees. It originated in Asia but in the 1950’s it made its way along the eastern coast of the United States and since into Ontario.  Changes in the northern climate has allowed the hemlock woolly adelgid to spread and survive in the central and eastern Canadian provinces, including Nova Scotia.

The pest can cause significant damage to hemlock trees, and it spreads rapidly due to its life cycle and ability to be carried by wind, animals, or humans. Therefore, it is essential to monitor and prevent the spread of this invasive species to protect hemlock populations.


HWA has a unique life cycle. It is active during the winter months,  reproduces in the early spring and remains dormant during the summer. The adult feeds on the nutrient and water storage cells at the base of the hemlock needles. The eggs are laid at the base of the needles, on the branches, in a white cottony protective mass, usually starting on the lower limbs of the tree. 

Once established, the HWA will rob the tree from its water and nutrients, causing the needles to fade in color and eventually fall off the limbs. The loss of needles will begin at the base and work it’s way to the top. Death of the tree will occur if the tree is left untreated. 

The Hemlock Woolly Adelgid
Mass of Hemlock Woolly Adelgid eggs

Treatment of Hemlock Woolly Adelgid

If left untreated, your hemlock trees will die within 4 – 15 years after infection, depending on the health of the tree prior to infection. 

There are currently only two possible treatments of HWA. 

Biological Treatments – This involves releasing insects into the environment that is a natural predator of HWA. This is this only long-term solution for combatting HWA in the area, but is not being used currently, as the release of additional insects into the ecosystem can have negative impacts on the native species. Extensive research is currently underway for different biological control options. 

Chemical Control – This involves using systemic insecticides. In Canada, only one option is available. This is done by directly injecting the tree with imidacloprid into the tree’s trunk. It then spreads through the tree’s vascular system (like a circulatory system) and kills the HWA as it feeds on the tree’s nutrients and water. According to NS Hemlock Initiative, under this treatment, hemlock trees can stay protected for up to 5 years. 

HWA can be treated by our certified pesticide professionals, through systemic insecticide injections with our Arborjet product.  We drill holes in the trunk and put a chemical insecticide right into the tree and it moves up the tree with the flow of water.  The  window of application is in September – December and March – June, when the  tree’s uptake of water is at its greatest. The insecticide will spread throughout the tree with the water, and kill HWA as it feeds. 

HWA can start dying off within 14 to 28 days up to 2 years. There may still be the presence of the cottony masses, but these will eventually turn gray and breakdown. 

 Additional nutrient injections of the tree is recommended after receiving the insecticide injections as the nutrient help the tree recover from damages caused by HWA, increase the insecticide and water uptake.  This can be done with additional tree injections with Arborjet.

Over time, you will see the Hemlock twig start to recover by  regenerate new, green needles

To learn more about our ArborJet system click here.

How to Reduce the Spread of HWA

HWA can spread rapidly, if the optimal conditions are present. IT has the ability to be carried by wind, animals and humans. Therefore it is essential to monitor and prevent the spread of this invasive species to protect hemlock populations throughout areas of Nova Scotia. 

The Invasive Species Center of Canada recommends:

  • Avoid placing bird feeders near hemlock trees  – birds can transport HWA
  • Keep firewood/cut onsite – avoid moving to other locations like campsites
  • Proper removal and disposal of infected tree
  •  Treatment of infected trees
  • Report sightings of HWA to the Invasive Species Center of Canada