This is Invasive Species Awareness Week in Canada (February 26, 2024). And because we're all about trees, we're going to focus on the nastiest -- the hemlock woolly adelgid (HWA) -- which is sweeping from west to east across the province decimating our biggest and oldest tree -- the Eastern Hemlock.
There are more invasive species that affect our trees -- like Emerald Ash Borer and the Beech Leaf Mining Weevil -- but we'll get back to them another day.
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Why should I care about hemlock?
Hemlock Ravine Park, HRM (c) toddbealphoto
Hemlock are not as commercially important as other trees like spruce and fir, altho their moisture resistance make them a good alternative to pressure treated wood.
But they have a significant role in the forest -- their dense treetops and branches slow rainwater and prevent erosion.
Those branches also provide shade which cools hemlock lined waterways encouraging fish populations.
They have extensive root networks -- particularly along lakes and rivers -- that stabilize the soil and prevent erosion.
As our oldest and largest trees, there are many species of plants and animals that rely on hemlocks for shelter and a place to build their homes.
They store a HUGE amount of carbon.
And they are found throughout Nova Scotia so your recent forest walk probably was underneath hemlocks. 😀
Where is HWA in Nova Scotia?
HWA in NS, (c) Canadian Food Inspection Agency
In 2017, HWA was first discovered in western Nova Scotia. As shown by the red dots in the map above, it has been steadily marching eastward across the province and we have just had the first confirmed sighting in the Halifax Regional Municipality.
Where the bug has been established the longest, there is extensive mortality of all ages and sizes.
Typically, from first infestation to death is between 5-10 years.
Without treatment, this will happen to most of our hemlocks.
How can your help?
Hemlock Woolly Adelgid on the underside of a hemlock branch
Right now you can help by being on the lookout for the telltale sign of HWA -- a white, cotton-like sac -- typically found on the underside of a hemlock branch where the needles meet the stem.
Until the first week of April is a very good time to spot the sacs. The momma -- who is inside the sac -- is feeding and making babies.
Later in spring and early summer, those babies will crawl out and each one will form a new egg sac. You will find this new, second generation of egg sacs on the new growth tips. This second generation of HWA is one of the reasons HWA is so prolific.
At other times of year, the symptoms of HWA are more easy to detect than tiny HWA. Look for branch tips that are devoid of needles and thin, yellowing canopies.
Early detection means that treatment costs less and should be more successful.
From April until August you can help by taking steps to make sure you do not accidentally spread HWA. During this time, HWA have hatched and the babies are looking for a new home.
The babies are very small -- about the size of a pencil dot -- and there can be a lot of them -- scientists have collected more than 1,000 falling off a a single tree in one day.
So if you travel in forests with hemlocks west of Halifax, you can help by making sure you are not transporting a baby HWA to a new location. Change your clothes or run a link brush over your clothes and hat (pets too!). And it's a good idea not to go directly to another forest with hemlocks.
How do I find HWA?
Look for the telltale white sacs where the needle meets the branch
There are a couple of ways to find the white cotton-like sacs:
1. Grab a branch, flip it over and look for the sacs where the needles meet the stem, typically close to the tips of the branches
2. After a wind storm there will often be hemlock tips lying on the ground. This is an excellent opportunity to check branches that came from higher up on the tree
3. I have had the best success inspecting branches that have fallen down.
4. From March to mid April when the sacs are large, you can shine a very bright light up into the tree just before dark and the sacs will be quite prominent in the light. Watch your step as you're looking up!
What do I do if I find HWA?
Get a pic to help confirm your sighting!
There are several things that look like HWA so it is best to get a picture if possible to help confirm that it is the bug.
And let us know!
1. You can send your pic with location info to hwa@nshemlock.ca
Recent screen capture of HWA in Nova Scotia on iNaturalist
2. Are you an
iNaturalist user? Post your pic and we'll find it!
How do we save a tree with HWA?
Applying pesticides to the tree bark
At this time, the only way to treat a hemlock infested with HWA is by using pesticides -- either injecting it into the stem of the tree or by spraying the bark.
Several groups in Nova Scotia are currently treating trees including:
1. Kejimkujik National Park
2. The Medway Community Forest Coop which includes the Hemlock Heroes
3. The Nova Scotia Department Natural Resources
There are also some commercial pesticide operators and a growing number of private landowners who have learned the steps required to treat their own trees
What is the long-term plan for dealing with HWA?
Treating hemlock with pesticides is time consuming, expensive and the treatments only last 5-7 years before the trees have to be treated again.
Fortunately, there is a natural solution being researched and trialed.
So what is this "natural solution"? In British Columbia there are hemlock trees and HWA that live together. But there is another piece to this puzzle ... bugs that feed on HWA keeping it in check.
In the fall of 2023, Nova Scotia imported one of the bugs from BC -- a beetle nicknamed "Little Larry" -- to evaluate if they will survive and flourish here. The beetles have been used in other locations with encouraging results and we are hopeful they will work here as well.
It will be at least 10 years before this makes a meaningful impact, but government scientists have started trials to evaluate this approach.
Wentzell Lake Park
As we wait for scientists to establish insect predators and build populations large enough to munch through HWA and keep hemlocks healthy like on the west coast, please help our irreplaceable old growth hemlocks by:
1. take steps to prevent the spread
2. identify infestations as early as possible
3. treat trees where possible.
If you'd like to take a more direct role, consider becoming a Hemlock Hero.
And if you'd like to learn more about HWA, checkout www.nshemlock.ca
Many thanks to Donna Crossland and Ron Neville for their assistance preparing this post.
