Researchers race to protect Nova Scotia's hemlocks from invasive pest

In a shaded, mossy hemlock stand in Kejimkujik National Park, trees span generations — from young hemlocks with spindly trunks to multi-centenarians thicker around than two people can reach.

But these trees have something in common. All are at risk.

Like many of the hemlocks in Nova Scotia, these trees are threatened by an invasive insect known as the hemlock woolly adelgid.

"It's a spectacular tree — it really is the redwood of the East," said park ecologist Matthew Smith. "And I think, still, a lot of people don't realize that these trees are in danger of disappearing."

To counter the fast-moving threat, scientists are investigating options from chemical treatments to changing the composition of the forest to save hemlock stands from destruction. That includes Kejimkujik, where some of the province's largest hemlock stands and oldest trees are found.

Over the last seven decades, the hemlock wooly adelgid, which feeds on nutrient and water storage cells at the base of hemlock needles, has left a swath of destruction across eastern North America.

The HWA was originally imported into Virginia on nursery stock. Since then, it's travelled northward and was detected in five counties in Nova Scotia in 2017, along with a sixth — Lunenburg County — where the presence of the adelgid was confirmed this summer. 

The insect is a threat to hemlocks across Nova Scotia, but the need for a solution is particularly urgent in Kejimkujik given the presence of rare old-growth hemlock forest. Some trees are 300 years old. 

This past year, park officials launched a five-year strategy to address the adelgid in the park, where the pest was detected in 2018. 

"The project that we have in the park has a number of different focuses and one of them is on keeping track of what's happening with the infestation," said Smith. "We don't have a very good handle on how quickly the HWA spreads in Nova Scotia.

The adelgid can kill hemlocks in as little as three years, and can wipe out 95 per cent of the trees in a hemlock stand.

"So we're wanting to watch the trees carefully so we understand how fast it's spreading and how quickly the trees decline," Smith said.

Once the infested trees are identified, researchers are investigating the possibility of countering the adelgid with chemical controls, in the form of insecticides injected directly into the trunk. 

There is one treatment already approved, said Smith, but it isn't an option for widespread use because it is expensive.

"It's not practical for most people, unless you were just doing a single-tree application, and we've got a lot of hemlock. So we're looking at some other options."

Scientists are conducting trials on less expensive insecticides. Another option is silviculture, or cutting hemlocks to allow more sunlight to reach the trees, which some research suggests helps hemlocks resist the adelgid.

Restoring balance

Scientists are also considering solutions with broader application on the landscape.

The adelgid is killing trees in and outside of the park because of an absence of checks and balances that would exist in its native range. 

"Because it's not a native insect, it's not held in check by natural enemies and tree resistance," said Jeff Fidgen, a biologist with the Canadian Forest Service, one of the agencies involved in the work to save hemlocks. 

In an attempt to restore some balance, Fidgen and other scientists are investigating ways to recreate the conditions found in the adelgid's native range, including the presence of insect predators, and are working on a framework for the use of biological controls for eastern hemlocks.

Building up natural resistance through selective breeding of trees, and the implementing the use of natural enemies — such as a beetle species and two species of silver fly — are "definitely our best option for saving hemlocks," said Fidgen. "It's very urgent, but it does take time to vet these natural control agents."

Fortunately for Nova Scotian hemlocks, researchers can draw on work done in the eastern United States, where scientists have been investigating similar approaches for decades. 

"We think that could save us a bit of time, in terms of being able to see some impacts." 

Even with American research helping to expedite the process, it could be at least 15 years before this approach yields measurable results in Nova Scotia.

In the even longer term, scientists are investigating changing the genetic composition of hemlock forests as a protective mechanism. 

Identifying and breeding hemlocks that are genetically resistant to the adelgid, and replanting hemlock stands with trees that have those traits, would help recreate the natural resistance that hemlocks have in the adelgid's native range.

To preserve the hemlocks that are currently found in Nova Scotia, especially in places like Kejimkujik, some combination of all of these approaches will be necessary, Smith said. "There's no silver bullet."

In the meantime, individuals have a role to play.

Not too far from Kejimkujik, in his family's woodlot in Lunenburg County, Tom Rogers said he grieved the potential loss of the hemlocks on his property when he first found out about the threat.

The trees "are almost like friends," he said. "And the idea of this little bug coming in and in very short order and eliminating all of these trees, it just kind of broke my heart, to think they could all be taken away."

But there are ways Nova Scotians can forestall this future, he said, including to report sightings of the insect and respecting CFIA restrictions on the movement of certain wood products out of affected counties, including a firewood importation ban into Kejimkujik. 

"If we can slow this thing down, then we stand a better chance of preserving the trees that we have," Rogers said. "If in some small way I can help, then I'm happy to do that."

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