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TORONTO — When Emma Hudgins was born, her parents planted an ash tree. 

It's still standing 29 years later in New Maryland, N.B., but it's under threat from the emerald ash borer, a shiny green beetle that kills almost every ash tree it encounters. 

The insect was first detected near Windsor, Ont., in 2002, and has since spread to parts of Manitoba, Quebec, Nova Scotia and New Brunswick. 

"It's only a few kilometres away," said Hudgins, a post-doctoral fellow at Carleton University who studies invasive forest insects. "But it hasn't gotten to the tree yet." 

In the 20 years since the emerald ash borer was first found in Canada, the centimetre-long insect has wiped out hundreds of thousands of ash trees, many of them in cities, where trees are heavy lifters in climate change adaptation, offering shade and lessening what's known as the "urban heat island" effect. 

"It's going to be one of the major factors influencing vulnerability of Canadian communities to climate change moving forward, because it's just this huge force of destruction in urban areas," Hudgins said.

The ash tree, beautiful and hardy, was long a favourite of urban planners, she said. It could stand up to the stressors of the city: road salt, soil compaction, construction and vandalism. 

But Hudgins said overreliance on the ash has left the urban canopy particularly vulnerable to the emerald ash borer, whose larvae feed on the inner bark of the tree and cut off its circulation, preventing it from delivering nutrients up its trunk. 

Within eight to 10 years of its introduction to a region, Natural Resources Canada says, the emerald ash borer kills as many as 99 per cent of unprotected ash trees. 

That's a problem, Hudgins said.

"Climate change adaptation is really where urban trees have an important role in terms of limiting the number of people who are going to be dying of extreme heat events, mitigating floods, those types of outcomes," she said.

Moving forward, she said, cities should safeguard urban forests against similar threats.

"The gravity of the problem is partly because of this reliance on ash trees as an urban tree and people not thinking about biodiversity as being an important component of urban planning," Hudgins said.

Cities tend to be hotter than their rural counterparts, as urban building materials such as asphalt and concrete retain more heat and reflect less radiation than vegetation, the National Research Council Canada's Construction Research Centre found. 

But trees can help reduce the heat, in part by offering shade and preventing solar radiation from reaching the ground, and also by moving water from the earth into the atmosphere, according to their research, which was published in the journal "Buildings" in May.

For that reason, cities have committed to bolstering their urban canopies. 

Montreal currently has 23 per cent tree cover, and has pledged to increase that to 25 per cent by 2025. Vancouver's tree cover also sits at roughly 23 per cent, and it plans to increase that to 30 per cent by 2030.

In Toronto, meanwhile, there's somewhere between 28 per cent and 31 per cent tree cover, and the city wants to increase that to 40 per cent by 2050.

Progress, however, has been set back by the emerald ash borer.

Before the pest was found in Toronto in 2007, the city estimates it was home to an estimated 860,000 ash trees on both public and private land. Of those, 32,000 were "street trees," located alongside roads. Others were in parks, on private property and in urban forests. 

Now, there are just 6,000 ash trees remaining on publicly owned land, all of which are treated regularly with insecticide, said Jozef Ric, supervisor of forest health care for the city. 

It's not clear how many ash trees have survived on private property, but Ric said the number is likely quite low, considering the mortality rate of infected trees and the insect's ability to spread. 

When the bug initially appeared, Ric said, city arborists sought to control the issue. 

"We had identified 150 trees, roughly, and we thought maybe we could try to contain them," he said. But soon after, they learned that the insect had already spread to all corners of the city. 

"It became clear that we wouldn't be able to eradicate that pest from our environment," he said. 

They turned to treatment, Ric said. Now, all surviving ash trees are injected with an insecticide every other year — a costly and labour-intensive process. 

Hudgins said the most effective way to prevent the spread of the emerald ash borer is by controlling the movement of ash lumber, wood chips or nursery stock out of areas where the bug has already been found — something the Canadian Food Inspection Agency has done for years.

Some communities have also brought in a millimetre-long invasive parasitoid wasp, which lays its eggs inside the emerald ash borers' larvae, eventually killing them — Ridley Scott's "Alien" on an entomological scale.

"The goal is not necessarily to save ash trees locally but allow for a population of those parasitoids to grow and then spread to neighbouring communities and potentially save the ash trees there," she said. 

There are drawbacks to this system, however. Introducing any invasive species is a risk, as it could have unforeseen side-effects, Hudgins noted. 

She said she has spoken with different groups, including Indigenous knowledge holders, who have raised concerns that the practice could impact other species. Extensive research has so far shown no such consequences, she added.

"But it's hard to do an exhaustive test on all of the impacts, and there's still really big open questions about how effective this is actually going to be."

Given the gravity of the situation, she said, many have deemed it worthwhile.

"We're facing the extirpation of entire species of trees, so the tools that we would use would probably be more extreme than what we would do for a less impactful pest."

This report by The Canadian Press was first published Nov. 24, 2022.

Nicole Thompson, The Canadian Press