Toronto's "urban canopy" is under threat, with as many as 860,000 of the city's ash trees projected to die within the next four years unless they're treated this summer against the scourge of a green beetle from Asia.

Volunteer and freelance writer Jane Clark has devoted her summer to protecting Toronto's ash trees, which she said account for about 8.3 per cent — or roughly one in every 12 — of the city's total tree population.

Clark, who often writes about tree conservation issues, said she's aware of citywide plans to treat as many of the ash trees as possible this summer before an infestation of emerald ash borer beetles wipes them out.

That municipal effort can only go so far, however.

"One of the big issues is that about 60 per cent of those trees are on private property," she told CBC Radio's Matt Galloway Tuesday on Metro Morning.

"The city can't afford to treat anything other than its own trees, and it falls to home owners to choose to treat their trees or lose them."

TreeAzin to protect against beetles

Clark describes the urgency of the matter as a "now or never" scenario, as the city has no further plans to treat its ash trees beyond this summer due to the seriousness of the epidemic. She's been showing up at community events to inform the public of the problem and also stopping door-to-door at homes, urging Torontonians to educate themselves on whether they have ash trees on their property.

If they decide they do have ash trees and would like to protect theirs or their neighbours' trees, they can purchase a biological insecticide called TreeAzin, which can be injected directly into the trunks. The insecticide disrupts the beetles' lifecycle.

A tree with a roughly one-metre circumference could cost about $175 to inject with TreeAzin. The treatment would have to be applied again in another two years. 

"Number one is find out if you have an ash tree or even if your neighbours have an ash tree, and then you have to decide whether to treat or have it removed, that's the now or never part," she said.

Aside from the environmental significance of losing the trees, Clark noted that the economic losses of having to replace them should get some people thinking about preservation.