It may look pretty, but the emerald ash borer is a killer. These insects kill ash trees by gobbling up their nutrients. First, as larvae, when the bugs hatch and burrow between the inner bark and the sapwood, forming S-shaped lines, and then, as adults, when they eat ash tree leaves.
The beetle, which is native to Eastern Asia, has already killed hundreds of millions of trees across North America. Late spring is when the emerald ash borer (EAB) emerges from trees and lays eggs.
“This insect will spread wherever there are ash trees, and it’s really threatening,” says Matt Mayr, the district manager of the Davey Tree office in northwest Toronto.
This year could be especially damaging for ash trees, as Canada is expected to see a warm summer with dry conditions, which makes trees more vulnerable to wood-boring insect damage.
Here’s what you need to know about the invasive species, and how to protect your ash trees.
Where is it active?
The emerald ash borer was first discovered in North America near Detroit, Mich., and Windsor, Ont., in 2002. Over the years, Mayr has watched as the invasive species spread throughout Ontario—and beyond.
“Several years ago, it was in the GTA. I spent two years of my career doing nothing but removing ash trees in Toronto,” says Mayr. “Then around 2019, it was detected up in Muskoka area, then it moved all the way up to Sault Ste. Marie and Thunder Bay.”
“It’s moving east, to Quebec and New Brunswick,” he says. “And then last year, it was even detected in Vancouver.” Indeed, in May of 2024, larvae of the EAB were found in Vancouver by the Canadian Food Inspection Agency, the first official sighting in British Columbia.
The emerald ash borer is now present and regulated all over the southern and middle areas of Ontario, Quebec, and New Brunswick, plus in the cities of Thunder Bay, Winnipeg, Halifax, Vancouver, and Burnaby, according to the Government of Canada.
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Which trees are at risk?
Five species of native ash have been attacked by ash borers in Canada, including green ash, white ash, black ash, pumpkin ash, and blue ash (though blue ash appears to be less susceptible to damage).
According to the federal government, ash trees have been extensively planted in municipalities throughout Canada, and “all are at risk.”
What to look out for
“In my experience, you’re going to start to notice the canopy dieback of the tree, which typically will start in the top third,” says Mayr. Look out for yellow, thin, or wilted foliage, or woodpecker activity, as birds go after insects inside.
“If you’re very observant, you might notice D-shaped holes in the bark,” says Mayr. “That’s from emerging adults when they come through the bark.” Sucker shoots or water sprouts growing out of the tree trunk are other indications of a tree under stress.
As soon as you notice any signs or symptoms, Mayr says it’s important to act quickly and get your trees assessed by a certified arborist before it’s too late. “If you get in there relatively early, there are systemic treatments and other holistic treatments that we can do.”
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How to prevent damage
The beetle is spread primarily through the transportation of firewood, so buy and burn your firewood locally. Transporting firewood across Canada can help spread the invasive species.
Adult EAB are most active during the mid-to-late summer and fall, so signs of infestation will be apparent during this period. Inspect your trees and act quickly. The longer you wait, says Mayr, the more difficult and dangerous it will be to remove. And, with early intervention, ash trees could be treated and saved.
“Within two or three years, the tree could be beyond saving,” says Mayr. “You’ve got to get in there early if you want to save it.”
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