An Asian beetle that has been killing ash trees in Ontario and the United States has been found for the first time in Montreal, where it now threatens thousands of trees of that species on the island.

The emerald ash borer (agrilus planipennis fairmaire) was first spotted in Quebec in 2008 and has spread so quickly that Montreal officials believe the island's estimated 45,000 ash trees could be decimated.

City workers spotted the beetle in some trees in the southeast borough of Hochelaga-Maisonneuve, near the intersection of Haig and Notre-Dame streets in early July.

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Infected trees have telltale ridges in their trunks. (Morgan Dunlop/CBC)

The tiny green insect colonizes a tree by laying eggs in the folds and ridges of a tree's bark and branches.

The larvae burrow under the bark and feed off cambium, the tree's vegetal lining.

The exotic beetle carves distinctive trails under the bark that eventually inhibit sap flow.

Deprived of its nourishing sap, trees starve from a lack of nutrients and can die within three years.

If the beetles are allowed to spread unfettered, it could cause significant damage to the ash tree population — about 20 per cent of the trees lining the streets of Montreal.

"If you lost a canopy of trees, you increase pollution, you increase noise," said Jacques Audette, an authority on the emerald ash borer at the Canadian Food Inspection Agency.  The biodiversity of the region would be affected, he added.

"There is no natural enemy here [for the beetles] so they spread quite rapidly," Audette said. "That's why we are concerned."

The CFIA is investigating the infestation.

Beetle likely imported by boat or train

Residents who believe they may have an infected tree are urged to contact the city.

"Citizens can contribute by reporting declining ash trees," said city of Montreal science advisor Anthony Daniel.

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The emerald ash borer is harmless to human health, but deadly for ash trees. (Radio-Canada)

It's likely that the beetle was brought to Montreal through a shipment of foreign goods, Daniel said.

"There is the port, there is a road, there is a train that passes just by here, so there are plenty of ways the infestation can be transported."

The city of Montreal will run tests on infected trees and may destroy some to contain the beetle's spread.

Residents can also contribute to reducing the spread of emerald ash borers by buying local firewood and avoiding imported hardwood.

Infested trees have thin foliage with branches sprouting from the base of the trunk.

The beetle is believed to have killed hundreds of thousands of ash trees in Ontario  and northeastern United States in the last decade.