Technology & Science

Hemlock trees under threat by invasive pest, the woolly adelgid

The woolly adelgid, an invasive pest that infests and eventually destroys hemlock trees, has made one incursion into Eastern Canada and biologists say it's likely to turn up again.

Forests of Eastern Canada are susceptible as pest is already eating through trees in neighbouring U.S. states

The hemlock woolly adelgid appears as white, woolly sacks on the base of needles of the hemlock. (CFIA)

The woolly adelgid, an invasive pest that infests and eventually destroys hemlock trees, has made one incursion into Eastern Canada and biologists say it's likely to turn up again.

Like the Asian long-horned beetle and the emerald ash-borer, it's a foreign invader that's threatening forests in Eastern Canada and the urban tree canopy.

The first sign of hemlock woolly adelgid is often a white woolly mass at the base of needles of a hemlock tree, says Erin Bullas-Appleton, a plant survey biologist with the Canadian Food Inspection Agency.

The tiny insect, less than 1.5-mm long, produces a wool-like wax to protect its eggs, which are usually deposited on the underside of hemlock needles at the tips of the branches.

Nymphs eat the trees

The nymphs that hatch feed on the new growth of the hemlock tree and over three or more years, and an infestation can kill the tree. After a year or so, the hemlock may have brown sections as it comes under stress.

"If people see those white woolly masses at the base of needles, they should act quickly," Bullas-Appleton said. "They should report it to the CFIA at"

Right now, the first line of defence to control the woolly adelgid is to take out affected hemlocks, Bullas-Appleton said.

That's what was done in 2013, when two infestations were found in Ontario on the Niagara Peninsula and in Toronto. After the trees were cut and removed, the infestation was controlled.
The hemlock woolly adelgid has been in North America for 95 years, but the variant now affecting eastern hemlocks seems to be more damaging. (CFIA)

But Canada has since created a technical advisory committee to seek out alternative strategies to control the pest, because of the potentially devastating effect it could have on forests.

Eastern Canadian forest threatened

The eastern hemlock grows from Ontario east to the Maritimes and throughout the Canadian shield, often on the shores of lakes and rivers.

"It has an important impact as a regulator of water temperatures," Bullas-Appleton says, as it shades streams, preserving wildlife habitat.

It is also widely planted as an ornamental tree, which is why urban forests may be even more at risk.

"Urban forests are most at risk because they are at close proximity other trees and to people. People move pests inadvertently," she said.

For expertise, Canada's working group is turning to forestry services and biologists in the eastern U.S., where the hemlock woolly adelgid has spread from Georgia to Massachusetts and also been found in the St. Lawrence region of New York state.

The woolly adelgid is native to Japan and first was reported in British Columbia in 1919, spreading throughout the west coast. In Western Canada, the trees are resistant to the pest.

However, the variant of the insect that first turned up in Virginia in the 1950s has proven more damaging to eastern hemlocks and is now affecting over half the range of the trees in the U.S.

Spread by animals, people

The insect is spread by birds and small animals, as well as by people moving wood products.

The CFIA already has rules demanding that hemlock seedlings or wood products, including wood bark, being imported to Canada must have a certificate showing they are free of the pest.

But that's not going to be enough to stop the insect from moving north. It's very adaptable, to cold winters as well as warm ones, Bullas-Appleton said.

She said members of Canada's technical advisory committee are studying strategies used by U.S. parks and forestry services, including chemical control and biological control, such as insects that evolved with adelgid in Asia and prey on it. 

"We need a management strategy and it has to be adaptable to our environment," she said.

At the same time, biologists and park staff are monitoring for the pest.