An invasive moth species is on the rise in Canada and, if you've planted a certain shrub, it could stand to ruin your garden.

The box tree moth, native to Asia and designated as an invasive species here in Canada, was first reported in the Greater Toronto Area in 2018. If left untreated, the moth's larvae destroy the only plant it is found on in Canada -- the boxwood shrub.

“Basically, they eat all the green material of the leaf and the tree starts to turn brown," Dr. Sandy Smith, professor in forest health at the University of Toronto, told CTV News Toronto. "It looks very raggedy."

 A second generation is hatched towards the end of summer, and the cycle repeats itself.

The shrubs, also non-native to Canada, are incredibly common in gardens across the country, Smith explained, largely because of their hardiness -- they stay green all year round -- and the ease with which they can be shaped into topiary.

Since the moth’s arrival in 2018, however, Ontario has declared an infestation. Quebec, Nova Scotia and New Brunswick, along with some U.S. states bordering Canada, have also reported a presence, prompting a warning from the Canadian government last week.

“This is very much an ornamental pest,” she said. “It’s not an issue of our natural landscapes.”

Box tree moth

With the rise of the box tree moth, some experts and gardeners have called for an end to the use of boxwood shrubs in gardens, but the pervasive shrub has been vital to the landscaping industry -- according to data provided to the federal government in 2020, the plants represent an estimated annual value of $40 million to the Canadian nursery sector.

"The moth only has one host," Smith said. If the boxwood continues to be planted, then it's going to probably require constant vigilance.”

How to treat a boxwood infestation

According to Smith and the latest advice from the Landscape Ontario Horticultural Trades Association, the leading agent for tackling a box tree moth infestation is a biological pesticide containing Bacillus thuringiensis subsp. Kurstaki, commonly known as BTK Spray. It can be found at your local garden centre.

"Once you start to see signs of browning, or larvae feeding and webbing on the plants, you should very quickly knock that population down," Smith said.

According to Landscape Ontario, regular inspections and maintenance should keep an infestation at bay.

"If you're up to the task of keeping an eye out for the signs of box tree moth, and keep a supply of BTK, you will be fine," Executive Director Joe Salemi told CTV News Toronto. "You need to be prepared to keep a close eye on them during larval stages, otherwise they may need to be replaced."

"By inspecting the plants and observing the small green caterpillars in the foliage, you will know that is the right time to spray. Boxwood plants rarely host other caterpillars, so it is safe to spray your boxwood plants with BTK without impacting other beneficial insects or pollinators," the association said in a recent news release.

The best times for treatment, according to the association, are when the larvae are in their early stages, when they are less than one inch long, ideally in mid-May or early July.

If gardeners want to avoid the boxwood shrub altogether, Salemi highlighted a number of alternatives, including yew, inkberry holly, and azalea, that can be used. All are available at your local independent garden centres.

Still, he says boxwoods continue to be the main choice for hedges because of their ability to be shaped and maintained.

As the moth continues to spread in Ontario and beyond, the association underlined that collaboration among landscape horticulture professionals, homeowners, and local authorities is crucial.

“By staying informed and implementing effective management strategies, we can mitigate the impact of this invasive pest on Ontario's urban landscape.”

box tree moth

Ontario researchers predict further spread

Researchers, on the other hand, say the box tree moth’s spread isn’t slowing anytime soon – according to a model worked on by a team of Ontario researchers, populations are expected in new parts of Europe, Africa, Canada and the U.S. in the coming years. In North America, the model predicts that climate change will soon render most of the region, with the exception of Alaska, the northern territories of Canada, higher elevations in the Rocky Mountains, and hot and dry areas in the south, “climatically suitable” for the species.

"The moth is not going anywhere," Smith, one of the participating researchers, said. "It's going to be very difficult to stop its movements."

According to the Canadian Food Inspection Agency, a national advisory committee has been established to explore the issue of the box tree moth in Canada and to raise awareness within the industry and the public. If you encounter an infestation outside of Ontario, the agency asks you to report it promptly.