They can strip a beautifully manicured boxwood shrub very quickly, rendering hours of effort worthless and ruining an investment. The larvae of Cydalima perspectalis, better known as the box tree moth, can’t get enough of the shrub that's a common sight on local residents' lawns.
However, the population of the pest in Ontario is larger than normal in 2024, leading to destruction on a massive scale in Halton Hills and Milton.
“It is absolutely and totally devastating boxwood hedges. You can lose your hedge in a season,” Georgetown Horticultural Society member and master gardener Susan Quirk said.
Her Milton & District Horticultural Society counterpart Jennifer Mirosolin says the moth is not a new pest.
"It's been around for a while. It's just absolutely exploded and I think it has exploded because of the very mild winter we had last year,” Mirosolin said.
Both boxwood shrubs and the moths that love eating them are not native to Canada. The bushes were brought to the country to be used as ornamental plants in gardens.
The moth was not far behind, however. Plants contaminated by the Asian native species mixed in with local flora, with sellers and owners being none the wiser. Here, they found an ideal climate, no predators keeping them in check and plenty of food.
The flying egg factories produce young with voracious appetites, causing their destructive power to spread far and wide. This year has been especially devastating for the province, with the Town of Halton Hills even providing advice on how to deal with them.
The Weather Network and the federal government have both sounded the alarm as well, with the latter saying that “Ontario is declared a place infested with box tree moth.”
A 2023 ministerial order under the Plant Protection Act is in place, which restricts the movements of boxwood shrubs.
Both Quirk and Mirosolin agree that homeowners should spray with BTK to kill the pests.
"If you spray BTK on your boxwood and try to get it underneath the leaves as well as on top, the caterpillars eat it. It kills them,” Mirosolin said.
“As soon as you know they're there, you need to spray and nothing else will help you, unfortunately,” Quirk added.
The bugs have three life cycles a year, meaning gardeners would need to spray at least as often during those times.
“The second wave is just about to hatch,” Mirosolin warned.
Silken webbing, the caterpillars themselves, defoliation and skeletonization of the bush are signs that the pests are present in a boxwood.
Landscape Ontario has lots of helpful information on how to tackle infestations. Both Mirosolin and Quirk are happy to answer any questions one might have about the bugs.
Quirk often answers questions about all things gardening on Facebook. She can be found in the Master Gardeners of Ontario and Halton Gardeners.
