Boxwoods have long been a top choice for landscape shrubs. Their deep green colour and year-round foliage make them especially appealing. They can be shaped into almost any form, and until recent years, they required minimal maintenance and experienced few issues. Unfortunately, with the growing population of the Box Tree Moth (Cydalima perspectalis) in Southern Ontario, many homeowners are now considering removing boxwoods from their landscapes and starting over.
History of the Box Tree Moth
Native to Asia, the Box Tree Moth (BTM) has become an invasive species in Southern Ontario. It was first identified and documented in August 2018.
The threat it poses to boxwoods is severe. Reports indicate that complete defoliation of a boxwood shrub can occur in less than a week. While many homeowners expect to see adult moths when their plants begin to decline, the damage actually occurs during the insect’s larval stage—when it exists as a caterpillar.
Life Cycle of the Box Tree Moth
The Box Tree Moth lays its eggs on the underside of boxwood leaves. Once the larvae emerge, feeding begins immediately, and this is when the most damage occurs.
As the caterpillars mature, they spin cocoons and enter the pupal stage. After approximately 1–2 weeks, they emerge as adult moths, which typically live for 2–4 weeks.
In Ontario, the BTM can produce between one and five generations per year, depending on location, temperature, and humidity. As colder weather approaches, the insect can overwinter in the larval stage, protected within cocoons spun between the leaves of its host plant.
Treatment Options
Treatment for the Box Tree Moth is relatively straightforward and can be effective if applied properly. A commonly used biological insecticide targets caterpillars specifically and is not harmful to humans.
The main challenge is timing. Applications must be made early, before significant damage occurs. Currently, a minimum of three treatments is recommended, with up to five applications depending on the season and severity of infestation.