by Nancy Burnett
Boxwoods are used widely in our gardens. Their versatility as hedges or specimens, ease of being pruned or shaped, and their ability to grow in a variety of soil types has made boxwood a go-to plant for the home gardener. The distinctive scent of boxwoods can transport the gardener to a sunny Mediterranean clime or to a classic English garden.
Boxwoods have a long history in Ontario. Landscape architect Howard Grubb and garden designer Lorrie Dunnington came to Canada from England in 1911 and used boxwoods in many of their garden designs. These founders of Sheridan nurseries saw the potential to develop boxwoods suitable for our colder climate and worked for 10 years to develop hardy cultivars that were introduced in the Sheridan catalogue in 1939. Until recently, boxwoods were considered to be relatively pest free.
But now danger lurks. In 2018, the first box tree moth (Cydalima perspectalis) was first sited in Canada in the Toronto area. The Box tree moth, a species native to Asia, is now found throughout Ontario and is also present in Quebec, Nova Scotia, New Brunswick and PEI. It has spread south to New York where it was first detected in 2021 and is now found in Michigan, Ohio, Massachusetts, Pennsylvania, West Virginia, Delaware Maryland and Kentucky.
Are Your Boxwoods Affected?
Symptoms of box tree moth damage include the skeletonization of leaves, defoliation, evidence of silken webbing, and frass (larva excrement). Multiple overlapping generations of this insect can be produced each year. It is important to monitor your plants from May-September. Landscape Ontario suggests you begin your monitoring when it is warm enough for lilacs to bloom. Be sure to pull back the branches and look inside the shrub canopy to effectively monitor for damage.
Become familiar with this insect through its life cycle. The adult moth is nocturnal and therefore rarely seen. The adult female lays clusters of up to 100 eggs on the underside of the leaves. Eggs hatch within a week. The larva is a voracious feeder and it is at this stage that most damage is done to the plant. The pupa is likely to be tucked deep within the shrub so look diligently. (See photos below.)
Is It Really Box Tree Moth?
There are other pests and diseases that prey on boxwood. Boxwood leafminer and Boxwood psyllid populations can be controlled with pruning at the right time to control their populations. There is a detailed video that you can watch to help with correct identification of the boxwood tree moth https://www.youtube.com/watch?v=bFyqWK9O1NI
Taking Action
You can physically remove eggs, larva, or pupa and place them in soapy water or rubbing alcohol or simply use the squish method.
Landscape Ontario suggests applying BTK (Bacillus thuringiensis kurstaki) during the larval stage. However, the difficulty with using BTK is that it is not specific to the box tree moth and will kill any butterflies or moths that come in contact with your plant. If you find larva actively feeding on your plant and decide to apply BTK, your plant needs to be inspected every 5-7 days and then the BTK must be re-applied if more larva are found. This treatment is considered to be most effective when the BTK is applied 3-4 times a year. The need for ongoing treatment and the ecological impact may help you decide that it’s time to rethink boxwood as a desirable plant choice.
If you have an affected boxwood, it is important to know how to handle the affected debris. Landscape Ontario advises that any clippings or branches be placed in a black plastic garbage bag and placed in direct sunlight for a 48-hour period in order to kill the larva and then dispose of the material.
Help raise awareness about detecting this insect and control its spread.
Boxwood Alternatives
Master Gardeners focus on sustainable, environmentally friendly garden practices and encourage home gardeners to consider replacing boxwood with a native species. Some native plant options include: Inkberry, Winterberry, New Jersey tea, Snowberry, Kalm’s St. John’s Wort or Virginia Sweetspire. Evergreen options could include cultivars of Eastern White Cedar or Yew. The Halton Master Gardeners have a chart that outlines some options, their growing conditions, and features https://haltonmastergardeners.com/2023/10/01/sustainable-solutions-for-the-boxwood-blues/
Submitted by: Nancy Burnett, Tri-County Master Gardeners
Resources
Box tree moth a Growing Concern for Ontario Gardens. (2024, May 15). Landscape Ontario. https://landscapeontario.com/box-tree-moth-a-growing-concern-for-ontario-gardens
Government of Canada, Canadian Food Inspection Agency. Box tree Moth. (2025, June 19). https://inspection.canada.ca/en/plant-health/invasive-pests-and-plants/insects/box-tree-moth
Sims, Claudette and Janet McKay. Time to Re-think Boxwood Part 1. (n.d.) Halton Master Gardeners. https://haltonmastergardeners.com/2023/08/11/time-to-rethink-boxwood-part-1-box-tree-moth/
Sims, Claudette and Janet McKay. Sustainable Solutions for the Boxwood Blues Part 2. (n.d.) Halton Master Gardeners. https://haltonmastergardeners.com/2023/10/01/sustainable-solutions-for-the-boxwood-blues/
Photos courtesy of Canada Food Inspection Agency (link in Resources)
