A few years ago, I remember talking to a work colleague about the box tree moth, a nocturnal butterfly native to Asia, known for its larvae that devour the foliage of box trees (Buxus). At the time, it was already attacking boxwoods in Ontario and the northeastern United States, and I wondered if it had reached Quebec. Boxwoods are a very popular choice for low, formal hedges, especially in cooler climates. So it might be time to think about options for replacing boxwoods before the moth does too much damage.
Recently, this former work colleague came back to warn me: they’ve arrived! At least here in Montreal. She had spotted the first damage caused by the boxwood borer. In fact, by the summer of 2023, boxwood borer had already been detected on several private properties in Quebec, Nova Scotia and New Brunswick.
Regulated Pest
The box tree moth is now a real problem in Canada, where it is classified as a regulated pest. This means that its spread is closely monitored, especially in provinces like Ontario, Quebec and the Maritimes. Strict rules prevent the transportation of boxwood plants and any plant material that might be infested.
The Canadian Food Inspection Agency’s (CFIA) Box Tree Moth Order, dated May 9, 2024, declares the provinces of Ontario, Quebec, New Brunswick, Nova Scotia and Newfoundland and Labrador to be officially infested with box tree moth (Cydalima perspectalis). Under this decree, it is forbidden to move boxwood plants (and their cuttings) out of infested areas without a movement certificate issued by an inspector.
In the United States, the moth has also been on the radar since it was discovered in 2021 in New York State. States such as Michigan and Massachusetts have also spotted it. There, the USDA is imposing restrictions to prevent the insect from moving from one state to another, and homeowners must follow instructions to manage the infestation before it does too much damage to ornamental boxwood. Basically, be vigilant and report any suspicious signs to help stop the invasion!
Distribution
The box tree borer (Cydalima perspectalis) originates from Asia, where it is mainly found in China, Korea, Japan and India. It was accidentally introduced into Europe in the early 2000s, probably through international trade in boxwood plants. The first European detection was reported in Germany in 2006. The insect rapidly spread across several European countries, affecting France, the Netherlands, Italy, the UK, Belgium and many others. Due to its lack of natural predators and the great popularity of boxwood in European gardens, the invasion accelerated, causing significant damage.
In North America, the box tree borer was first detected in Canada, in Toronto, in 2018. Since then, it has spread to several Canadian provinces, including Quebec, Ontario and the Maritime provinces. In the United States, it was first reported in 2021 in New York State, then spread to other states such as Michigan and Massachusetts. Its ability to fly over several kilometers and travel via infested plants sold commercially has contributed to its rapid expansion in North America.
Lifecycle
The box tree borer’s life cycle comprises several stages. Eggs are laid under boxwood leaves in groups of 5 to 20 and hatch in around 3 days. The larvae, initially small and yellow, turn green with black and white stripes. They feed on boxwood foliage, devouring it and leaving only the veins. They pass through 6 to 7 larval stages, then transform into chrysalises in silken cocoons hidden in the branches. Adults emerge in summer, flying up to 10 km and living for around 14 days. The moth can have from 1 to 5 generations per year, depending on the region. In winter, the larvae enter diapause in a cocoon.
Detection
The box tree borer is not very difficult to identify. The small, translucent eggs are laid in clusters under the leaves. You’ll see a small black dot inside before they hatch. Young larvae are greenish yellow at first, but turn green with black and white stripes when mature. They grow up to 4 cm (1 1/2″ long and feed avidly on your boxwood foliage. The chrysalises are hidden in the branches, turning from pale green to brown, but they are quite inconspicuous. Butterflies, on the other hand, are active at night and have white wings edged in brown. In short, as soon as you see these signs, act fast before they wreak havoc on your boxwoods!
Control
Take a few minutes every now and then to check your boxwoods, especially under the leaves, for eggs or larvae. If you see any caterpillars or eggs, remove them by hand – this may be enough if the infestation is light. If you have a major boxwood borer infestation, there are several safe options for dealing with it.
- Pheromone traps: These little gadgets attract male butterflies and help limit reproduction. Install them around your box trees to detect and catch butterflies before they lay their eggs.
- Organic control: Use an organic insecticide such as Bacillus thuringiensis var. kurstaki (BTK), which only attacks caterpillars and does not harm other insects. An easy application and you’re safe for a while.
- Nematodes: Entomopathogenic nematodes, such as Steinernema carpocapsae and Heterorhabditis bacteriophora, can be used to combat the box elder borer by parasitizing its larvae. However, their effectiveness depends on a moist environment and their ability to reach the larvae hidden in the boxwood.
- Cleaning up debris: Regularly remove fallen leaves and branches, especially in autumn. Caterpillars can overwinter there, so it’s best to clean up to limit their return in spring.
Having a diverse garden with native plants could encourage the presence of natural predators of the moth, such as certain birds or insects. In addition, research into biological and genetic control could offer promising innovations for the future.
Alternatives aux buis
Does all this sound like a lot of work to you? Well, it does to me! It’s high time to look for alternatives to boxwood, at least temporarily. If you already have some, it might be a good idea to replace them before a major infestation occurs. Anyway, it’s not as if boxwoods don’t already have plenty of problems!
The two main alternatives for low clipped hedges, privet (Ligustrum vulgare) and Japanese holly, are at the limit of their hardiness in Canada and are considered potentially invasive in some parts of North America.
The end of formal hedges? I wouldn’t mind! But you have to think about the historic and heritage gardens that contain them. So if you detect the presence of boxwood borer, act quickly by removing them or opting for a control measure to prevent their spread!
Why not opt for a free-form hedge, where shrubs are allowed to retain their natural habit, or a mixed hedge, composed of a variety of plants? Here are a few ideas:
- Leadplant (Amorpha canescens) – Height: 24 to 36 inches / Width: 24 to 36 inches / Canadian Hardiness Zone: 3 / USDA Hardiness Zones: 3-8
- Fringe Tree (Chionanthus virginicus) – Height: 60 to 72 inches / Width: 48 to 60 inches / Canadian Hardiness Zone: 5 / USDA Hardiness Zones: 4-9
- Black Chokeberry (Aronia melanocarpa) – Height: 36 to 60 inches / Width: 36 to 60 inches / Canadian Hardiness Zone: 4 / USDA Hardiness Zones: 3-8
- Bearberry (Arctostaphylos uva-ursi) – Height: 6 to 12 inches / Width: 24 to 36 inches / Canadian Hardiness Zone: 2 / USDA Hardiness Zones: 2-6
- New Jersey Tea (Ceanothus americanus) – Height: 24 to 48 inches / Width: 24 to 48 inches / Canadian Hardiness Zone: 4 / USDA Hardiness Zones: 4-8
- Sand Cherry (Prunus pumila) – Height: 24 to 48 inches / Width: 48 to 60 inches / Canadian Hardiness Zone: 3 / USDA Hardiness Zones: 2-6
- Sweetfern (Comptonia peregrina) – Height: 24 to 36 inches / Width: 36 to 48 inches / Canadian Hardiness Zone: 3 / USDA Hardiness Zones: 3-7
- Bush Honeysuckle (Diervilla lonicera) – Height: 24 to 48 inches / Width: 24 to 48 inches / Canadian Hardiness Zone: 3 / USDA Hardiness Zones: 3-7
- Leatherwood (Dirca palustris) – Height: 48 to 72 inches / Width: 48 to 60 inches / Canadian Hardiness Zone: 4 / USDA Hardiness Zones: 3-8
- Alpine Currant (Ribes alpinum) – Height: 39 to 59 inches / Width: 39 to 59 inches / Canadian Hardiness Zone: 4b / USDA Hardiness Zones: 3-7
- Creeping Juniper (Juniperus horizontalis) – Height: 6 to 12 inches / Width: 48 to 72 inches / Canadian Hardiness Zone: 2 / USDA Hardiness Zones: 3-9
- Smooth Hydrangea (Hydrangea arborescens) – Height: 36 to 60 inches / Width: 48 to 60 inches / Canadian Hardiness Zone: 3 / USDA Hardiness Zones: 3-9
- Kalm’s St. John’s Wort (Hypericum kalmianum) – Height: 24 to 36 inches / Width: 24 to 36 inches / Canadian Hardiness Zone: 4 / USDA Hardiness Zones: 4-7
- Ninebark (Physocarpus spp.) – Height: 59 to 118 inches / Width: 59 to 98 inches / Canadian Hardiness Zone: 2b / USDA Hardiness Zones: 2-7
- Dwarf Ninebark (Physocarpus opulifolius ‘Nanus’) – Height: 24 to 36 inches / Width: 24 to 36 inches / Canadian Hardiness Zone: 2b / USDA Hardiness Zones: 2-7
- Shrubby Cinquefoil (Dasiphora fruticosa) – Height: 24 to 39 inches / Width: 24 to 39 inches / Canadian Hardiness Zone: 2 / USDA Hardiness Zones: 2-7
- Flowering Raspberry (Rubus odoratus) – Height: 48 to 72 inches / Width: 48 to 72 inches / Canadian Hardiness Zone: 3 / USDA Hardiness Zones: 3-7
- Smooth Rose (Rosa blanda) – Height: 36 to 72 inches / Width: 36 to 48 inches / Canadian Hardiness Zone: 3 / USDA Hardiness Zones: 3-7
- Blizzard Mockorange (Philadelphus lewisii ‘Blizzard’) – Height: 59 to 79 inches / Width: 59 to 79 inches / Canadian Hardiness Zone: 3 / USDA Hardiness Zones: 3-7
- Meadowsweet (Spiraea alba) – Height: 36 to 60 inches / Width: 36 to 48 inches / Canadian Hardiness Zone: 3 / USDA Hardiness Zones: 3-8
- Vanhoutte Spirea (Spiraea x vanhouttei) – Height: 72 to 89 inches / Width: 69 to 79 inches / Canadian Hardiness Zone: 3 / USDA Hardiness Zones: 3-8
- Fragrant Sumac (Rhus aromatica) – Height: 36 to 72 inches / Width: 48 to 60 inches / Canadian Hardiness Zone: 3 / USDA Hardiness Zones: 3-9
- Common Snowberry (Symphoricarpos albus) – Height: 48 to 72 inches / Width: 48 to 72 inches / Canadian Hardiness Zone: 3 / USDA Hardiness Zones: 3-7
- Arrowwood Viburnum (Viburnum dentatum ‘Christom’) – Height: 36 to 60 inches / Width: 48 to 60 inches / Canadian Hardiness Zone: 3 / USDA Hardiness Zones: 3-8
- Weigela (Weigela spp.) – Height: 36 to 72 inches / Width: 48 to 60 inches / Canadian Hardiness Zone: 4 / USDA Hardiness Zones: 4-8
This was a great read. Thanks for the detailed information!
Thank you for this article Mathieu, I had to remove a boxwood hedge due to blight, no sign of moths or caterpillars. A planting opportunity.
Good to hear we can get rid of boxwood, not sorry that it has to be replaced by beautiful flowering, and fruiting shrubs or exciting healthy evergreens. Nope not sorry that it has to be replaced. Personally when I see boxwood growing in a garden it makes me think of invasive species similar to dog strangling vine, goutweed all really useless plants without any benefits. Oh yes the invasive moth is being supported by non native boxwood Ummm makes you wonder.
I, for one, love boxwood. The pretty, shiny little leaves are gorgeous. I’ll be watching out for this pest as we have quite a few boxwood and I would hate to lose them. I have a lot of the alternatives listed and I love them, but for me they don’t fill the place of boxwood.