Toronto is the epicentre of a North American infestation of the box tree moth, Cydalima perspectalis (Walker). The arrival of this invasive pest was officially announced by the Canadian Food Inspection Agency in November 2018. The moth crossed into the US at Niagara in the summer of 2021, so the US is still in the very early stages of responding to the infestation.
- Identification of Box Tree Moths and their damage
This pest (originally thought to have come from Asia) has now reached plague proportions. They are attacking (and sometimes killing) our pretty and widely-grown evergreen boxwood plants, Buxus spp. An infested boxwood plant is disfigured by the box tree moth by the loss of leaves, by webbing spun by the larvae, as well as larval excrements. Larvae feed principally on leaves of the host but may also attack the bark. They seem to prefer boxwood plants that receive partial shade but can also be found in full sun gardens.
To identify the box tree moth, carefully pull apart some webbed leaves and twigs and look for green larvae with black spots and black heads. Look first on the periphery and then deep inside the branches for this camouflaged larva. Webbing alone is not enough to ensure that you have box tree moth eggs; the webbing can be from spiders (good guys!) The larval webbing can be difficult to detect, and having some form of magnification will be helpful.
The insect has four stages. The eggs, which are laid in clusters, are greenish yellow in colour when first laid. Black dots start to show as the larval head capsule is forming. Eggs hatch in about 3 days. On hatching, larvae are greenish yellow in colour with a shiny black head. As they mature, they become more greenish and develop a striking pattern of thick black and thin white stripes along the length of the body. Mature larvae may be up to 4 cm long. Mature larvae pupate in a cocoon of white webbing spun among the leaves and twigs of the host. Pupae are on average 1.5 to 2.0 cm long. They are always hidden and rarely visible without really looking for them. Adults are described as medium-sized moths with a wing span of about 4 cm. Two colour variants have been recognized. The more common variant has white coloured wings with thick dark brown border trimmings. The ‘melanic’ variant is less common and has brown wings with small white streak on the forewing.
Here is a link to a poster with photos of the different stages of the box tree moth from the Canada Food Inspection Agency:
If you suspect that you have an infestation of box tree moth, you do need to treat the problem. When unmanaged, severe larval populations have consumed 100% of the plant foliage; hungry larvae may chew on branches and stems, causing plant mortality.
The horticultural association for landscape professionals, Landscape Ontario, has been monitoring and controlling the spread of box tree moth in Toronto for several years. Box tree moth larvae can be effectively managed with a safe biological insecticide, BTK ((Dipel 2X DF PCP#26508), that can be legally used in Toronto. This is the same biological insecticide that is applied by air over the City of Toronto to combat Gypsy moth larvae in the spring. Dipel 2X DF contains a naturally occurring bacterium, Bacillus thuringiensis (B.t.),
When the box tree moth larvae consume the spray residue on the leaves, they soon stop feeding and expire within a couple of days. However, this bacterial spray will not kill the eggs or pupae stage, so you need to time your spraying for when the larvae have hatched.
Frequent reinfestation is likely given the mobility of the adult moth. Treatment is not a “one and done”. You need to spray your box trees at least three times a year. Use a pressurized sprayer to spray, and focus on the bottom of the leaves. You can keep spraying every 5 – 7 days as long as you see any caterpillars or pupae on the plants. Note: only spray in the presence of larvae. Preventative spraying for long periods of time will affect good moth catepillars. OMAFRA (Ontario Ministry of Agriculture, Food and Rural Affairs) recommends that the first time to treat your plants and spray the larvae (worms) is between early May and mid-June. The second spraying is done in mid-July. The third spraying is done in mid September.
For additional information on this pest:
Landscape Ontario box tree moth resource page: https://landscapeontario.com/tag/box%20tree%20moth
OMAFRA’s blog: Ontario nursery crops blog (also for landscape plants) by Jen Llewellyn; search box tree moth: https://onnurserycrops.wordpress.com/tag/box-tree-moth/
- How to remove and dispose of infested shrubs
Your box tree plants may grow more leaves if they have been defoliated. However, it is important that you first look for bark damage and potential girdling. It may be necessary to remove damaged plant foliage and bark.
The following recommendations for cleaning up plants infested by the box tree moth come from the OMAFRA best practices factsheet (March 2022). Ensure complete cleanup and containment of plant debris, especially foliage.
Dispose or destroy the infested material and debris by:
(1) deep burial to a depth of 1 meter covered by soil, or
(2) incineration to ash; or
(3) heat treatment – place plant material in a sealed black plastic bag and expose it to 48 hours of direct sunlight. Following heat treatment, dispose of the sealed bag in your garbage (not compost) bin.
- Other boxwood problems to consider (in case it isn’t Box Tree Moth attacking your box trees)
Box tree moth is not the only potential villain attacking Boxwoods. Two other major pests are: (1) Boxwood blight (a fungus) that has been a problem in Ontario since 2014, and (2) the boxwood leaf miner.
Boxwood Blight: In the case of boxwood blight, some experts recommend not replanting boxwood in the same spot in the event that the blight fungal spores are still active in the soil. However, if care is taken to select more blight resistant varieties, then you should be able to proceed. There is a hybrid developed in Canada called the ‘Green’ series with cultivars such as Green Gem, Green Mountain and Green Velvet, that are resistant to boxwood blight.
For more information and pictures, here is a link to a Toronto Master Gardener factsheet on Boxwood Blight: https://www.torontomastergardeners.ca/askagardener/boxwood-blight/
More information can be found in this factsheet on Boxwood Blight from Landscape Ontario: https://landscapeontario.com/research-continues-on-management-of-boxwood-blight
Boxwood Leafminer (Monarthropalpus flavus): This is the larva (immature form) of a small, orangish gnat-like fly, whose larvae can cause severe damage to boxwoods. Using plant leaf minor resistant vazrieties is an excellent integrated pest management tactic for long term control.
Here is a link to a previous Toronto Master Gardener post that you may find informative on the Boxwood leafminer.
- Alternative Plants to Boxwood
Unfortunately, there aren’t many one-for-one substitutions for boxwood, which is why it’s so widely planted.
There are a number of alternatives that you could consider planting as a hedge if you are feeling adventurous. The University of Georgia has an interesting article on this topic: https://secure.caes.uga.edu/extension/publications/files/pdf/C%201107_2.PDF
They have a thorough table with suggestions that include non-traditional alternatives with distinct character; evergreen shrubs with fragrant or showy flowers; coarse-textured broadleaf evergreens or needle-leaf evergreens; and boxwood look-alikes with similar texture and compact form. The yew is included in their lists. Some of the suggestions on this website will only be appropriate to plant in Georgia and will not be good for our plant hardiness zone in Toronto (6a or 7).
There is also a Master Gardener Gardening Guide on “Evergreens Suitable for Hedging” that you may like to look over for ideas: https://www.torontomastergardeners.ca/gardeningguides/evergreens-suitable-for-hedging-a-toronto-master-gardeners-guide/
They list boxwood, but also add cedar, Ilex, ilex glabra, inkberry, juniper, spruce, white cedar, and yew.
Another Ontario native option might be an American holly, Ilex glabra or inkberry. It is hardy in Southern Ontario. Like boxwood, it is a broad-leaf evergreen but has the advantages of being both a native North American shrub and a berry producer for added garden interest. It works well as a mass planting, but prefers moist, acidic sandy to peaty soil and part shade conditions. Although this shrub bounces back after occasional hard pruning, the jury is out on whether it can take the place of boxwood in all its clipped forms.
Webinar: Jennifer Llewellyn webinar: Box Tree Moth Webiner with Jennifer Llewellyn, OMAFRA
Date published: July 2022
Prepared by the Toronto Master Gardeners, these Gardening Guides provide introductory information on a variety of gardening topics. Toronto Master Gardeners are part of a large, international volunteer community committed to providing the public with horticultural information, education and inspiration. Our goal is to help Toronto residents use safe, effective, proven and sustainable horticultural practices to create gardens, landscapes and communities that are both vibrant and healthy.
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