Time to Re-Think Boxwood – Part 1 Box Tree Moth – Halton Region Master Gardeners

Claudette Sims & Janet Mackey – Halton Master Gardeners

The Rise and Fall of Boxwood

Boxwood (Buxus spp.) are a hugely popular ornamental non-native shrub. They are prized in formal gardens because they can be neatly trimmed and shaped to frame other plants or features. They are the perfect size for growing under windows or around trees and in small front yards. They create a textured understory of shrubs and many stay green through the winter.

Boxwood can be decimated over a period of a few days

However, their popularity has become their downfall. When first introduced, boxwood was carefree and quite disease and pest resistant. The unintentional introduction of non-native pests and diseases has changed things significantly. Boxwood are now susceptible to a number of disease and pests (More info: Clemson Boxwood Diseases & Insect Pests):

Because boxwood were so overplanted, these diseases and pests are now able to spread rapidly from one garden to the next. One such example is box tree moth (Cydalima perspectalis), an invasive moth from Asia. It is currently spreading throughout southern Ontario. Entire boxwood hedges are being decimated by this new invasive pest in a matter of days. While many boxwood shrubs won’t die immediately when infested by the box tree moth, they will be severely weakened. If treated early, your boxwood may survive, but they will need ongoing treatment for box tree moth and other diseases for as long as you decide to maintain them.

There are many great substitutions for boxwood in the landscape which will be covered in a subsequent blogpost. Landscape Ontario has a guide on rejuvenating infested boxwood shrubs.

Interesting fact: This pest was reported by a citizen scientist in Toronto in 2018 using iNaturalist. Consider joining to identify and report your own sightings.

Making the Call

Given the number of pests and diseases that are now affecting boxwood, gardeners will have to decide if it is worth keeping this non-native shrub, or whether it is time for a change.

Consider the following:

Boxwood will need to be monitored and sprayed indefinitely to keep pests and disease away.
Boxwood are the hosts for several invasive species and diseases. Keeping your boxwood will only help in their spread.
While some boxwood pests can be controlled with pesticides, the same pesticides will also kill non-target insects.
Btk will kill all lepidoptera (butterflies and moths), not just the box tree moth.
Insecticidal soap will kill boxwood psyllids, but it will also kill many other insects that are caught in the spray.

Master Gardeners follow sustainable gardening practices. Boxwood has so many pests and diseases, that growing it is no longer considered sustainable.

We now recommend that infested boxwood not be treated or replanted and that they be replaced over time. Gardeners should consider alternative plants that have similar characteristics.

Choosing native plants will ensure that complex food webs, important for pollinators and birds, are supported.

“I’m not giving up my boxwood!“

We understand that this may be a difficult decision for some gardeners. For those wanting to keep their boxwood, here are some suggestions:

When treating boxwood plants, it is important to identify the pest(s) involved before taking any action. Since boxwood may be attacked by more than one pest or disease at the same time, different treatments may be required on the same plant. E.g., Btk can be used to kill the caterpillar of the box tree moth. But Boxwood leafminer (Monarthropalpusi flavus), is a midge (fly maggot), not a moth, so it should NOT be treated with Btk.

Integrated Pest Management Strategies is a decision-making process for managing pests in an effective, economical and environmentally sound way:

Natural control – Encourage natural predators such as green lacewings and spiders.
Cultural control – Healthy plants are more tolerant of insect damage.
- Maintain plant vigour by proper watering, fertilizing and pruning.
Mechanical control (boxwood leafminer) –
- Boxwood Leafminer: Prune the foliage before adults emerge or right after adult flies lay their eggs in May. This reduces the overall population of the leafminer.
- Maggots: Pinch leaves hard enough to kill maggots in the infested leaves when practical.
- Boxwood psyllid: “If the infestation is small, do nothing. The damage caused by boxwood psyllids is only aesthetic and rarely affects the long-term health of the plant.
  - Prune out and destroy the affected tips. If done before the nymphs mature to adults, this will decrease the number of eggs for next year. The damage caused by psyllids is complete by early summer and the shrub will outgrow the injury.
  - Spray horticultural oil or insecticidal soap.
    - Apply in early May when new growth occurs.
    - Pesticides sprayed before or after that time will not be effective as the eggs are protected by the bud scales and the nymphs are protected by the cupped leaves.” More Infor: Missouri Botanical Garden – Help for Home Gardener Boxwood Psyllid)

Maintaining & Treating Existing Boxwood Shrubs for Box Tree Moth

Step 1: Monitor

Inspect boxwood plants weekly from May to September.

Look for signs of infestation, e.g., webbing, black-headed caterpillars, rolled/yellowing leaves, chewed foliage. (See: Box tree moth look for these signs this spring)

Step 2: Treatment

Boxwood can be treated with a Btk (Bacillus thuringiensis subsp. kurstaki) spray as soon as the black-headed green caterpillars appear. Btk works by killing any caterpillars that have eaten the leaves sprayed with Btk. Spraying is most effective on days where no precipitation is expected for at least 24 hours. Make sure you follow directions on the product label.

Timing of Treatment

Btk does not impact the eggs, pupae (cocoons) or adults moths. The Btk spray is only effective when caterpillars consume the leaves.

Continue to inspect plants 5 to 7 days after spraying and reapply if actively feeding caterpillars are present. It is likely that ongoing monitoring and continual treatment will be needed each week throughout the growing season to keep your boxwood healthy.

Since there are multiple generations of box tree moth each year, plants should be inspected during the following dates:
- May 15 – June 15
- July 15 – August 15
- September 1 – 20

Step 3: Fertilize

Landscape Ontario recommends fertilizing twice each year – early June and mid-October.

Step 3: Pruning

Pruning and removal of affected plant parts are essential to control box tree moth populations
Pruning is best done in late winter, but can be done before the first of August
Dispose of clippings as though they may have been infected by the moth. (See below)

Remove & Report

Report your infestation to the Canadian Food Inspection Agency

To determine the extent and distribution of boxwood moth, the CFIA (Canadian Food Inspection Agency) is asking all stakeholders to report sightings of this pest. It can be done online here. The CFIA will be interested in reporting of areas that are new rather than areas where box tree moth is already present (see map).

*The Distribution of Box Tree Moth (Cydalima perspectalis) in Southern Ontario.*

*Bag infested plants and place in direct sunlight for 48 hours*.

How do I dispose of infested plants?

Dig up the plants, roots and all.
Place in tied, sturdy black plastic bags.
Leave bags in the sun for a minimum of 48 hours.
Dispose of plants in your municipal yard waste stream.

Landscape Ontario Horticultural Trades Association

The Good News

Thankfully, Ontario has some amazing low-growing native shrubs that stay under 1.8m, or 6ft, tall. Sure, you could choose Boxwood or Spirea but these exotic ornamentals are grown for looks alone and they don’t support the wildlife that share our yards.

Look for the next post on Beautiful Boxwood Alternatives