Problems with your Boxwood Trees? Read on to find out what the problem could be, and the solution!
For decades, Niagara gardeners have relied on boxwoods as versatile landscaping shrubs, for hedges and as easy-care evergreens which can be planted in sun or shade. There were boxwoods in almost every neighbourhood. However, all that began to change last summer when the Box Tree Moth invaded the Niagara region. Within weeks, people found that their boxwoods had dead spots, had turned brown or were completely defoliated. The problem appeared quite suddenly, possibly from boxwoods brought into the region from a distant infected region. People have been searching for ways to treat their affected boxwoods or have had to dispose of plants they’ve had for many years.
Only affects boxwoods, not other plants
One small blessing is that in Canada, the Box Tree Moth only affects boxwoods, not other plants, so while it’s devastating to your boxwood hedge, it won’t decimate your entire garden as some other pests such as tent caterpillars can. That makes it easier to target the culprit, the moth larvae (caterpillar) that eats the boxwood leaves and makes it easier to find replacement shrubs as well.
Life cycle of the moth
The Box Tree Moth goes through several life stages in a season. In spring, larvae that spent the winter in webbing-wrapped cocoons in boxwood branches emerge in early May as tiny white and brown moths, which fly around laying eggs on the underside of boxwood leaves. These hatch in just a few days, and the tiny green caterpillars feed undetected under the leaves until they grow into larger caterpillars with black heads and black stripes down their backs. At this point they’re visibly destroying foliage, sometimes just leaving the centre strip and outside edge of a leaf. The caterpillar stage lasts about two weeks. If untreated, they’ll pupate again in summer to emerge as moths, lay another round of eggs and launch a second wave of destructive caterpillars. The cycle continues until the weather turns cold and they spin webbing to spend the winter as dormant pupae.
The timing of when the life cycle begins and ends depends on weather conditions and temperature. This year, with our especially warm spring and no late frost, they got off to an early start. They become active in spring once the temperature hits 15C.
Box Tree Moth is considered an invasive species in Canada and the federal government has some useful information about it at this link. https://www.invasivespeciescentre.ca/box-tree-moth/
Here’s a link to information from Landscape Ontario what to look for if you suspect Box Tree Moth is in your garden. https://landscapeontario.com/box-tree-moth-look-for-these-signs-this-spring
Treatment (BTK, how it works, when to apply)
There is a biological spray treatment called BTK which specifically targets caterpillars. It’s a concentrated liquid which contains a bacteria, Bacillus thuringiensis (BT) subspecies kurstaki (BTK), which occurs naturally in the environment in soil and water. It’s used in forestry to control spruce budworm and is also used agriculturally for pest caterpillars such as cabbage loopers. When caterpillars consume plant material sprayed with a dilution of the bacteria concentrate, it destroys their digestive system. It is toxic only to caterpillars, not flying insects, pollinators, pets, wildlife or people.
Here is a link which explains in detail how BTK works. https://www2.gov.bc.ca/gov/content/industry/forestry/managing-our-forest-resources/forest-health/invasive-forest-pests/spongy-moth/what-is-btk
A popular retail form of the treatment, Safers BTK, advises an application rate of 30 mL of concentrate to 10 litres of water, which will cover about 100 square metres. If you only have a small area to spray, use a smaller amount of water and corresponding smaller amount of BTK concentrate (15mL/5L of water). When you see the early stages of caterpillar activity on your boxwoods, spray all leaves to thoroughly wet them, and reapply after 7 to 10 days. Try to avoid spraying during the hottest part of the day or during a windy day.
With the first batch of eggs hatching in early to mid-May, spray right now, and watch for the next batch of caterpillars to appear. You may need to spray multiple times during the summer season.
How to dispose of badly infested plants
Sadly, if there’s more brown than green on your boxwoods, they will not recover from the devastating attack, and it’s best to dispose of them rather than spray them. Rather than putting them in yard bags or a compost pile, where any pupating moths can emerge and fly away to spread the infestation, put all boxwood plant material in garbage bags, seal them up and put them out with your trash. Wash your pruning tools and rake well afterward to ensure no caterpillars remain.
Other boxwood issues (Boxwood Blight)
Prior to last summer when the Box Tree Moth hit Niagara, people sometimes had issues with their boxwoods involving a fungal infection caused boxwood blight. This disease, which could occur after a spell of very humid warm weather, would cause boxwood leaves to die and fall off, which also gave a brown defoliated effect. However, the presence of webbing and caterpillars will quickly tell you if the problem is the moth or the blight. In either case, dispose of infested plant material in sealed garbage bags and disinfect your pruning tools with rubbing alcohol.
Replacement shrubs
Fortunately, there are several other shrubs which can replace boxwoods in your landscape. For sunny locations, globe cedars and inkberry holly are evergreen options if you want a line of round, fairly uniform plants. Globe cedars range in size from 24 inches high and wide (the Danica and Tater Tot varieties), to three or four feet high and wide (Little Champ or Little Giant in green, and Golden Globe, Sunkist or Fire Chief in gold and orange shades).
Inkberry Holly, particularly the Gem Box variety, is a good replacement for boxwood as the leaves and plant size and shape closely resemble boxwood. It has glossy, evergreen leaves and can be lightly pruned.
For a trimmed hedge look in a sunny location, consider Privet. While not evergreen, it grows quickly and can be pruned to give a solid hedge with flat edges or left to its natural leafy look to allow the fragrant white flowers to bloom in spring.
If your spot tends to be in the shade, Yews are a great solution. They’re evergreen and can be pruned to the size and shape you need. Spreading yews give an informal look while Hicks and Hills yews can be pruned into a solid formal hedge.
Here at Country Basket Garden Centre, we do have a few boxwood shrubs in stock which were inspected before being shipped to us from growers within the Niagara region, and we check them regularly for signs of the moth larvae. We carry Safers BTK, and we have a wide range of great shrubs which provide reliable alternatives to boxwoods. We just got in a shipment of Gem Box inkberry holly, and we also have yews, globe cedars, privet and a variety of other evergreen shrubs and hedge trees and shrubs. Our nursery staff would be happy to help you select which options would work best in your garden.
By Charlene MacKenzie, Nursery Manager
Country Basket Garden Centre
